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Soil mineralizable nitrogen as an indicator of soil nitrogen supply for grain corn in southwestern Ontario
A Thesis submitted to the Committee of Graduate Studies in Partial Fulfillment of the requirements for the Degree of Master of Science in the Faculty of Arts and Science
TRENT UNIVERSITY
Peterborough, Ontario, Canada
© Copyright by Jessica Lucie Stoeckli 2015
Environmental and Life Science M.Sc. Graduate Program
September 2015
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ABSTRACT
Soil mineralizable nitrogen as an indicator of soil nitrogen supply for grain corn in southwestern Ontario
Jessica Lucie Stoeckli
Soil mineralizable nitrogen (N) is the main component of soil N supply in humid
temperate regions and should be considered in N fertilizer recommendations. The
objectives of this study were to determine the potentially mineralizable N parameters, and
improve N fertilizer recommendations by evaluating a suite of soil N tests in southwestern
Ontario. The study was conducted over the 2013 and 2014 growing seasons using 19 field
sites across southwestern Ontario. The average potentially mineralizable N (N0) and
readily mineralizable N (Pool I) were 147 mg kg-1 and 42 mg kg-1, respectively. Pool I
was the only soil N test that successfully predicted RY in 2013. The PPNT and water
soluble N (WSN) concentration (0-30cm depth) at planting were the best predictors of
fertilizer N requirement when combing data from 2013 and 2014. When soils were
categorized based on soil texture, the relationships also improved. Our findings suggest
that N fertilizer recommendations for grain corn can be improved, however, further field
validations are required.
Keywords: nitrogen, nitrogen mineralization, corn, southwestern Ontario, soil nitrogen supply, soil N test
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ACKNOWLEDGEMENTS First, I would like to thank my supervisor, Dr. Mehdi Sharifi for providing me with this
opportunity. I would also like to thank my committee members Dr. Catherine Eimers and
Dr. Raul Ponce Hernandez for their guidance and participation. I would like to thank my
collaborators Greg Stewart, Craig Drury, Dave Hooker, Bao-Luo Ma and their
technicians Kenneth Van Raay, Scott Jay, Henk Wichers, Ben Rosser, Scott Patterson
and Lynne Evenson for their collaboration, patience and support throughout the course of
this project, they were a valuable asset to the completion of this project. I am grateful for
my lab partner and friend Samantha Halloran for all the help and support she has
provided me over the past two years and Liana Orlovskaya for technical assistance.
Finally I would like to thank Ben Thomas for the endless intellectual conversations and
support, and my family for their encouragement.
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TITLE PAGE /i ABSTRACT /ii ACKNOWLEDGMENTS /iii TABLE OF CONTENTS /iv LIST OF FIGURES /vi LIST OF TABLES /vii LIST OF ABBREVIATIONS /ix CHAPTER 1. Review of methods for estimating soil N supply for grain corn in Ontario
1.1 Introduction /1 1.2 Nitrogen in crop production /2
1.2.1 Forms of N in soil /2 1.2.1.1 Soil mineral N /2 1.2.1.2 Soil organic N (SON) /3 1.2.2 Nitrogen transformation /6 1.2.2.1 N mineralization and immobilization /6 1.2.2.2 N losses /9 1.2.3 Soil N supply /11 1.3 Current status of N recommendations for corn in Ontario /12 1.3.1 Pre-plant nitrate test (PPNT) /13 1.3.2 Pre-sidedress nitrate test (PSNT) /14 1.4 Methods of predicting soil N supply /15 1.4.1 Laboratory- based measures of soil N supply /15 1.4.1.1 Biological tests /15 1.4.1.2 Chemical tests /17 1.4.2 Field-based measures of soil N supply / 25 1.4.2.1 Crop response /25 1.6 Conclusion /26 1.7 Thesis Objectives /27 CHAPTER 2. Evaluating laboratory-based indicators to predict N availability to corn in southwestern Ontario 2.0 Abstract /28 2.1 Introduction /29 2.2 Materials and Methods /31 2.2.1 Site description and plot setup /31 2.2.2 Soil sampling and analysis /35 2.2.3 Laboratory methods of predicting crop N availability /35 2.2.4 Field-based indicators of crop N availability /39 2.2.5 Statistical analysis /40 2.3 Results / 40 2.3.1 Laboratory methods of predicting corn N availability /40 2.3.2 Field-based indicators of corn N availability /47 2.3.3 Relationships between laboratory and field-based indicators /49 2.4 Discussion /55 2.4.1 Laboratory methods of predicting corn N availability /55
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2.4.2 Field-based indicators of corn N availability /58 2.4.3 Relationships between laboratory and field-based indicators of crop N availability /59 2.5 Conclusion /61 CHAPTER 3. Assessing the ability of laboratory-based indicators of mineralizable N to predict fertilizer N recommendations for corn in southwestern Ontario 3.0 Abstract /63 3.1 Introduction /64 3.2 Materials and methods /68 3.2.1 Field site description and plot setup /68 3.2.2 Soil sampling and analysis /71 3.2.3 Soil N test parameters /71 3.2.4 Field indicators of corn N availability /73 3.2.5 Statistical Analysis /77 3.3 Results /78 3.3.1 Soil N test parameters /78 3.3.2 Field indicators of corn N availability /80 3.3.3 Soil test correlation and calibration /85 3.4 Discussion /99 3.4.1 Soil N test parameters /99 3.4.2 Field indicators of corn N availability /100 3.4.3 Soil N test correlation and calibration /102 3.5 Conclusion /104 GENERAL CONCLUSIONS/106 REFERENCES /108 APPENDIX /125
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LIST OF FIGURES Figure 2.1 Map of locations of experimental field sites in southwestern Ontario in 2013. n=12. Figure 2.2 The relationship between PNU0N and DY in zero N plots for experimental corn sites in Ontario. Figure 2.3. Relationship between PPNT and RY for experimental sites in Ontario in 2013. Figure 2.4 Relationship between Pool I and RY for (a) the whole data set, (b) Cs-T soils (clay ≤ 240 g kg-1), and (c) Md-T soils (clay > 240 g kg-1); *=data point not included in correlation and regression analysis. Figure 2.5. Relationship between Pool I + SMNp in CT soils and RY. *=data point not included in correlation and regression analysis. Figure 3.1 Map of locations of experimental field sites in southwestern Ontario in 2014, n=7. Figure 3.2. A scheme of corn N calculator spreadsheet developed by OMAFRA for general recommendation of N rates for corn in Ontario. Figure 3.3 Corn grain yield response curves to N applied at all individual experimental corn trials that obtained maximum yield, each point is the mean of replicates (n=4). Figure 3.4 Relationship between soil N supply and relative yield (RY) for corn N trials in Ontario in 2013 and 2014. (n=49). Figure 3.5. Relationship between relative yield and a PPNT, b WEMN and c WSN for the whole data set (n=49). Figure 3.6. Relationship between RY and a Pool I + SMNp and b WEMN in Cs-T soils (n=28). Figure 3.7. Relationship between RY and a WEMN, b WEOC:N and c POMC:N for Md-T soils (n=21). Figure 3.8 Relationship between a PPNT and MERN b PPNT and MYRN c WSN and MERN and d WSN and MYRN; x=Elora site.
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LIST OF TABLES Table 2.1. Summary of characteristics for 12 experimental sites established in southwestern Ontario (n=4). Table 2.2. Mean values for N mineralization parameters (standard deviation in parenthesis) measured during the long-term aerobic incubation for soils from each experimental site sampled in southwestern Ontario (n=4). Table 2.3 Mean cumulative N mineralized on day 2, 4 7 and 14 of the long-term aerobic incubation for each experimental field site (n=4) and the correlation coefficient (r) with RY at each time step for Cs-T (n=21) and Md-T (n=21) soil texture group. Table 2.4. Mean values for mineralizable N tests (standard deviation in parenthesis) measured in soils from 12 experimental sites (n=4). Table 2.5. Mean values for field-based indicators of crop N availability (standard deviation in parenthesis) measured at 12 experimental sites in southwestern Ontario (n=4). Table 2.6. Correlation coefficients (r) between laboratory indicators for predicting crop N availability and RY for whole data set (n=42), Cs-T soils (clay ≤ 240 g kg-1, n=21) and Md-T soils (clay>240 g kg-1, n=21). Table 3.1 Summary of site characteristics for 2014 corn N response trials in Ontario (n=4). Table 3.2 Means for the proposed soil N tests (standard deviation in parenthesis) from 7 corn N response trials in Ontario in 2014 (n=4). Table 3.3 Mean values for crop yield, crop response indicators and soil N supply (standard deviation in parenthesis) from 13 corn N response trials across Ontario (n=4) established in 2013 and 2014. Table 3.4. Recommended rate of N fertilizer (MERN and MYRN) for each site in 2013 and 2014 using the quadratic equation based on corn yield response to fertilizer N rates (Figure 3.3), and recommended rate based on the corn N Calculator. Table 3.5. Correlation coefficients (r) between soil N tests and RY for 2013 and 2014 Category I field sites field sites for the whole dataset (DS), coarse textured soils (Cs-T soils, clay ≤ 240 g kg-1) and medium textured soils (Md-T soils, clay > 240 g kg-1). Table 3.6. Correlation coefficients (r) between soil N test parameters and MERN and MYRN for 2013 and 2014 field sites for the whole dataset (DS), coarse textured soils (Cs-T soils, clay ≤ 240 g kg-1) and medium textured soils (Md-T soils, clay > 240 g kg-1).
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Table 3.7 Interpretation table for successful soil N tests PPNT and WSN using linear response curves derived from the relationship between RY and soil N test. For PPNT, RY= 3.24(PPNT)+34. For WSN, RY=1.37(WSN)+3.4. Table 3.8 Recommended N fertilizer rates based on the linear relationship between PPNT concentration and MERN (MERN=-5.16 (PPNT)+203; R2 =0.47) and MYRN (MYRN=-4.72(PPNT)+167; R2=0.56). Table 3.9. Recommended N fertilizer rate based on the linear relationship between WSN concentrations and MERN (MERN=-2.02(WSN)+250; R2=0.60) and MYRN (MYRN=-1.65(WSN)+197; R2=0.48).
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LIST OF ABBREVIATIONS 2M KCl extractable nitrogen following hot water extraction (KCl-WEOM)
95% of the maximum yield (95% RY)
Adenosine triphosphate (ATP)
Ammonia (NH3)
Ammonium (NH4)
Carbon (C)
Cation exchange capacity (CEC)
Crop heat unit (CHU)
Delta yield (∆Y)
Dissolved organic matter (DOM)
Economic optimum nitrogen (EONR)
Electrical conductivity (EC)
Extractable organic matter (EOM)
Hot water extractable organic matter (Hot-WEOM)
Hot water extractable organic carbon (Hot-WEOC)
Illinois soil nitrogen test (ISNT)
Light fraction organic matter carbon (LFOMC)
Light fraction organic matter nitrogen (LFOMN)
Maximum economic rate of nitrogen (MERN)
Mineralization rate constant (k)
Microbial biomass (MB)
Microbial biomass carbon (MCB)
x
Microbial biomass nitrogen (MBN)
Nitrate (NO3)
Nitrogen (N)
Nitrogen use efficiency (NUE)
Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA)
Organic matter (OM)
Particulate organic matter (POM)
Particulate organic matter carbon (POMC)
Particulate organic matter nitrogen (POMN)
Potassium Chloride (KCl)
Potentially mineralizable nitrogen (N0)
Net nitrogen mineralized (Net N min)
Nitrogen mineralized during the first 2 weeks of the aerobic incubation (Pool I)
Nitrogen mineralized during weeks 2 and 24 of the aerobic incubation Pool II
Nitrogen predicted to mineralize during the aerobic incubation minus Pool II (Pool III)
Pre-plant nitrate test (PPNT)
Pre-sidedress nitrate test (PSNT)
Relative yield (RY)
Residual soil nitrate (RSN)
Salt extractable organic matter (SEOM)
Sodium bicarbonate (NaHCO3)
Soil mineral nitrogen at harvest (SMNh)
Soil mineral nitrogen at planting (SMNp)
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Soil organic matter (SOM)
Soil organic nitrogen (SON)
Total carbon (TC)
Total nitrogen (TN)
Ultraviolet (UV)
Ultraviolet absorbance of a sodium bicarbonate extract at 205 (NaHCO3-205)
Ultraviolet absorbance of a sodium bicarbonate extract at 260 (NaHCO3-260)
Water extractable mineral nitrogen (WEMN)
Water extractable organic matter (WEOM)
Water extractable organic nitrogen (WEON)
Water extractable organic carbon (WEOC)
Water filled pore space (WFPS)
Water soluble nitrogen (WSN)
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Chapter 1. Review of methods for estimating soil N supply for grain corn in Ontario 1.1 Introduction
Soil mineralizable nitrogen (N) is an important but variable component of the soil N
supply to grain corn (Zea Mays) in southwestern Ontario as residual mineral N and
fertilizer N is highly susceptible to leaching. The soil N supply is the amount of soil N that
will be available to the field crop during a growing season and consists of residual mineral
N from the previous growing season and N mineralized from soil organic N (SON) during
the current growing season (HGCA, 2012; Whalen et al., 2013). In humid temperate
environments the soil N supply is dominated by SON mineralization due to the high
mineral N losses during the off-season months (Zebarth et al., 1996; Wu et al., 2008). A
large portion of the total N in the soil is in the organic form but only 1-4% will mineralize
into plant available nitrate and ammonium (NO3 and NH4) during a growing season
(Tisdale et al., 1985; Warren, 2014). The contribution of SON to the plant available pool
throughout the growing season is difficult to predict due to variations in soil physical and
chemical properties, environmental factors (temperature and precipitation), and
management practices (Ma et al., 2004).
For corn production in Ontario, soil N mineralization is not accounted for when providing
producers with N fertilizer recommendations as rates rely on the pre-plant nitrate (NO3)
test (PPNT) or the pre-sidedress NO3 test (PSNT) (Verhallen and LeBoeuf, 2009). The
PPNT and PSNT estimate NO3 concentrations in the soil at one point in time (St. Luce et
al. 2011; Dayegamiye, et al., 2012), however it is well known that NO3 is highly
susceptible to leaching which results in an economic loss for growers and has
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environmental implications. One approach to minimize N losses is through consideration
of the SON mineralized throughout the growing season prior to mineral N fertilizer
application. Limited information is available for a pre-plant soil N mineralization test and
the effectiveness of such a test for grain corn in southwestern Ontario. Nitrogen fertilizer
recommendations could be improved by developing indicators of the contribution of soil
N mineralization to crop N demand, which could have economic benefits and reduce
environmental pollution. This chapter will cover the process of N mineralization and its
importance to crop N uptake, introduce the concept of the soil N supply and summarize
laboratory and field-based indicators for assessing and predicting plant available N for
corn production in southwestern Ontario
1.2. Nitrogen in crop production
1.2.1 Forms of N in Soil
1.2.1.1 Soil mineral N
Nitrogen is an essential nutrient in crop production and deficiencies can result in
substantial yield losses (St.Luce et al., 2011). The major source of soil N for corn is
mineral N (NH4 and NO3). Total N content in mineral soils is approximately 1% of which
5% is in the mineral form (Havlin et al., 2005). Ammonium can originate from the
mineralization (transformation of organic N into mineral N by soil microorganisms) of
SON or be added to the soil as ammonia N fertilizers or organic amendments such as
composted animal manures and slurries (Myrold and Bottomley, 2008). In agricultural
soils of Ontario, NO3 is the most abundant and extractable form of mineral N (Whalen
and Sampedro, 2010) and the majority originates from the nitrification of NH4 by
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chemoautotrophic microorganisms in the soil. Additionally, atmospheric deposition of N
can be a source of mineral N. Ammonia volatilization from land application of manure,
animal grazing and manure storage can be deposited and transformed into NO3 on
agricultural fields or be directly assimilated by crops through their stomata (Bittman and
Mikkelsen, 2009). Zbieranowski and Aherne (2012) found that NH3 concentration in
southern Ontario was highest in the springtime and varied from 0.3 𝜇g m3 (0.0004 ppm)
in low intensity agricultural areas to 2.8 𝜇g m3 (0.00387ppm) in areas of intensive
agriculture. The concentration was directly related to cattle and pig numbers. Watmough
et al. (2014) found that N concentration could also be predicted by road density (R2=0.32
for NH3 and 0.79 for NO2) and Aherne and Posch, (2013) estimated that N deposition can
exceed 20 kg N ha-1 yr-1 in some areas of southern Ontario.
In conventional corn production in Ontario the main sources of mineral N are mineral N
fertilizers and the decomposition and mineralization of crop residues and/or SON. The
Ontario Ministry of Agriculture Food and Rural Affairs (OMAFRA) reported average
spring NO3 concentrations of 9.8 mg N kg-1 soil in 2014, which is lower than the
historical average of 11 mg kg-1 and the 2012 average of 12.2 mg kg-1 (warm spring), and
higher than the 2011 average of 9.5 mg kg-1 (cool spring).
1.2.1.2 Soil organic N (SON)
The SON in soils is approximately 95% of the total N and is held in the soil organic
matter (SOM) (Schulten and Schnitzer, 1998; Olk, 2008). In southwestern Ontario soils
the SOM content ranges between 1 and 6% (OMAFRA, 2009). The N compounds found
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in the SOM are generally complex compounds including free amino acids, peptides and
proteins bound to clay minerals and humic colloids, amino sugars, nucleic acids,
chlorophyll related phospholipids, amines, vitamins, and heterocyclic N compounds
(Stevenson 1982,1994). The concentration of SON is typically a reflection of
management practices- mainly input and tillage (Wanniarachchi et al., 1999). The main
input of organic N in conventional corn production in Ontario is crop residues (Gregorich
et al., 1996).
The three main sources of crop residues include corn, soybean (Glycine max) and wheat
(Triticum spp.) stalks and in some cases, red clover and hay. Residues can be left on the
field, removed for straw or removed for bio-processing (OFA, 2012). A direct interaction
between fertilization rate and crop residue biomass is apparent and by increasing the
amount of residue returned to the soil, the SOM content and soil N supply tend to
increase. Gregorich et al. (1996) found that mineral N fertilization over 30 years in a
continuous corn rotation doubled the crop residue input compared with no fertilization.
The high crop residue input in this system also resulted in a 10% increase in soil carbon
(C). The plant availability of N from crop residues is dependent on their C: N ratio
(Jensen, 1994; Willson et al., 2001; Sanchez et al., 2004). Residues with low C: N ratio
(eg. legumes such as red clover) are expected to decompose rapidly, potentially
increasing soil N supply during the beginning of the growing season (Wilson et al.,
2001). When residues with a high C: N ratio are added (eg. corn and wheat stalks),
immobilization of N occurs at first followed by a period of mineralization as the C: N
ratio of the residue decreases (Green et al., 1995). The contribution of crop residues to
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soil N supply depends on the residue type and percent residue cover, tillage, soil
properties and environmental conditions (Thorup-Kristensen et al., 2003).
Tillage can affect SOM levels and consequently SON dynamics through several
processes (Six et al., 1999; Mikha and Rice, 2004). Incorporation of crop residues
increases the rate and degree of decomposition (Douglas et al., 1980; Christensen, 1986)
by disrupting soil structure and increasing the oxidation of SOM (Paustian et al., 1995).
Contrasting results on SOM conservation under no tillage or minimal tillage have been
observed. Several studies have shown SOM content to increase in the surface soil under
no tillage (Kern and Johnson, 1993; Angers et al., 1997), while others have reported no
significant differences (O’Halloran, 1993). The effect of tillage is more prominent on
active fractions of SON rather than the total SON (Soon et al., 2001; Liang et al., 2004;
Sharifi et al., 2008). Sharifi et al. (2008) found that the active fraction of N (N
mineralized during a controlled aerobic incubation) increased by 21% on average under
no tillage compared to conventional tillage in soils collected from Saskatchewan and
Quebec.
The SON can be fractionated into various pools depending on stability and turnover rates;
a young readily available or labile pool, an intermediate pool which influences the soils
physical status, and a recalcitrant pool that is relatively inert to microbial breakdown and
relates to the physicochemical reactivity of soils (Ros et al., 2011). The labile pool has a
turnover time of days to months and the recalcitrant pool has a turnover time of years
(Biederbeck et al., 1994; Gregorich et al., 2003). The labile pool is of most interest in
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crop production as it relates to compounds that are easily extractable and soluble in soil
solution rendering them more prone to microbial decomposition. This labile N pool
constitutes a small portion (about 2%) of the total soil N and is composed of water-
extractable organic matter N, particulate organic matter N and microbial biomass N (St.
Luce et al., 2013). The labile N pool has also been recognized to be the most dynamic
and bioavailable and have been used as indicator of N supplying capacity of soils (Ros et
al., 2010). The more stable fractions have been identified in the humus fraction due to
changes in chemical structure, accumulation in recalcitrant components and decrease in
accessibility to microorganisms (eg. aggregate formation) as the SOM begins to age.
1.2.2 Nitrogen transformations
1.2.2.1 Nitrogen Mineralization and Immobilization
For SON to become available to plants, it must first be mineralized into mineral N or
small molecular weight organic compounds (Whalen et al., 2013). Nitrogen
mineralization is linked to the decomposition of SOM as the heterotrophic bacteria
responsible for this process require C, N, and other nutrients. The decomposition begins
as the depolymerization of soil polymers by extracellular enzymes followed by
breakdown into monomeric compounds such as amino acids, amines, amino sugars and
urea (Schimel and Bennett, 2004). The process then proceeds by aminization and finally
ammonification to yield NH4 (Havlin et al., 2005). Ammonium does not reside in the soil
solution for an extended period of time in humid temperate climates but either
immediately undergoes nitrification (under optimal oxygen and moisture levels) to yield
NO3, undergoes NH4 fixation, is taken up by plant roots or is immobilized by soil
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microbes (Havlin et al., 2005). Only a small fraction of the organic N is mineralized
during a growing season as the majority is protected from decomposition (Whalen et al.,
2013) and the extent to which N mineralization occurs under field conditions is
dependent on soil physical and chemical properties, soil biotic processes and
management practices (Zebarth et al., 2009; St. Luce et al., 2011).
Chemical properties such as pH and electrical conductivity (EC) directly influence the
activity of microorganisms (Hartel, 2005; St.Luce et al., 2011). The optimum pH for
mineralization is between 5 and 7 and decreases with increasing pH and/or EC. Greater
cation exchange capacity (CEC) reduces leaching losses, therefore increasing the soil N
supply and soils with higher SOM content have greater microbial populations and activity
and therefore higher mineralization rates (Schnurer et al., 1985; Sharifi et al., 2008). Soils
with higher clay content are less susceptible to leaching as they retain water and have the
ability to fix NH4 within clay lattices (Chantigny, 2004), however clay has the potential
to slow N mineralization as clay particles are known to physically protect SOM from
microbial decomposition (Angers et al., 1997; Yoo and Wander, 2006, Kölbl et al., 2006,
Chivenge et al., 2011; Nyiraneza et al., 2012). Sandy soils on the other hand have the
ability mineralize more of their organic N due to better aeration and less physical
protection of the SOM (Griffen, 2008).
Soil biotic processes, including microbial activity and biochemical processes associated
with N mineralization, are mainly controlled by soil moisture and temperature (Kolberg et
al., 1999). Temperature controls oxygen consumption by microorganisms and aerobic
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volume of the soil (Sierra, 1997). Soil moisture regulates oxygen diffusion in the soil and
controls microbial mobility and diffusion of soluble substrates to microbes (Agehara and
Warncke, 2005). When the soil becomes too dry, mobility of the microbes’ decreases and
when soils are too wet, oxygen content decreases resulting in reduced activity (St.Luce et
al., 2011). Under field conditions fluctuating moisture and temperature occur which can
lead to soil rewetting events that cause a flush of N mineralization as microorganisms
resume activity (Griffen, 2008). Optimal conditions for mineralization occur between
25oC and 35oC, but can occur at -2oC in loam soils and -6 oC in clay soils (Clark et al.,
2009), and at a soil moisture content between 50 and 80% field capacity (Whalen and
Sampedro, 2010). Effects of environmental factors on N availability are evident through
current year growing season rainfall and temperature. In years where rainfall is high, the
potential for N losses through leaching and denitrification increases compared to years
with lower than average rainfall. The two are not mutually exclusive and therefore a cooler
growing season with higher rainfall will require higher N fertilizer supplementation for
desired grain yields.
Agricultural practices such as tillage and addition of organic (crop residues) and mineral
fertilizers impact N mineralization and soil N supply through their effect on the SON
fractions (section 1.2.1.2). Other management practices such as including cover crops or
legumes into the rotation increases N availability through N fixation and retention of
residual soil NO3 following harvest of the main crop (Vyn et al., 2000; Mueller and
Thorup-Kristensen, 2001; St.Luce et al., 2011). Carpenter-Boggs et al. (2000) found that
inclusion of alfalfa (Medicago sativa) into the rotation increased the net N mineralized
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during 189-day field temperature incubation by 56 kg ha-1 compared to a continuous corn
rotation and by 47 kg ha-1 compared with a corn soybean rotation. Inclusion of wheat
into the rotation has also shown to increase the total N content of soils (Van Eerd et al.,
2014) and can be attributed to the high N rhizosphere deposition from wheat. If the
current years crop is corn however the late harvest date (September to October) generally
does not allow a cover crop to establish prior to snowfall, exemplifying the importance of
matching fertilizer rates to corn N demand.
1.2.2.2 Nitrogen losses
In humid temperate environments, N can be lost from the root zone through several
pathways: NO3 leaching, denitrification, ammonia (NH3) volatilization and NH4 fixation
by soil clay. The latter, NH3 volatilization and NH4 fixation are significantly less
compared to leaching and denitrification in humid environments (Janzen, 2003).
Ammonia volatilization, the transformation of NH4 to NH3, is mainly dependent on pH
(Cameron et al., 2013) and N fertilizer incorporation (Havlin et al., 2005; Griggs et al.,
2007; Soares et al., 2012). Ammonium fixation is dependent on clay content as up to 34%
of added NH4 can be fixed within clay lattices (Chantigny et al., 2004).
In humid temperate regions like Ontario where average growing season precipitation is
greater than 300 mm (Environment Canada, 2014), NO3 leaching from the root zone into
surface waters is the most significant pathway of N loss and has been linked to
environmental concerns such as eutrophication of freshwater bodies and nitrate poisoning
of drinking water (Mulvaney et al., 2008). In Ontario, residual soil NO3 following crop
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harvest increases the risk of NO3 leaching. De Jong et al. (2009) found that 47 kg N ha-1
of NO3 was leached from agricultural fields during the off season in Ontario between
1981 and 2006. Nitrogen fertilization exceeding crop N requirements is the major
contributor to increased concentrations of residual soil NO3 (Ziadi et al., 2012) and the
presence of tile drainage increases losses by 30-50% compared with native drainage
(Randall and Goss, 2008). The amount of residual soil N and subsequent leaching also
depends on precipitation, soil texture, crop rotation and tillage (St.Luce et al., 2011;
Rasouli et al., 2014).
Corn in Ontario receives high amounts of mineral fertilizers to reach maximum yields
and over fertilization is one of the leading causes of high residual soil NO3. The two most
common forms of N fertilizer for corn in Ontario are urea (solid) and urea ammonium
nitrate (UAN, liquid). Producers typically apply N in the spring at planting, at the pre-
sidedress stage (when corn in at the 6 leaf stage) or as a split application (pre-plant and
pre-sidedress) to avoid N losses (OMAFRA, 2009). More accurate rates that match crop
N demand can significantly reduce residual soil NO3 and its loss from fields to surface
waters (Mitsch et al., 2011; Rasouli et al., 2014).
Denitrification, the transformation of nitrate to atmospheric N (N2, N2O, NOx), occurs
mainly in soils that are waterlogged for an extended period of time (2-3 days) resulting in
low oxygen concentrations allowing denitrifying bacteria to utilize NO3 for metabolic
processes (Bremner and Shaw, 1958). The majority (30-90%) of denitrification losses in
humid temperate climates occurs during the spring thaw (Smith et al., 2004; Wagner-
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Riddle et al., 2007) and increases with N fertilization rate (Chantigny et al., 1998).
Incomplete denitrification results in nitrous oxide emissions and is influenced by tillage,
N source and timing of application (Drury et al., 2012).
1.2.3 Soil N Supply
Soil N supply is defined as the sum of the residual soil mineral N (NH4 and NO3) present
in spring from the previous growing season plus the amount of N that is mineralized
throughout the current years’ growing season (Zebarth et al., 2005; Whalen et al., 2013).
As stated in section 1.2.2.2 leaching of residual soil NO3 in the off season is substantial in
humid temperate climates and therefore the soil N supply is dominated by SON
mineralization during the growing season. The soil N supply has the potential to supply
large amounts of N to the growing crop (Wu et al., 2008; Zebarth et al., 2009; St.Luce et
al., 2011). Nyiraneza et al. (2012) found the soil N supplying capacity of 19 sites under
diverse cropping systems across Ontario, Quebec and New Brunswick ranged between 13
and 198 kg N ha-1. Dessureault-Rompre et al. (2011) measured the soil N supply for
potato in eastern Canada to be between 50 and 175 kg N ha-1, Wu et al. (2008) measured
the soil N supply through the use of resin bags to range between 96 and 120 kg N ha-1 for
corn in Ottawa and St. Luce et al. (2013) calculated the soil N supply (measured as
Canola N uptake) in western Canada to range between 26 and 229 kg N ha-1. The soil N
supply is ultimately controlled by factors that affect soil N mineralization potential
(Zebarth et al., 2009; St.Luce et al., 2011).
Besides the adverse effects of over-application of mineral fertilizers, proper N
management can also lead to a higher nutritional value of field crops. Nitrogen is one of
12
the essential macronutrients required by plants to grow and reproduce through its
presence in DNA, RNA, proteins (enzymes), chlorophyll, ATP, auxin and cytokins
(Andrews et al., 2013). Upon application of high amounts high synthetic fertilizers (N-
phosphorus (P)- potassium (K)), nitrates can accumulate in plant tissues and be
carcinogenic upon consumption to both humans and animals (Albrecht, 1992;
Santamaria, 2006; Sutton et al., 2011). Increasing the SOM content has other beneficial
effect besides increased soil N supply as the SOM is a natural chelator for micronutrients
essential for the production and synthesis of carbohydrates, proteins, vitamins, hormones
and other complex N compounds necessary in plant physiology (Watson, 2012). In
addition, mineral fertilizers have shown various other negative effects to the soil
including: detrimental effects on soil biology by inhibiting mycorrhizal fungi and N2
fixing symbionts; the loss of SOM which is responsible for proper soil aeration, structure
and plant nutrient availability; decrease in soil pH; and can cause a decline in crop yields
(Paungfoo-Lonhienne et al., 2012). Maintaining the capacity of soils to supply N, which
consists mainly of organic N present within the organic matter portion, is the next step in
sustainable crop production.
1.3 Current status of N recommendations for corn in Ontario
Corn is grown on 890 thousand hectares of cropland in Ontario and constitutes
approximately 63% of the province’s annual grain harvest (OMAFRA, 2013). The main
cropping system in Ontario is a corn- soybean-wheat rotation and the high N demand of
corn (180 kg N ha-1 on average) results in large applications of mineral N fertilizers. It is
estimated that 85-90 million tons of nitrogenous fertilizer are applied to the soil every year
13
across the globe (Warren, 2014). However, N use efficiency (NUE) in Ontario remains
low (40-65%) and a large portion (87%) of N taken up by the plant remains originated
from mineralization of SON (Stevens et al., 2005; Wu et al., 2008). This low NUE
combined with corn’s high N demand can result in substantial economic losses for
growers.
An accredited soil test for the contribution of SON mineralization to crop N uptake
currently does not exist in Ontario. Nitrogen fertilizer recommendations for corn
production in Ontario are computed using NO3 concentrations in the soil at planting
(PPNT) and/or at the side-dress stage (PSNT) or using the corn N calculator established
by OMAFRA (2010). This tool determines N fertilizer rates based on soil type, previous
management history, crop heat units (CHU), projected corn market prices and N fertilizer
cost.
1.3.1 Pre-plant nitrate test (PPNT)
For the PPNT, soil samples are taken within 10 days of planting (5 days before or after)
and analyzed for NO3. The concentration of NO3 in the soil prior to planting can be a
useful indicator of carryover N and early season N mineralization and can be used to
adjust for fertilizer application at planting (Greenwood, 1986; Bundy and Andraski,
2004; Sharifi et al., 2007). This test has proven successful in dry climates where residual
N is a major component of the soil N supply (Zebarth et al., 2001) but the high and
variable early season rainfall in humid climates decreases the accuracy of N
recommendations (Sharifi et al., 2007;; St. Luce et al., 2011). In addition O’Halloran et al.
(2004) found that soil NO3 from 0-30cm was not a good predictor of corn yield or yield
14
response to fertilizer application in southwestern Ontario due to within-field variability.
1.3.2 Pre-sidedress nitrate test (PSNT)
The pre-sidedress nitrate test (PSNT) has gained popularity because of its accuracy to
determine NO3 levels when the corn plant is 15-30 cm tall (V5 stage) (Fox et al., 1989),
just prior to rapid plant N uptake and the application of side-dress N fertilizers. Unlike
the PPNT, this method allows more time for sampling for the farmer, allows
determination of NO3 that has mineralized from organic sources during the spring months
and more accurately reflects total available N than the PPNT. The PSNT has been most
useful in predicting non-responsive areas (prevents over-fertilization) and has effectively
predicted corn response to applied N fertilizer over a wide geographic range (Magdoff et
al., 1984; Blackmer et al., 1989; Magdoff et al., 1990; Magdoff, 1991). Results from 52
corn N response trials conducted in Ontario from 1986 to 1990 showed that NO3 taken at
the pre-sidedress from 0-60cm was a good predictor of recommended N (R2=0.73) with a
critical concentration of 23 mg kg-1 (OMAFRA, 2009). In 2015, the PSNT was modified
by OMAFRA to include expected yield, omitted in previous PSNT test. Critical levels for
the new PSNT range from 22.5 to 32.5 mg kg-1 soil depending on expected yield
(OMAFRA, 2015). Although the PSNT has shown greater accuracy in predicting crop N
needs its accessibility to producers due to the requirement of side-dressing equipment, the
large soil spatial variability and changes in concentration over a short period of time has
hindered its widespread use (Beauchamp et al., 2004; Ma et al., 2007).
1.4 Methods of predicting soil N supply
A variety of laboratory and field based methods exist to determine plant available N from
mineralization of SON. Laboratory based methods can be separated into two categories:
15
biological and chemical. These methods attempt to isolate a fraction or pool of organic N
that will become available to the crop over the growing season. The majority of studies
use chemical methods as a relatively rapid alternative to the biological method which
measures potentially mineralizable N (N0) using a long-term aerobic soil incubation (>20
weeks) under controlled conditions. The acceptance of a rapid measure of plant available
N has in the past been focused on satisfying its ability to predict N0 and N supply in the
field (Sharifi et al., 2007; Schomberg et al., 2009). To date, no robust measure has been
adopted as a soil N test that is both a predictor of mineralizable N and plant available N
in the field. This is due to the inconsistencies across studies in extraction conditions and
furthermore the large variability in climate and soil characteristics across a wide
geographical area (St.Luce et al., 2011).
1.4.1 Laboratory-based indicators of soil N supply
1.4.1.1 Biological tests
The capacity for a soil to supply N is measured using soil incubation. These incubations
can be short or long term (2-24+ weeks), can be performed under aerobic or anaerobic
conditions, and can vary in incubation moisture and temperature conditions (St Luce et
al., 2011). Typically, the amount of N mineralized over 20+ weeks under optimal
moisture and temperature conditions is run through a first order kinetic model, first
developed by Stanford and Smith (1972) and later modified by Curtin and Campbell
(2008), to predict potentially mineralizable N (N0). The first order kinetic model can be
expressed as a single exponential model with the following equation:
Nmin= N0 [1-e-kt]
16
where Nmin is the cumulative amount of N mineralized at time t, N0 is potentially
mineralizable N, and k is the mineralization rate constant. The N0 is an indicator of the
soils capacity to supply N and it is hypothesized to release the fraction of SON
responsible for the release of mineral N from microbial action over a growing season.
Several modifications have been made to more accurately predict N0. Most
recently, Sharifi et al. (2007) characterized three different pools of mineralizable N:
Pool I: the flush in mineral N that occurs in the first 2 week period following
rewetting (represents a labile organic-N pool)
Pool II: cumulative amount of N mineralized between weeks 2 and 24 (represents
an intermediate pool of organic N)
Pool III: the amount of N that was potentially mineralizable but did not mineralize
throughout the incubation period (calculated by different between N0 predicted from
curve fitting and the cumulative amount of N mineralized between weeks 2 and 24 (Pool
II))
Such laboratory incubations are time consuming and laborious and therefore not suitable
for routine soil testing. Therefore, relatively rapid chemical tests have been developed in
attempt to predict N mineralized during the aerobic incubation. Besides being time
consuming, incubation conditions are also not representative of field conditions. This
method excludes naturally occurring cycles including drying and rewetting (Mikha et al.,
2005; Appel, 1998; Cabrera, 1993), the role of soil macro-fauna (Whalen et al., 2013)
and priming effects (Kuzyakov et al., 2000; Kuzyakov, 2010) on N turnover. Overall, this
laboratory method is effective for measuring the size of the mineralizable N pool,
17
however its use as an indicator of plant available N under field conditions is not well
supported by the literature (Sharifi et al., 2007; Dessureault-Rompre et al., 2011;
Nyrianeza et al., 2012).
Pool I defined by Sharifi et al. (2007) is a labile mineralizable N pool released in the first
two weeks of the long-term aerobic incubation and is generally excluded from the first
order kinetic model, as it is known to represent an initial flush of N due to rewetting.
Sharifi et al. (2007) and Dessureault et al. (2010, 2011) found a significant relationship
between plant available N and Pool I and the relationship was improved when including
soil properties and climatic factors using multiple regression. Nyiraneza et al. (2012) also
found a stronger a correlation between soil N supply and Pool I (r=0.41) than with Pool II
(r=0.28) or N0 (r=0.09). Various studies measuring the size of Pool I have shown the
concentration to vary from as low as 5.7 mg N kg-1 in soils from Spain (Villar et al.,
2014) to as high as 61 mg N kg -1 soil (Nyiraneza et al., 2012) in soils from across
Canada. In these studies, Pool I represented 7% to 45% of the total N released during the
remaining 2-24 weeks. It has been suggested that determination of the release pattern of
Pool I has the potential lead to a better understanding of this labile pool of organic N and
its validation as a plant available N index (Dessaureult-Rompre et al., 2011).
1.4.1.2 Chemical tests
Chemical tests are relatively rapid alternatives to the long-term aerobic incubation and
can be tailored to predict the crop available mineralizable soil N (Ros et al., 2011).
Chemical methods in most studies are compared to both parameters measured during the
18
long-term aerobic incubation and in-field N mineralization parameters to validate their
use as an indicator of mineralizable N. A brief explanation of promising chemical tests in
terms of their ability to predict soil N supply will be reviewed.
Hot-KCl-NH4
The hot-KCl-NH4 extraction involves heating of soil and 2M KCl to 100oC for 4h and is
said to extract soluble and exchangeable pool of NH4, hydrolyzed organic N and N
compounds released from lysed microbial cells (St. Luce et al., 2011). In some cases, the
concentration of KCl-NH4 extracted at room temperature is subtracted from hot-KCl-NH4
and the difference termed hydrolyzed NH4. A significant relationship was observed
between N0 measured in a 24-week aerobic incubation at 35oC and Hot-KCl-NH4
(R2=0.78, P<0.001) in soils from Saskatchewan, Canada. McDonald et al. (2014) also
observed a moderate correlation between hot-KCl-NH4 and N mineralized using 7-day
anaerobic incubation (r=0.49, P<0.01) in grassland soils in Ireland. In soils sampled from
cornfields across Canada Nyiraneza et al. (2012) found hot-KCl-NH4 correlated with N0
(r=0.31, P<0.05) and soil N supply (r=0.28, P<0.05) and this relationship improved for
fine textured soils (r=0.68, P<0.01) but not for coarse to medium textured soils (r=0.34).
This indicates that the chemical parameter is influenced by clay particles, as clay is
known to have more exchange sites to hold NH4, which may be extracted upon heating.
Predictive ability of this test has shown mixed results with field based measures of N
mineralization and the chemical nature of organic N hydrolyzed during the extraction is
still unknown (St. Luce et al., 2011).
19
Illinois Soil Nitrogen Test (ISNT)
The Illinois Soil Nitrogen Test (ISNT), developed by Khan et al. (2001) and later
modified by Mulvaney et al. (2001) involves treatment of soil with sodium hydroxide to
release hydrolysable N in the form of amino sugar-N, amino acid-N and NH4+ as
ammonia that is caught by boric acid placed above soil extract in a sealed mason jar at
48oC. The ISNT estimates the amino acid N fraction and was first used to identify soils
that were unresponsive to fertilizer N at a success rate of 94% (Khan et al., 2001;
Mulvaney et al., 2001). This method is sensitive to soil properties and management
practices but has to date only shown positive results for corn fertilizer recommendations
(St.Luce et al., 2011). It has been related to corn yield response to N fertilization and has
shown relatively good correlations with N0 (St.Luce et al., 2011; Sharifi et al., 2007,
Mulvaney et al., 2001). The ISNT showed significant correlations with N0 (0.39>r>0.68)
in soils taken from corn fields across Canada (Nyiraneza, et al., 2012) and with N
mineralized during a 7-day aerobic incubation (r=0.83, P<0.001) from grassland soils
(Macdonald et al., 2014). Its relationship with the soil incubation test parameters may
indicate that compounds extracted using the ISNT are a food source for microbes.
From an in-field standpoint, it has shown promise in predicting the economic optimum N
rates (EONR) for corn in poor and well-drained soils in southeastern USA (R2=0.87 and
0.78, respectively) (Williams et al., 2007). Recent research in New York has also led to
the development of a modified ISNT curve that incorporates organic matter estimated by
loss-on-ignition (LOI) and was successful in predicting the N response of corn (83%) but
only in its second year following sod or soybean (Lawrence et al., 2007).
20
Ultraviolet (UV) bicarbonate extraction
The UV bicarbonate extraction involves extracting soil with a 0.01M bicarbonate
solution and measuring the absorbance of the extract at 205nm and 260nm (Maclean,
1964). The UV absorbance at 205nm is a measure of soil mineral and organic N, while
absorbance at 260nm is a measure of soluble organic N. Mixed results of the UV
absorbance method have been observed (St.Luce et al., 2011) and the interpretation of
absorbencies and how they relate to N availability to crops has not been thoroughly
researched. In a study done on soils from corn fields across Canada, absorbance at 205nm
had significant correlations with both N0 (r=0.41) and soil N supply (r=0.41) but the
absorbance at 260 was more strongly correlated to N0 (r=0.38) than soil N supply (r=-
0.02) (Nyiraneza et al., 2012).
Soluble labile organic N fractions
According to Ros et al. (2011) soluble organic matter, dissolved organic matter (DOM)
or extractable organic matter (EOM) can be used interchangeably and is measured by
extracting a fraction of SOM based on its solubility in water, with and without the
addition of salts. In contrast, Chantigny et al. (2003), Herbert and Bertsch (1995) and
Zsolnay (1996) operationally define DOM as the organic matter <0.45 𝜇m in size present
in the soil solution. These fractions can include but are not limited to the cold water
extractable OM (WEOM), hot-water extractable (hot-WEOM) and salt extractable OM
(SEOM). The organic C (WEOC) and organic N (WEON) are typically measured within
these fractions and are used as indicators of plant available N in soils. It has been
hypothesized that the WEOM and hot-WEOM may be precursors for DOM which can be
21
mineralized by microbes present in to the soil solution (Zsolnay, 2003) and are the most
dynamic and bioavailable fraction of OM within soils (Haynes, 2000) as a large portion
of N mineralized throughout the growing season originates from labile organic N
fractions (Gutser et al., 2005; Sharifi et al., 2007; Chantigny et al., 2008).
Approximately 0.75% of total soil N can be composed of WEOM and 2.6 to 8.7% as hot-
WEOM (Curtin et al., 2006). Gregorich et al. (2003) measured the WEOC, WEON, hot-
WEOC and hot-WEON in a corn monoculture receiving no amendment and in a corn-
soybean rotation receiving 100 kg N ha-1. They found that the WEOC and WEON in the
monoculture was lower (283 mg C kg-1, 22 mg N kg-1) than the corn-soybean rotation
(307 mg C kg-1; 24 mg kg-1 N) and the hot-WEOM concentrations were on average twice
as high as the WEOM. Studies have shown that such labile OM fractions are sensitive to
management changes (Chantigny, 2003) and the WEOC: WEON ratio is a robust
indicator of a soils potential to mineralize N (Haney et al., 2012). The biodegradability
of these fractions were assessed by Gregorich et al. (2003) for soils under corn
production receiving manure, fertilizer or no amendment. The authors found that the
WEOM extracted in hot water was more biodegradable than that extracted in cold water.
The WEOM fraction generally contains 2 pools of OM, one that is rapidly decomposed in
<1day and another than has a turnover of about 80 days. These results indicate that the
SOM extracted using water is an N rich source of labile OM for microbes and the
composition changes with management practices.
22
Microbial Activity
The microbial community present and their activity is important in SON mineralization
and therefore quantifying the size and the capability of the microbial community can
result in an indication of soil fertility (Harmel and Haney, 2013). Currently there are two
widely used methods that determine either microbial biomass (MB) (chloroform
fumigation) or microbial activity (CO2 emissions). The fumigation procedure measures
the amount of microbial biomass carbon (MBC) and nitrogen (MBN) to attempt to
quantify the size of the microbial community present within soils. The fumigation
method has shown mixed results to predict soil N supply (Deng et al., 2000;
Franzluebbers et al., 2000; Willson et al., 2001; Sharifi et al., 2007; St Luce et al., 2011)
and contains two major short falls: over estimation of MBC and N from inclusion of C
and N not originating from MB, and its potential to misrepresent the importance of
biomass size on organic matter mineralization as the size has been shown to remain stable
throughout the growing season (Hassnick et al., 1993; Puri and Ashman, 1998; Alessi et
al., 2011).
The CO2 emissions test measures C mineralization by the amount of CO2 produced in a
specified period of time and directly relates to microbial respiration during the
decomposition of SOM. Various methods have been used in the past that differs in
method of quantification (titration (Stotzky, 1965), gas chromatography (Mondini et al.,
2010), infrared gas analyzer for CO2 detector (Haney et al., 2008)), length of incubation
(1 day to 28 days; Franzluebbers, 1999) and preparation of soils (field-moist vs. rewetting
of dried soils; Haney et al., 2004). The CO2 emission following a 1-day incubation period
has shown strong correlation with the standard 28-day incubation period (Marumoto et
23
al., 1982; Sparling et al., 1997; Haney et al., 2001). Haney et al. (2008) reported the 1 day
CO2 from chemical titration and the Solvita soil test in treated and un-treated soils has a
strong correlation to 28 and 7-28 day C mineralization incubation experiments, indicating
that the initial flush may be a strong indicator of the quality and amount SOM present
within the soil.
Marumoto et al. (1982), Sparing et al. (1997) and Haney et al. (2008) suggest that the
CO2 emissions test can provide an estimate of N mineralization as microbial
decomposition of SOM is responsible for the availability of N to plants. Haney et al.
(2008) found that the 1-day CO2 emission was significantly related to the initial WEON
(R2=0.86) and C (R2=0.76) indicating that this fraction of SOM may be a source of food
for microbes and merits further investigation into the link between these two pools of
labile organic N. More recently the 1-day CO2 flush following soil rewetting showed
strong relationships to grain yield in Texas (R2=0.84) and to grain N uptake in Oklahoma
(R2=0.63) (Briton and Haney, 2013). This test has the potential to provide a better
indicator of the size of the labile soil organic pool, the quality of this pool and properties
of soil biota (Gestel et al., 1992), important factors in maintaining the integrity and
productivity of agricultural soils.
24
Light fraction organic matter carbon and nitrogen (LFOMC and
LFOMN) and Particulate organic matter carbon and nitrogen (POMC
and POMN)
The light fraction organic matter C and N (LFOMC, LFOMN) and particulate organic
matter C and N (POMC, POMN) fractions can be isolated through density and size
fractionation, respectively. The POM is separated through sieving and is the size fraction
between 53 and 250𝜇m (Gregorich and Beare, 2008). The LFOM is more related to fresh
residues and has a higher C:N ratio than the POM fraction, which is composed of
partially decomposed plant residues together with microbial byproducts. Both have been
defined as an intermediate pool of OM between freshly added plant residues and
stabilized organic matter, a major source of C and N for microbes and the pathway
through which N and C are transformed into stabilized SOM (Gregorich et al., 2006;
Wander, 2004; Haynes, 2005; St. Luce et al., 2011).
The LFOMC and N and POMC and N has shown to constitute 8 and 5% for LFOM and
22% and 18%for POM of total soil organic C and N, respectively (St.Luce et al., 2011).
The POMC and N fractions have shown sensitivity to management practices in the short
term (Christensen, 1992; Angers et al., 1993; Franzleubbers and Stuedemann, 2008) and
the POMC fraction has been shown to be affected by tillage. Six et al. (1999) found that
POMC was 51% lower under conventional tillage than no tillage, while the same was not
observed for the LFOC fraction (Liang et al., 1998). Inclusion of crop residues over the
long-term also increases the POM fraction (Biederbeck et al., 1998; Spargo et al., 2011).
It’s use as a predictor of soil N supply for Ontario is scarce. The POMN has shown a
moderate relationship with plant N uptake (R2=0.51) for potato production in Maine,
25
USA, a strong correlation to corn grain yield (r=0.72) and corn N uptake (r=0.63) in
Maryland, USA and a moderate relationship with canola yield and N uptake (R2=0.56,
P<0.001 and 0.69, P<0.001) in western Canada (St.Luce et al., 2014). St. Luce et al.
(2011) suggest that the nutrient availability and composition of these fractions is required
in order to understand their contribution to the soil N supply.
1.4.2 Field-based indicators of soil N supply
Plant based measures of N availability can have an advantage over laboratory indicators
as they are direct indicators of the availability of N to plants and can evaluate the
synchrony of plant N demand and N supply (Zebarth et al., 2009). The effectiveness of
plant diagnostics is often hindered due to other factors other than N availability (Olfs et
al., 2005) such as disease, drought and other nutrient limitations.
1.4.2.1. Crop response
In the field, N availability can be measured using grain yield, plant N concentration and
soil mineral N at harvest. Since N is the major limiting factor for plants, grain yield is a
direct reflection of N availability for cereal crops assuming no other factors are limiting
(Zebarth et al., 2009). The plant N uptake is a measure of the N available under field
conditions, as the amount of nutrient in the soil does not always reflect its availability to
plants (Binkley and Vitousek, 1989). By combining the plant N uptake and soil mineral N
at harvest, the soil N supply or the amount of N mineralized over the growing season
under field conditions can be estimated (Zebarth et al., 2009; Whalen et al., 2013).
Crop response indicators are measurements that evaluate the soil N supply to the field
crop and allow accurate prediction of the economic rate of N when developing fertilizer
26
N recommendations (Lory and Scharf, 2003). The most popular methods to evaluate the
fields soil N supply through relative yield (RY) or delta yield (∆Y), defined as the
increase in grain yield over zero N application obtained from applying fertilizer at the
most economic rate (Kachanoski et al., 1996) and is also a method of farmers to evaluate
their fertilizer application rates (Lory and Scharf, 2003). Two useful measurements can
be made from RY or ∆Y, 1) the ability for that soil to supply N and 2) site specific
response to N. This will provide an indicator of the pool of soil N that is available for
crop uptake and the response to applied N will provide an indicator of the effectiveness
of additional N.
1.6 Conclusion
A large group of laboratory and field-based methods exist for assessing the contribution
of N mineralization to corn N uptake. These methods however have not been extensively
evaluated under the climate, management practices and soil characteristics for corn
production in southwestern Ontario. An ideal laboratory method would consider the
mineralizable N as this approach has the potential to relieve environmental and economic
burdens associated with over application of N fertilizers.
27
1.7 Thesis Objectives
This thesis has three primary objectives
1) To predict corn N availability in the humid temperate climate of southwestern
Ontario
2) To evaluate the ability of a series of laboratory-based indicators of mineralizable
N
3) To assess the ability of successful laboratory-based indicators of mineralizable N
to predict N application rates for corn in southwestern Ontario
To achieve these objectives, the study will be split into two field seasons. In the first year,
laboratory-based indicators will be evaluated using correlation with relative yield (soil
test correlation) and in the second year, promising laboratory-based indicators will be
calibrated to determine N fertilizer recommendations (soil test calibration) using corn N
response trials.
28
Chapter 2. Evaluating laboratory-based indicators to predict N availability to corn in southwestern Ontario 2.0 Abstract
The pre-plant soil nitrate is a variable component of soil N supply in the humid temperate
climate of southwestern Ontario. The objective of this study were to quantify the capacity
of southwestern Ontario soils to supply N (N0) to the crop, and evaluate a suite of
chemical and biological N mineralization soil tests as indicators of crop N availability for
twelve southwestern Ontario soils. Soils were collected from the 0-30-cm depth and
aerobically incubated at 25°C for 24-wk and analyzed for selected soil N tests. Nitrogen
availability indices were tested against relative yield as the field index of N supply. The
average potentially mineralizable N was 147 mg kg-1 and the Pool I mean was 42 mg kg-
1, representing 31% of the total N mineralized during the incubation. The pre-plant nitrate
test (PPNT) was weakly correlated to crop N availability indicators. The top
mineralizable N tests for predicting crop N availability were Pool I and Pool I + soil
mineral N at planting (SMNp) but this relationship was dependent on soil clay content
(coarse textured soil (clay ≤ 240 g kg-1): r=0.79; medium textured soil
(clay > 240 g kg-1): r=-0.59). This study showed that southwestern Ontario soils have a
high capacity to supply N to corn and Pool I can better predict relative yield in
southwestern Ontario compared to the PPNT in 2013.
29
2.1 Introduction
Soil nitrogen (N) mineralization is an important but variable component of soil N supply
to crops in humid temperate environments (Zebarth et al., 2009; Whalen et al., 2013).
Soil N mineralization can supply 20-80% of crop N requirement during the growing
season (Broadbent, 1984; Zebarth et al., 2009). No robust index of soil organic nitrogen
(SON) mineralization exists and has resulted in less accurate fertilizer N
recommendations and low N use efficiency in field crops (Lobell, 2007; Sharifi et al.,
2007). Pre-plant N fertilizer recommendations for corn (Zea Mays) in Ontario are based
on the pre-plant nitrate (NO3) test (PPNT) (Verhallen and LeBoeuf, 2009) or solely on
crop N requirement. Development of a laboratory index of mineralizable soil N can
improve the accuracy of fertilizer N recommendations and consequently optimize corn
yields while reducing the environmental risks of excess N.
The current fertilizer N recommendation tool, PPNT, provides a snap shot of NO3
concentration in the field and can provide an indication of residual soil N from the
previous growing season and early season N mineralization when limited leaching has
occurred (Sharifi et al., 2009). In humid temperate climates the large spatial variability
and unpredictable rainfall can rapidly alter NO3 concentrations resulting in residual soil
mineral N and applied N being leached from the root zone (Tan et al., 2002; Scharf et al.,
2006; Lobell 2007; Zebarth et al., 2009). Therefore, N originating from in-season SON
mineralization is an important source of N for crops.
30
The standard laboratory method to measure SON mineralization is the long-term aerobic
incubation (>20 weeks) (Standford and Smith, 1972). The measured potentially
mineralizable N (N0) can be sub-divided into three pools: Pool I, and Pool II and Pool III
(Sharifi et al., 2007). Pool I is a readily mineralizable N pool and represents between 7 to
45% of the total N mineralized throughout the incubation (Sharifi et al., 2007; Dessureault
Rompre et al., 2010, 2011; Nyiraneza et al., 2012; Villar et al., 2014). Pool I has been
reported as the most sensitive pool to management practices (Sharifi et al., 2008). Pool II
is the intermediate pool of mineralizable N and represents the remaining of total N
mineralized. Pool III is a more stable pool of mineralizable N that does not release during
the incubation.
The long-term aerobic soil incubation is time consuming, laborious and not usually a
strong indicator of in field N availability to crops (St. Luce et al., 2011). Therefore, the use
of relatively rapid laboratory indicators that can predict mineralizable N is a more practical
alternative (Ros et al., 2011). For example, St. Luce et al. (2013) found particulate organic
matter N (POMN) to be significantly related to canola (Brassica napus L.) yield and N
uptake (R2=0.56 and 0.69) in western Canada. Also, the soil microbial activity, measured
as the 1-day CO2 flush following soil rewetting, has also shown strong relationships with
grain yield in Texas (R2=0.84) and to grain N uptake in Oklahoma (R2=0.63) (Briton and
Haney, 2013). Haney et al. (2012) found that the flush of CO2 after rewetting of dried soil
was related to the amount of soil carbon (C) present in a water extract (WEOC, r=0.87).
They reported that the ratio between WEOC and organic N in a water extract (WEON)
was a strong indicator of the quality of a readily available pool of substrate for microbes,
31
in soils collected throughout the US. Other promising tests include the Illinois soil N test
(ISNT; Williams et al., 2007), UV absorbance of a sodium bicarbonate extract (Sharifi et
al., 2007) and hot-KCl extractable ammonium (NH4; Nyiraneza et al., 2012).
Laboratory indicators of mineralizable N have not been thoroughly evaluated as indices of
soil N supply in Ontario and therefore it is hypothesized that a soil mineralizable N test
can better predict the soil N supply for grain corn in Ontario compared to the current pre-
plant N test (PPNT). The primary objectives of this research were to i) determine soil N
mineralization parameters of selected Ontario soils to determine the potential of soils to
supply N using the long-term aerobic soil incubation, ii) extract soils using a series of
mineralizable N tests iii) evaluate soil N mineralization parameters and mineralizable N
tests to predict field-based indicators of crop N availability to grain corn across
southwestern Ontario. To accomplish this, soils from a large region in southwestern
Ontario, Canada cropped to grain corn with varying soil properties were used.
2.2 Materials and Methods
2.2.1 Site description and plot setup
Twelve field sites were selected across southwestern Ontario in 2013. Sites were selected
to encompass a broad range of soil chemical and physical properties to properly represent
the variability in growing conditions across southwestern Ontario (Figure 2.1, Table
2.1). These sites were located on growers fields in Ilderton, Strathroy, Woodstock (Hart
and Rutherford), Moorefield, Mount Hope, Bornholm and Lucan, ON and at the Elora
Research Station (ERS) with the Ontario Ministry of Agriculture and Food (OMAFRA)
32
and the University of Guelph (Lauzon), at the Agriculture and Agri-Food Canada
research center in Woodslee (Woodslee) and at the University of Guelph Ridgetown
Campus (U of G-Ridgetown). Soil properties were measured on 0-30 cm pre-plant
composite soil samples. Soil texture varied from 86 g kg-1 to 440 g kg-1 clay and pH
ranged from 6.6 to 8.5 with an average of 7.6. Total C ranged between 11.8-25.4 g kg-1
with an average of 20.7 g kg-1 and total N ranged between 0.94 and 2.1 with an average
of 1.7 g kg-1. The C:N ratio of soils ranged from 10 to 20 with an average of 13. Previous
year (2012) crops included soybean, winter wheat and corn under conventional
management receiving only mineral fertilizer as the N source. Crop heat units (CHU)
varied from 2680 (Elora) to 3560 (Woodslee) and all sites were located in a humid
temperate climate regime with a 2013 average growing season air temperature of 16 oC,
ranging from 15.3 to 17.8 oC, and growing season rainfall of 573 mm, ranging from 422
to 748 mm, close to the 30 year historical average of 16.1oC and 530 mm of rainfall
(Environment Canada, 2015).
At each field site, plots were set up in a randomized complete block design with an area
of at least 32 by 16m. Two N fertilizer rates were applied at planting (a zero N control
and an optimal rate of N) as treatments in replicate of four. Optimal N rates were applied
based on corn N requirement and/or spring soil test and ranged between 92 and 224 kg N
ha-1 (Table 2.1). All sites were then cropped with grain corn and fertilizer was applied at
planting as 28% urea ammonium nitrate (UAN-28-0-0).
33
Figure 2.1 Map of locations of experimental field sites in southwestern Ontario in 2013. n=12.
34
Table 2.1. Summary of characteristics for 12 experimental sites established in southwestern Ontario (n=4).
Site Location Latitude and
Longitude
Growing season
rainfall§
Mean growing season air
temperature§
Crop Heat Unit
(CHU)# Previous Crop
Optimal N rate
(kg N ha-1) Soil Classification Soil Texture† pH¶ Total
C Total
N C/N ratio
oN, oW Clay Silt Sand
g kg-1‡
mm oC g kg-1 g kg-1 Ilderton 43o11’, 81o30’ 748 N/A 2900 Winter wheat 224 London loam 194 438 367 7.7 20.7 1.6 13
Strathroy 42o96’, 81o65’ 654 17.3 2900 Soybean 151 Berrien sandy loam 191 157 652 6.6 11.8 0.94 12
Hart 43o13’, 80o82’ 492 16.9 2890 Soybean 224 London loam 192 320 431 7.7 23.0 2.1 11
Rutherford 43o13’, 80o82’ 492 16.9 2890 Grain corn 224 Perth silt loam 112 373 487 7.6 16.1 1.6 20
Moorefield 43o75’, 80o77’ 670 15.3 2700 Winter wheat 224 Perth loam 225 546 228 7.8 21.1 1.8 12
Mount Hope 43o15’, 79o89’ 422 16.6 3210 Soybean 224 Brantford silt loam 270 491 411 7.9 19.5 1.7 11
Bornholm 43o52’ 81o13’ 720 N/A 2820 Winter wheat 224 Perth clay loam 1 274 584 141 6.8 23.6 1.8 13
Lucan 43o20’, 81o39’ 601 16.9 2900 Winter wheat 184 Huron clay loam 1 359 529 112 6.5 16.1 1.5 11
ERS-OMAFRA 43o65’, 80o39’ 701 15.3 2680 Soybean 224 Woolwich silt loam 1 87 470 442 8.0 19.2 1.7 12
ERS-Lauzon 43o65’, 80o39’ 701 15.3 2680 Soybean 92 Woolwich silt loam 1 86 459 454 8.4 19.2 1.7 20
Woodslee 42o13’, 82o44’ 344 17.8 3560 Grain corn 200 Brookston clay loam 1 406 336 258 7.0 19.1 2.0 10
U-of-G-Ridgetown 42o44’, 81o87’ 479 16.9 3340 Wheat 200 Brookston clay loam 1 440 250 310 7.5 25.4 2.0 13
Mean (n=42)∞ 573 16.1 2847
230 378 411 7.6 20.7 1.7 13
SD (n=42) 127 1.40 406.8
112 109 134 0.6 6.5 0.51 40 § From May to October: http://climate.weather.gc.ca/prods_servs/cdn_climate_summary_e.html # OMAFRA Factsheet: Crop Heat Units for Corn and other Warm Season Crops in Ontario † Pipette method (Gee and Bauder, 1986) ¶ pH in water (1:2 soil/water ratio, Hendershot et al., 1993) ‡ Dry Combustion (VarioMAX Cube, Elementar Analysensysteme GmbH, Hanau, Germany) 1 Indicates site is tile drained ∞ Grand mean and SD, (n=12 sites*4 replicates-6 outliers=42). There were 6 data points identified as outliers.
35
2.2.2. Soil sampling and analysis
At each of the field sites, one composite soil sample was taken at a depth of 0 to 30 cm
from the zero N plots five days before or after planting. A subsample was kept moist and
stored at 4oC until analysis. On the remaining soil, soil moisture content was determined
by drying soil at 105 oC for 24 hours and the remaining was air-dried and sieved (<2mm)
before laboratory analysis. On air-dried soils, soil pH was determined in a 1:2 soil:
deionized water suspension (Hendershot et al., 1993). Particle size analysis was
determined using the pipette method following organic matter removal (Gee and Bauder,
1986). Total soil C and N was measured with the dry combustion method using a CNS
analyzer (VarioMAX cube, Elementar Analysensysteme GmbH, Hanau, Germany).
2.2.3 Laboratory-based indicators for predicting crop N availability
Soil N mineralization parameters
A modified method of the long-term aerobic incubation (Curtin and Campbell, 2008) was
used to measure potentially mineralizable N (N0) and mineralizable N pools (Pool I, Pool
II and Pool III). In brief, 30g of soil were mixed with an equal amount of acid-washed
Ottawa sand for coarse-textured soils and with twice the amount of sand for fine-textured
soils and packed into 5 cm diameter plastic Buchner funnels. The soil and sand mixture
was then re-wetted to 55% water filled pore space (WFPS) by adding 175 mL 0.01M
CaCl2 and applying vacuum. Soils were incubated at 25 oC for 24 weeks. Leachates were
collected at day 2, 4, 7 and 14 after incubation and every 2 weeks for the first 12 weeks
and every 4 weeks thereafter with 150 mL 0.01M CaCl2 followed by 25mL of zero-N
nutrient solution (Curtin and Campbell, 2008). Leachates collected during the incubation
36
experiment were analyzed for NH4 and NO3 using the indophenol blue method (Sims et
al., 2005) and an Epoch microplate spectrophotometer (BioTek Instruments Inc.,
Winooski, VT, USA). Three mineralizable N pools were calculated from the results of
the 24-week incubation. Pool I was the cumulative N mineralized in the first 2 weeks
following rewetting of the soil, Pool II was the cumulative amount of N mineralized
between week 2 and 24 and Pool III was the amount of N that was potentially available
but did not mineralize during the 24-week incubation. Pool III is therefore estimated as
the difference between N0 and Pool II. Net N mineralized was also calculated as the sum
of N mineralized throughout the 24 weeks incubation (Pool I + Pool II). In addition,
cumulative N leached at each sampling date between 2 and 24 weeks were fitted to the
first-order kinetic model as follows to determine the potentially mineralizble N (N0):
N = 𝑁 (1 − 𝑒 )
where N0 is the potentially mineralizable N, Nmin is the cumulative N mineralized at time
t, k is the mineralization rate constant. The N mineralized in the first 2-week period was
excluded from curve fitting as it represents an initial flush of N due to rewetting.
Mineralizable N tests
The KCl extractable NH4 and NO3 (KCl-NH4 and KCl-NO3) were extracted from moist
soil using 2 M KCl (1:5 soil to extractant ratio). Concentrations of NH4 and NO3 were
determined colorimetrically as described above. The KCl-NO3 was hereafter referred to
as the PPNT and the soil mineral N at planting is hereafter referred to as SMNp
37
(calculated as the sum of KCl-NH4 and KCl-NO3). In addition, composite soil samples
were collected from zero N plots at a depth of 0 to 30 cm at corn harvest and stored at -
20˚C until analysis. The soils were extracted for mineral N (NH4 and NO3) using 2M
KCl. Soil mineral N at harvest was calculated as the sum of KCl-NH4 and KCl-NO3 and
is hereafter referred to as SMNh.
Hot KCl extractable NH4 (Hot KCl-NH4) was determined by heating 3.0 g of dry soil
with 20 mL of 2M KCl to 100oC for 4 hours using a digestion block (Gianello and
Bremner, 1986) followed by analysis of NH4 as described above. The ultraviolet
absorbance of a 0.01M NaHCO3 extract at 205nm (NaHCO3-205) and 260nm (NaHCO3-
260) as described by Fox and Piekielek (1978) and Hong et al. (1990) using an Epoch
microplate spectrophotometer (BioTek Instruments Inc., Winooski, VT, USA). The ISNT
was determined using the method described by Khan et al. (2001). Briefly, 1g of air-dried
soil was mixed with 10mL of 2M NaOH in a mason jar and heated for 5h at 50oC using a
water bath. Released ammonia (NH3) was collected in a boric acid indicator solution
(4%, w/v) and titrated using 0.02N H2SO4 to indicator endpoint. Carbon mineralization
was measured as an indicator of microbial activity (Hopkins, 2006). Briefly, 40g field-
moist soil was incubated at 25oC in a 1L mason jar containing a CO2 trap (10mL 2M
NaOH). Additionally, 10mL deionized water was placed in a vial at the bottom of the jar
to maintain humidity. The CO2 traps were exchanged at day 1 (CO2-1 Day) and day 7
(CO2-Day 7). Nitrogen mineralization following the 7-day incubation (CO2-N min) was
also measured by exacting mineral N using the 2M KCl extraction as described above.
38
The water-extractable organic C (WEOC) and N (WEON), hot water-extractable organic
C (Hot-WEOC) and KCl extractable N (WEOM-KCl) was determined sequentially as per
Curtin et al. (2006) and Chantigny et al. (2009). In the first step, 4 g of air-dried soil was
shaken with 20 mL of room temperature deionized water for 60 min. Extracts were then
centrifuged at 4500xg for 20 min and the supernatant was decanted and analyzed for
organic C (WEOC) using an Schimadzu TOC-VCPH (Schimadzu Scientific Instruments,
Columbia, MD, USA). Total water soluble N (WSN) was determined using the persulfate
oxidation method as described by Cabrera and Beare, 1993. The water extractable
organic N (WEON) was calculated by subtracting the water extractable mineral N
(WEMN) from the WSN. Following decanting, 30 mL of deionized water was added to
the soil and the solution was heated to 50oC for 16h using a water bath. Hot water
extracts were then centrifuged as per WEOM, decanted and analyzed for organic C (Hot-
WEOC) as described above. Finally, 2 M KCl was added to the soil, shaken for 60 min,
filtered and analyzed for mineral N (WEOM-KCl).
Particulate organic matter C and N (POMC and POMN) was determined by shaking 25g
field-moist soil overnight in a 5 g L-1 sodium hexametaphosphate solution. Soil was then
passed through a 53-𝜇m sieve (Gregorich and Ellert, 1993). Retained sand and macro-
organic matter were dried and weighed and total C and N concentrations were determined
using a CNS analyzer (VarioMAX cube, Elementar Analysensysteme GmbH, Hanau,
Germany).
39
2.2.4 Field-based indicators of corn N availability
Corn plants were harvested at maturity in October 2013. Eight to 10 plants were
randomly harvested within a 16 m2 subplot in each of the zero N rate and optimal N rate
treatments. Corn plants were separated into their kernel, cob and stover portions. Grains
were threshed and weighed, and yields (GY) were adjusted to 15% moisture content. The
stover and grain from each zero N rate treatment was then dried in a drying oven at 60 oC
and a subsample was weighed to determine DM yield. The grain and stover was then
ground (<1mm) for determination of total N concentration by dry combustion using a
CNS analyzer (VarioMAX cube, Elementar Analysensysteme GmbH, Hanau, Germany).
The plant N uptake in zero N plots (PNU0N) was calculated from the grain and stover
tissue N concentrations from zero N rate treatments, corrected for any starter fertilizer N
applied with the seeder at planting. Relative yield (RY) and delta yield (∆Y) were then
calculated as follows:
RY = (
) × 100
∆Y = GY optimal N − GY zero N
The RY was used to determine crop response to N fertilizer addition and used as an
indicator of crop N availability to minimize the variability in soil productivity and
management, and site characteristics across experimental sites (Williams et al., 2007). A
larger value of RY indicates that the field was less responsive to fertilizer addition and
therefore assumed to have higher crop N availability. In contrast, a smaller RY value is
indicative of a more responsive field to N fertilizer addition and therefore lower crop N
availability can be assumed. Finally, the soil N supply was calculated as PNU0N plus
SMNh and was used as an estimate of the N available for plant uptake.
40
2.2.5 Statistical Analysis
Statistical analyses were performed using SAS (SAS Institute Inc., 2012). Data was first
tested for normality using the Kolmogorov–Smirnov test. Outliers were identified and
removed from the data set if the data point was ± 3 standard deviations from the mean.
The ward clustering method was performed using PROC CLUSTER and TREE to group
together soils similar in soil texture (clay, silt and sand), chemical properties (pH, TN, TC
and C: N) and site characteristics (growing season rainfall and temperature). The PROC
CORR was used to determine significant correlations between laboratory-based
indicators and RY. Significant correlations (P<0.05) were then evaluated for functionality
by graphing in excel and tested for significance using PROC REG. Regressions were
accepted as significant at P<0.05. The correlation analysis was first done on the whole
data set followed by analysis based the results of the clustering method. Additionally,
PROC CORR was performed between Pool I results and RY to determine the best
incubation time.
2.3.Results
2.3.1 Laboratory-based indicators for predicting corn N availability
Soil N mineralization
The long-term aerobic incubation results showed that the potential for selected soils to
mineralize N (N0) was in the range of 95 to 199 mg kg-1 with an average of 147 mg kg-1
(Table 2.2). The associated k values ranged from 0.022 to 0.048 week-1. The NA
associated with the Ridgetown site was because it did not converge with first order
41
kinetics. The Pool I ranged from 16 to 70 mg kg-1, with an average of 42 mg kg-1 and
represented approximately 31% of the net N mineralized. The Pool II was less variable
among sites (56-121 mg kg-1) with an average of 89 mg kg-1. The average net N
mineralized (Pool I + Pool II) was 117 mg N kg-1. The N0 represented 9.8% of the total
soil N and the 3 distinct pools: Pool I, Pool II and Pool III represented on average 2.5, 5.3
and 4.8% of total soil N.
42
Table 2.2. Mean values for N mineralization parameters (standard deviation in parenthesis) measured during the long-term aerobic incubation for soils from each experimental site sampled in southwestern Ontario (n=4).
Pool I§ Pool II
Net N Min Pool III N0 k
Site location mg kg-1 week-1 Ilderton 36(10) 99 (4.9) 135(15) 57(18) 155(17) 0.047(0.008)
Strathroy 44(11) 90(7.8) 134(16) 109(37) 199(42) 0.029(0.009)
Hart 31(16) 121(13) 153(28) 77(26) 198(36) 0.048(0.009)
Rutherford 24(3.8) 73(2.0) 97(4.7) 109(50) 181(48) 0.027(0.013)
Moorefield 34(3.0) 59(6.3) 93(7.7) 22(8.2) 81(13) 0.060(0.011)
Mount Hope 29(6.0) 57(4.8) 86(3.0) 44(15) 103(18) 0.035(0.007)
Bornholm 53(4.2) 68(2.6) 121(5.6) 49(9.5) 117(10) 0.041(0.005)
Lucan 26(6.4) 82(6.3) 108(13) 76(25) 161(24) 0.036(0.008)
ERS-OMAFRA 35(4.0) 74(13) 109(11) 140(42) 224(44) 0.022(0.005)
ERS-Lauzon 16(5.4) 56(20) 73(19) 51(40) 95(56) 0.032(0.01)
Woodslee 70(3) 105(7.3) 176(15) 87(45) 193(38) 0.041(0.01)
U of G-Ridgetown
53(13) 71(13) 123(16) NA NA NA
Mean (n=42) # 42 89 117 66 147 0.04
SD (n=42) 21 40 16 43 56 0.01
§Pool I= concentration of N mineralized within the first two weeks of the aerobic incubation; Pool II= N mineralized between 2 and 24 week of the aerobic incubation; Net N Min=Pool I +Pool II; Pool III= N not mineralized during the aerobic incubation but may be potentially available (calculated as the difference between N0 and Pool II); N0= potentially mineraliable nitrogen calculated using N mineralized between weeks 2 and 24 and first order kinetic model; k=mineralization rate constant; NA=not available; SD=standard deviation # Grand mean and SD (n=12 sites*4 replicates-6 outliers=42). There were 6 data points identified as outliers
43
Results from the Pool I incubation (leaching at days 0,2,4,7 and14) showed the
cumulative amount of N mineralized at day 14 was the best predictor of RY for both
coarse-textured soils (Cs-T) and medium textured soils (Md-T) (Table 2.3). Although the
amount leached at day 2 was significantly related to RY for Cs-T soils (r=0.34) the
correlation was almost twice as strong at day 14 (r=0.64). In Md-T soils the amount
released at day 4 was significantly correlated to RY (r=0.27) but the correlation was
again substantially stronger at day 14 (r=-0.59).
Table 2.3 Mean cumulative N mineralized on day 2, 4 7 and 14 (Pool I) of the long-term aerobic incubation for each experimental field site (n=4) and the correlation coefficient (r) with RY at each time step for Cs-T (n=21) and Md-T (n=21) soil texture group. Cumulative N Mineralized (mg kg-1) Day 2 Day 4 Day 7 Day 14 Cs-T Ilderton 15 28 32 36 Strathroy 16 27 36 44 Hart 15 28 30 31 Rutherford 19 22 23 24 ERS-OMAFRA 19 25 34 35 ERS-Lauzon 10 14 15 16 Correlation Coefficient (r) RY 0.34** 0.20 0.28* 0.64** Md-T Helmuth 20 27 30 34 Mount Hope 14 25 28 29 Bornholm 22 33 37 53 Lucan 10 20 23 26 Woodslee 22 42 56 70 UofG-Ridgetown
24 41 47 53
Correlation Coefficient (r) RY <0.10 -0.27* -0.23* -0.59**
*= P<0.05 and **=P<0.01
44
Mineralizable N tests
Methods for predicting crop N availability from N mineralization are summarized in
Table 2.4. The PPNT ranged from 3.13 to 13.5 mg kg-1 and represented 70% of the
SMNp. The SMNp across sites ranged between 5.56 and 12.8 mg kg-1 with an average of
9.9 mg kg-1 and constituted from 15.8% (Strathroy) to 30% (Mount Hope) of the soil N
supply. The Hot-KCl-NH4 ranged between 8.86 and 26.1 mg kg-1 and extracted 1.6 to 10
times more NH4 than the 2M KCl extraction. The N mineralized over a 7-day incubation
period (CO2-Min N) was a magnitude of 1.2 to 16 times lower than the amount released
over the 14-day incubation with periodic leaching events (Pool I). The CO2 evolved over
a 7-day period was not reported, as concentrations were too small to be measured using
manual titration resulting in no difference being observed between soils.
The ISNT extracted the most organic N (215-436 mg N kg-1) followed by POMN (4.74-
177 mg N kg-1) and WEON (24-82 mg N kg-1). The WEOC was between 163 to 408 mg
kg-1 and extracted 50% more C than the hot-WEOC (61-183 mg kg-1). The WEOC and
WEON fractions represented 1.3% and 2.1% of the total C and total N, respectively and
the POMC and POMN fractions represented 9% and 4.3% of the total C and total N,
respectively. The WEOC: N values ranged from 3.1 to 11 and were between 0.8 and 6.8
times lower than the whole soil C: N and the POMC: N ranged from 14.7 to 147 and
were between 0.9 and 3.4 times higher than the whole soil C: N. The POMC was the
most variable chemical test observed in this study (123-5657 mg kg-1)
45
Table 2.4. Mean values for mineralizable N tests (standard deviation in parenthesis) measured in soils from 12 experimental sites (n=4).
Continued…
Mineralizable N tests SMNp§
KCl- NH4
PPNT
NaHCO3- 205
NaHCO3- 260
Hot-KCl- NH4
CaCl2-N Extractable N
CO2- MinN ISNT
Site Location mg kg-1 mg kg-1 Ilderton 9.80
(1.5) 3.80
(0.40) 6.10 (1.2)
0.41 (0.07)
0.10 (0.05)
20.0 (6)
25.6 (1.8)
17.6 (7.2)
218 (121)
Strathroy 6.98 (1.8)
3.80 (1.2)
3.13 (0.59)
0.37 (0.04)
0.13 (0.04)
8.86 (2.7)
16.9 (2.18)
11.2 (1.3)
258 (60)
Hart 8.16 (2.3)
3.17 (1.8)
4.98 (0.61)
0.51 (0.02)
0.15 (0.01)
18.9 (5.2)
24.8 (3.0)
14.3 (3.8)
360 (47)
Rutherford 9.12 (2.2)
4.80 (1.6)
4.30 (0.79)
0.40 (0.05)
0.07 (0.01)
12.5 (2.7)
18.2 (4.0)
10.5 (2.7)
273 (11)
Moorefield 12.2 (4.3)
3.60 (2.3)
8.60 (2.1)
0.51 (0.05)
0.14 (0.01)
14.2 (4.5)
26.0 (5.8)
14.7 (2.8)
329 (83)
Mount Hope 15.7 (7.4)
2.19 (0.64)
13.5 (6.9)
0.56 (0.09)
0.12 (0.03)
16.0 (6.4)
8.17 (0.36)
25.1 (10.9)
351 (152)
Bornholm 11.0 (1.5)
2.87 (1.6)
8.10 (0.49)
0.54 (0.05)
0.15 (0.02)
12.5 (1.7)
20.4 (2.0)
16.0 (3.4)
372 (42)
Lucan 7.67 (0.95)
2.75 (0.50)
4.90 (0.53)
0.49 (0.04)
0.18 (0.01)
10.9 (3.8)
2.83 (0.06)
13.9 (5.6)
255 (55)
ERS-OMAFRA 10.1 (1.2)
1.80 (0.98)
8.20 (1.3)
0.47 (0.04)
0.13 (0.02)
16.6 (3.8)
22.0 (2.5)
11.3 (3.5)
317 (40)
ERS-Lauzon 5.56 (0.43)
0.88 (0.16)
4.68 (0.53)
0.34 (0.05)
0.12 (0.01)
11.5 (6.0)
14.8 (3.6)
10.3 (1.8)
215 (36)
Woodslee 12.8 (1.2)
5.13 (1.1)
7.60 (1.3)
0.52 0.04)
0.20 (0.02)
26.1 (3.0)
26.1 (3.0)
20.6 (7.8)
374 (30)
U of G-Ridgetown
9.40 (3.0)
0.93 (0.17)
8.40 (2.9)
0.49 (0.06)
0.16 (0.04)
10.3 (2.2)
7.49 (3.5)
4.50 (3.3)
437 (21)
Mean (n=42)# 9.88 2.99 6.89 0.47 0.14 16.1 17.6 14.6 317 SD (n=42) 2.3 1.00 1.60 0.07 0.04 6.63 7.64 5.40 67.7
46
Table 2.4. Continued
§SMNp= soil mineral N at 0-30cm soil depth prior to planting; KCl-NH4= extractable NH4-N with 2M KCl at 0-30cm depth prior to planting; PPNT= extractable NO3-N with 2M KCl at 0-30cm soil depth prior to planting; NaHCO3-205= ultraviolet absorbance of 0.01M NaHCO3 extract at 205nm; NaHCO3-260=unltraviolet absorbance of 0.01M NaHCO3 extract at 260nm; Hot KCl-NH4= extractable NH4 with 2M KCl at 100oC for 4 hours; CaCl2 = soil mineral extracted in 0.01M CaCl2; CO2-Mineral N= mineral nitrogen extracted with 2M KCl following 7 day incubation; ISNT=Illinois Soil N Test for amino-sugar-N; WEOM-C= water-extractable organic carbon; WEOM-N=water extractable organic nitrogen; WEOM-C:M= water extractable organic carbon to nitrogen ratio; Hot-WEOM-C: hot water extractable organic carbon; WEOM-KCl= mineral N extracted using 2MKCl following cold and hot water extraction in sequence; POM-N= particulate organic matter nitrogen; POM-C=particulate organic matter carbon; POM-C:N= particulate organic matter carbon to nitrogen ratio; SD=standard deviation # Grand mean and SD. (n=12 sites*4 replicates-6 outliers=42). There were 6 data points identified as outliers.
Mineralizable N tests
WEOC WEON WEMN WSN WEOC: N Hot- WEOC
WEOM- KCl POMN POMC POMC: N
Site Location mg kg-1 mg kg-1 Ilderton 204
(8.5) 25.8 (9.5)
11.8 (5.5)
37.5 (6.2)
8.9 (1.7)
117 (0.4)
16.17 (1.6)
12.7 (8.2)
332 (261)
24.2 (5.5)
Strathroy 188 (36)
23.9 (6.1)
3.92 (0.9)
27.8 (5.8)
8.4 (1.4)
109 (9.3)
7.04 (0.75)
103 (6.8)
1774 (304)
17.2 (1.9)
Hart 246 (4.1)
28.4 (6.2)
9.34 (2.0)
37.7 (5.2)
9.0 (0.91)
118 (11)
19.2 (4.3)
142 (45)
2351 (600)
17.1 (4.4)
Rutherford 185 (4.9)
26.6 (2.1)
8.33 (1.0)
35.0 (1.5)
7.0 (0.14)
93 (7.4)
11.1 (0.97)
111 (35)
1975 (400)
18.4 (4.1)
Moorefield 188 (5.2)
31.6 (3.4)
10.5 (2.5)
42.2 (3.8)
6.0 (0.41)
79 (5.2)
11.5 (3.8)
11.6 (4.6)
319.0 (144)
27.6 (4.4)
Mount Hope 252 (8.6)
33.0 (2.9)
13.8 (5.1)
46.7 (6.0)
7.7 (0.39)
108 (2.0)
11.1 (5.7)
110 (50)
2625 (57)
37.4 (10)
Bornholm 312 (7.1)
32.5 (2.5)
8.82 (2.1)
41.3 (3.2)
9.8 (0.51)
129 (7.0)
19.0 (12)
8.42 (3.0)
191 (73)
22.7 (2.8)
Lucan 343 (8.6)
32.4 (2.9)
5.76 (0.8)
38.2 (3.3)
11 (0.75)
137 (5.9)
9.6 (3.8)
4.74 (1.5)
122.7 (45)
25.6 (1.8)
ERS-OMAFRA 192 (3.6)
26.6 (1.5)
8.85 (1.4)
28.8 (14)
7.3 (0.24)
92 (0.13)
14.1 (2.2)
60.2 (46)
2488 (2378)
33.2 (14)
ERS-Lauzon 163 (8.3)
27.3 (1.0)
20.7 (18)
34.4 (4)
5.7 (0.27)
63 (1.6)
6.4 (2.7)
64.6 (53)
5657 (2653)
147 (115)
Woodslee 408 (18)
36.6 (4.9)
5.27 (0.27)
41.9 (4.7)
11 (0.60)
95 (0.24)
15.2 (3.5)
177 (69)
2596 (1126)
14.7 (0.85))
U of G-Ridgetown
238 (22)
82.4 (21)
7.2 (2.4)
89.6 (21)
3.1 (0.66)
183 (3.7)
9.08 (6.5)
146 (18)
3057 (472)
21.0 (2.7)
Mean# (n=42) 243 33.9 9.52 41.8 4.0 58.8 13.4 73.2 1904 34.6 SD (n=42) 52.8 5.3 3.5 6.5 1.33 19.6 5.2 55.5 1597 34.4
47
2.3.2 Field-based indicators of corn N availability
Dry matter yield and GY ranged between 11.48- 23.40 and 4.8-10.12 Mg ha-1,
respectively and ∆Y and RY showed that a wide variation in crop response to N fertilizer
addition was observed. Strathroy and Moorefield sites exhibited less response to fertilizer
addition (∆Y=1.8 and 2.6 Mg ha-1, respectively) than Ridgetown and Lauzon sites where
yields increased by 7.8 and 8.9 Mg ha-1, respectively. Plant N uptake from the zero N
plots ranged from 47.52 to 131.5 kg N ha-1 (Table 2.5). The soil N supply for all sites
ranged from 68 to 189 kg N ha-1. A large portion (17-46%) of the soil N supply was
remaining in the soil at harvest (SMNh), but these values were highly variable across
sites. The Lucan and Mount Hope sites showed substantially more SMNh (74 and 63.1
kg N ha-1, respectively) than the Lauzon and Strathroy sites (26.5 and 27.6 kg N ha-1,
respectively).
48
Table 2.5. Mean values for field-based indicators of crop N availability (standard deviation in parenthesis) measured at 12 experimental sites in southwestern Ontario (n=4).
§PNU0N=Plant N uptake in zero N treatments; SMNh= Soil mineral N at harvest; Soil N Supply=PNU0N + SMNh; DM= dry matter yield; GY=grain yield; ∆Y=(GY optimal N-GY zero N); RY=[(GY zero N/GY optimal N) x100]; SD=standard deviation. # Grand mean and SD
PNU0N§ SMNh
Soil N Supply DM
GY ∆Y RY
kg N ha-1 Mg ha-1 %
Ilderton 66.9(12) 55.7(4.8) 123(12) 23.8(4.2) 6.93(1.9) 5.0(1.1) 61.9(15) Strathroy 132(18) 27.6(4.5) 159(20) 27.1(2.2) 10.1(1.1) 1.8(1.1) 93.9(11)
Hart 97.7(24) 46.9(11) 145(22) 30.2(3.3) 7.04(0.5) 4.8(1.1) 55.5(5.0) Rutherford 116(29) 40.0(2.1) 156(29) 31.3(6.3) 7.07(0.1) 4.8(0.5) 55.5(6.9) Moorefield 126(13) 44.4(4.8) 170(14) 27.3(3.2) 6.93(0.5) 2.6(1.0) 70.1(7)
Mount Hope 127(32) 63.1(15) 189(36) 33.5(6.0) 6.54(0.9) 3.8(0.7) 64.1(3.5) Bornholm 94.0(14) 61.3(9.5) 156(17) 19.4(2.0) 6.93(0.9) 3.8(0.6) 60.4(7.2)
Lucan 113(24) 74.0(7.6) 166(22) 23.4(2.8) 7.57(1.4) 3.3(1.0) 65.9(6.2) ERS-OMAFRA 87.8(20) 52.2(11) 140(21) 21.3(1.8) 7.00(0.9) 3.1(0.3) 63.7(2.2)
ERS-Lauzon 47.5(15) 26.5(20) 74.0(15) 14.3(3.3) 4.33(1.1) 8.9(1.2) 39.2(3) Woodslee 68.4(10) - 68.4(10) 11.5(2.2) 4.80(1.1) 4.4(1.4) 51.4(15) U of G-
Ridgetown 73.7(31) 63.2(9.5) 137(29) 13.9(5.7) 5.74(2.1) 7.8(1.7) 45.9(9.0)
Mean# (n=42) 97.3 48.5 140 24.0 6.70 4.30 59.3 SD (n=42) 32.0 13.0 33.2 6.23 1.67 2.10 10.4
49
2.3.4 Relationships between laboratory indicators and RY
Results from the correlation analysis done between laboratory indicators and RY are
presented in Table 2.6. A significant correlation was observed between PNU0N and DY
(Figure 2.2, r=0.72, P<0.001). The data set was successfully grouped based on clay
content to form two groups: the Coarse Textured (Cs-T) soil group (clay ≤ 240 g kg-1,
n=6) and the Medium Textured (Md-T) soil group (clay> 240 g kg-1, n=6). The PPNT
showed no significant correlation with RY for the whole data set (r=-0.13; Figure 2.3)
and when separated based on soil texture (Table 2.6). Pool I was the only parameter to
show a significant functional relationship with RY but only when the data set was
separated based on clay content (Figure 2.4). In Cs-T soils, Pool I showed a significant
positive linear relationship with RY (R² = 0.55, P<0.001, n=21) and a negative
relationship in Md-T soils (R² = 0.35, P=0.004, n=21) (Figure 2.4). Other parameters that
had significant correlations with RY included WEON, WEOC: N, POMC and POMC: N,
however upon further assessment showed no functional relationship (refer to Appendix).
50
Table 2.6. Correlation coefficients (r) between laboratory indicators for predicting crop N availability and relative yield (RY) for whole data set (n=42), Cs-T soils (clay ≤ 240 g kg-1, n=21) and Md-T soils (clay>240 g kg-1, n=21).
*=P<0.05; **=P<0.01.
Indicator RY Whole
DS Cs-T Md-T Mineralizable N parameters Pool I 0.12 0.64** -0.59** Pool II 0.17 0.34 -0.31 Net N Min -0.21 0.51* 0.50* N0 0.30 0.44 -0.14 Pool III 0.27 0.17 0.14 k 0.10 0.13 0.42 Mineralizable N tests PPNT -0.13 0.07 -0.02 KCl-NH4 0.06 0.20 0.15 SMNp -0.10 0.16 0.04 NaHCO3-205 0.35* 0.33 0.34 NaHCO3-260 0.05 0.31 -0.24 Hot KCl-NH4 0.009 -0.06 -0.27 CaCl2 –N 0.06 0.22 -0.17 CO2-Nmin 0.30 0.06 0.04 ISNT -0.13 0.40 -0.18 WEOC 0.08 0.18 -0.24 WEON 0.18 -0.54** -0.41* WEMN -0.02 -0.30 0.33 WSN 0.01 -0.23 -0.36 WEOC: N 0.41* 0.67** 0.12 Hot WEOC 0.19 0.43 -0.36 WEOM-KCl 0.12 0.13 0.17 POM-N -0.29 0.07 -0.40 POM-C -0.54** -0.54** -0.51* POM-C: N -0.42** -0.54** 0.45
51
Figure 2.2 The relationship between PNU0N and DY in zero N plots for experimental corn sites in Ontario.
r = 0.72; P<0.05, n=48
0
5
10
15
20
25
30
35
40
45
0 50 100 150 200
DY
(Mg
ha-1
)
PNU0N (kg ha-1)
52
Figure 2.3. The relationship between PPNT and RY for all experimental sites in Ontario in 2013.
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25
RY (%
)
PPNT (mg kg-1)
r = -0.13, P>0.05, n=42
53
Figure 2.4 Relationship between Pool I and RY for (a) the whole data set, (b) Cs-T soils (clay ≤ 240 g kg-1), and (c) Md-T soils (clay > 240 g kg-1); *=data point not included in correlation and regression analysis.
0
20
40
60
80
100
0 20 40 60 80 100
RY (%
)
Pool I (mg kg-1)
a
y = 0.77x + 31R² = 0.55, P<0.001, n=21
0
20
40
60
80
100
0 20 40 60 80 100
RY (%
)
Pool I (mg kg-1)
*b
y = -0.36 + 76R² = 0.35, P<0.01, n=21
0
20
40
60
80
100
0 20 40 60 80 100
RY (%
)
Pool I (mg kg-1)
c
r = 0.12, P>0.05, n=42
54
As the soil N supply consists of pre-plant soil mineral N and mineralizable N and in
attempt to improve the relationship between Pool I and RY, SMNp was included with
Pool I. Results from regression analysis showed the relationship improved in Cs-T soils
from R2=0.55 to 0.63 (Figure 2.5).
Figure 2.5. Relationship between Pool I + SMNp in Cs-T soils and RY. *=data point not included in correlation and regression analysis.
y = 0.78x + 23.903R² = 0.63, P<0.001, n=21
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70
RY (%
)
Pool I + SMNp (mg kg-1)
*
55
2.4 Discussion
2.4.1 Laboratory indicators for predicting corn N availability
Soil N mineralization
Selected Ontario soils exhibited a wide range of N supplying potential which is consistent
with the wide distribution of site characteristics, especially pH. The optimum pH for
microbes is between 6-7 (Hartel, 2005). The majority of our sites had a pH between 7 and
8. Strathroy had a pH of 6.6, which may explain its high N mineralizaion potential. The
Lauzon site on the other hand had a pH of 8.5 and may explain its low mineralization
potential and crop yield (Table 2.1, Table 2.5).
The cumulative N mineralized during the incubation were higher than the values reported
for soils taken from 17 experimental field sites in New Brunswick, Quebec, Manitoba
and Saskatchewan, Canada and Maine USA under various field crops (54-197 mg kg-1;
Sharifi et al., 2007a), and closer to the values reported for 19 arable soils in Ontario,
British Columbia, Quebec and New Brunswick (18-174 mg kg-1; Nyiraneza et al., 2012).
The Pool I values were similar to those measured by Nyiraneza et al. (2012; 3-61 mg kg-
1) across Canada and larger than the values reported by Sharifi et al., (2007a,b; 4.3-41,
20-37 mg kg-1) for annual crops in eastern and western Canada and Maine, USA. The
higher values in this study may be due to the difference in leaching method (buchner
funnels vs. leaching tubes) and 0.01M CaCl2 volume (175 vs. 125 mL; Sharifi et al.,
2007a; Nyiraneza et al., 2012).
56
Furthermore, the potential of a soil to supply N depends on the total N content (Ros et al.,
2015). The greater the total N, the higher the capacity of a soil to supply N. A similar
portion of total N as Pool I and II was mineralized in study compared to Sharifi et al.
(2007a) (Pool I=2.25% and Pool II=5.25%) however the higher total C and total N in this
study (20.7 and 1.7 g kg-1) compared to theirs (17.5 and 1.3 g kg-1) may explain the larger
amount of mineralizable N. Sharifi et al. (2007a) and Nyiraneza et al. (2012) also found
that Pool I represented a similar portion of the net N mineralized (23 and 27%)
supporting the developing theory that short term-incubations may be a more realistic
representation of N mineralized in a growing season than the long-term aerobic
incubation (Standford and Smith, 1972; Jalil et al., 1996; Schomberg et al., 2009; Ros et
al., 2011).
Mineralizable N tests
The SMNp in this study contributed a similar portion to the soil N supply as observed for
corn in Eastern Ontario (16-24%, Wu et al., 2008; 19-27%, and Ma et al., 2005) and for
corn in Quebec in unfertilized plots (25%, Nyiraneza et al.,2009). This suggests that a
large portion of the crop available N (approx. 70% of the soil N supply) remains to
originate from the mineralization of SON in Ontario soils.
The concentration of Hot-KCl-NH4 was within the range found in arable soils (Sharifi et
al., 2007, Nyiraneza et al., 2012). The higher concentrations of Hot-KCl-NH4 compared
with 2M KCl-NH4 can be explained by the high capacity of these soils to supply N (Jalil
et al., 1996; Sharifi et al., 2007a). The ISNT method extracted the largest fraction of
57
organic N in this study with values similar to the average reported for Canadian soils
(190 mg kg-1; Nyiraneza et al., 2012), well-drained soils of North Carolina (150 mg kg-1,
Williams, 2007) and from soils taken from 96 N rate studies across the Midwest US
(Laboski et al., 2008). The greater of amount N extracted in the ISNT compared to
WEOM or POM is directly related to the nature of the extraction as ISNT measures total
hydrolysable N, hydrolysable NH4+ and hydrolysable amino acid and amino sugar N
present within the soils (Mulvaney et al., 2001).
The WEON values were larger than those reported for soils collected in Western Canada
receiving pea, faba bean or wheat residues (3.9-6.4 mg kg-1, St. Luce et al., 2014), but
similar to values reported for non-amended soils under a corn monoculture (Gregorich et
al., 2003. The WEOC concentrations were similar to those reported for maize cropped
soils in Ontario (280-570 mg C kg-1; Gregorich et al., 2003), however the WEOC: N was
lower (Gregorich et al., 2003 (13) Curtin et al., 2006 (10.4-23.9); Haney et al., 2012 (20))
and WEON proportion of TN (2.5%) was higher compared to the literature (0.75-0.9%,
Curtin et al., 2006). The high proportion of total N as WEON indicates that these soils are
high in soluble N compounds, a biologically available form of organic N (Herbert and
Bertsch, 1995), and the low C: N ratio of the WEOM fraction could be the result of long-
term N fertilization leading to a larger soluble N pool and stimulation of the microbial
community resulting in increased mineralization of SON (McDowell, 2003).
The POMN values were lower than those reported for Western Canada (174-643 mg kg-1;
St. Luce et al., 2014) in soils receiving legume and non-legume crop residues and for
58
potato trials in Eastern Canada and US (POMC=2050 mg kg-1, POMN=101 mg N kg-1;
Sharifi et al., 2007). The proportion of POMN of total N was lower in this study (4.3%)
compared to the average reported in the literature (18%; Gregorich et al. 2006), which is
indicative of a management system not receiving large inputs from organic sources
(Wander, 2004). The POMC: N (34) was larger than the whole soil C: N (13) which is
typically seen due to the addition of high C: N residues (corn, wheat and soybean stalks)
in conventional systems (St.Luce et al., 2011). These results merit further investigation
into the labile fractions of organic N (WEOM and POM) for soils under conventional
management in Ontario soils is required, as both of these fractions are known to be
readily available substrates for microbes (Wander, 2004).
2.4.2 Field-based indicators of corn N availability
The soil N supply was highly variable across southwestern Ontario and a significant
portion, between 28 and 74 kg N ha-1, was remaining as mineral N in the soil at harvest.
This is consistent with De Jong et al. (2009) who found that for agricultural land in
Ontario between 1981 and 2006, on average 57 kg N ha-1 was remaining in the soil as
residual soil NO3 (RSN) at harvest. This high RSN at harvest provides evidence that there
is asynchrony between soil N supply and crop N demand resulting in the potential of
significant N losses over the fall and spring months (Power et al., 1998; Dinnes at al.,
2002). Management practices such as inter-seeding cover crops can synchronize N
mineralization to crop N uptake and reduce N losses following harvest (Loecke et al.,
2012; Rasouli et al. 2013).
59
2.4.3 Relationships between laboratory indicators and RY
The PPNT showed no relationship with RY for the whole data set which is in agreement
with Belanger et al. (2001) who reported NO3 alone to be a poor predictor of RY for a
potato-forage-grain rotation in Atlantic Canada (r=0.37-0.55) and Nyiraneza et al. (2009)
who determined it was not a consistent predictor of the most economic rate of N for corn
in Quebec (r=-0.24, P>0.05 in 2007; r=-0.46, P<0.05 in 2008). The poor relationship
observed between NO3 and RY can be explained by the temporal and spatial variability
across landscapes arising from early season soil temperature and moisture (Nyiraneza et
al., 2009) soil texture (Cambouris et al., 2005) and cropping history (Andraski et al.,
2000).
Pool I had a significant positive relationship with RY in Cs-T soils while in Md-T soils,
Pool I had a significant negative relationship with RY. It is hypothesized that this lower
N availability in the field for Md-T soils, indicated by the inverse relationship, can be
explained by two factors: soil clay content and soil pre-treatment. Sites with the highest
clay content (Woodslee- 406 g kg-1 and U of G-Ridgetown- 440 g kg-1) had the lowest
RY (51 and 45%, respectively) but relatively high net N mineralization (176 and 123 mg
kg-1). This may be a result of slower actual release of SON in the field from aggregate
formation and physical protection from clay particles. (Angers et al., 1997; Yoo and
Wander, 2006; Kölbl et al., 2006; Chivenge et al., 2011; Nyiraneza et al., 2012). Kölbl
and Kögel-Knabner (2004) attribute this decrease in mineralization due to the
contribution of partially decomposed plant material to macro-aggregate formation in soils
with higher clay content.
60
Soil pre-treatment in this study consisted of air-drying, crushing and sieving (2mm) soils
prior to the incubation. Beare et al. (1994) found that crushing macro-aggregates in a
sandy clay loam increased C and N mineralization in the laboratory, compared to cores
that were left intact. This effect may be more prominent in clay soils as clay particles
have been shown to physically protect SOM through the formation of macro-aggregates.
These results indicate that determination of this labile pool of organic N may be more
informative in sandy rather than clay soils as a large proportion (50-75%) of SOM may
be stabilized by clay particles (Christensen, 2001).
Pool I was the best indicator of N availability for both coarse and medium textured soils.
Pool I was more related to RY (r=0.12-0.64) than Pool II (r=-0.31-0.34) and N0 (r=-0.14-
0.44) in this study. This is consistent with Villar et al. (2014), who found Pool I was more
strongly correlated to apparent mineralization from elongation to harvest in wheat
(r=0.53, P<0.05) than with Pool II (r=0.40) and N0 (r=0.32) in the humid Mediterranean
climate of Northern Spain. Nyiraneza et al. (2012) also found a stronger a correlation
between soil N supply and Pool I (r=0.41, P<0.01) than with Pool II (r=0.28, P<0.05) in
soils collected at depth of 0-15cm from cornfields across Canada and incubated at 25oC
for 24 weeks.
The addition of SMNp to Pool I explained an extra 10% of the variability in RY for CT
soils, but not MT soils. This indicator of N availability has been proposed for potato
production in Eastern Canada and Maine, USA (Sharifi et al., 2007). Pool I in
61
conjunction with mineral N content at sowing has also showed a strong correlation to
wheat straw yield (r=0.57, P<0.05; Villar et al., 2014) in the humid Mediterranean
climate of northern Spain and recent research indicates that more labile pools (SMN and
Pool I) are depleted throughout the growing season and can contribute up to 72% plant N
uptake (Dessureault-Rompré et al., 2013).
Compared with the whole data set, grouping soils based on soil texture revealed
relationships dependent on clay content. These results are consistent with other work that
has found clay content can explain a substantial proportion of the variation in soil N
mineralization (Nyrianeza et al., 2012; Dessureault-Rompré et al., 2010, Ros et al., 2011;
Villar et al., 2014).
2.5. Conclusion
In conclusion, this study indicated that Ontario soils have a high potential to supply N,
N0=147 mg kg-1. Laboratory indicators of mineralizable N were highly variable across
sites and the SMNp contributed between 15 to 30% of the soil N supply throughout the
growing season indicating that a large portion of the soil N supply originates from
mineralization of SON in humid temperate climates. Field indicators of crop N
availability indicated that management practices should be tailored to reduce residual N
at harvest. The current PPNT was a poor predictor of field based indicators of crop N
availability and Pool I, the cumulative amount of N released in a 2-week aerobic
incubation at 25 oC, was a more robust predictor of crop N availability (RY). Grouping
soils based on soil texture revealed relationships dependent on clay content indicating
62
that this parameter should be taken into account when predicting N availability under
field conditions. Pool I in coarse textured soils had a stronger relationship with crop N
availability indicators. The relationship between Pool I and crop N availability indicators
also improved with the addition of SMNp . Overall, this study demonstrated the
importance of a readily mineralizable N pool when predicting N availability to corn over
the growing season in southwestern Ontario.
63
Chapter 3. Assessing the ability of laboratory-based indicators of mineralizable N to predict fertilizer N recommendations for corn in southwestern Ontario
3.0 Abstract
Mineralizable nitrogen (N) is an important component of soil N supply with potential to
improve N fertilizer recommendations for grain corn in southwestern Ontario. However,
a robust indicator of mineralizable N has not been developed as a soil test for N fertilizer
recommendations in this region. The objective of this study was to evaluate readily
mineralizable N (Pool I- N mineralized in 2 wk), water soluble N (WSN) and particulate
organic matter N and C (POMN-C) as potential soil N tests for corn in southwestern
Ontario. The soil test calibration method was used to estimate fertilizer N
recommendations based on maximum economic rate of N (MERN) and nitrogen rate at
95% maximum yield (MYRN). Corn N response trials were established at 13 field sites
across southwestern Ontario in 2013 and 2014 and soil samples were collected (0-30-cm
depth) before planting. The pre-plant nitrate test (PPNT) and WSN were significantly
correlated with relative yield, MERN and MYRN and used for N fertilizer
recommendations. The minimum MERN and MYRN was 72 kg N ha-1. When PPNT
concentrations were between 1 and 18 mg kg-1 N fertilizer recommendations could be
successfully calculated using the equation MERN=-5.16 (PPNT)+203 (R2 =0.47) and
MYRN=-4.72(PPNT)+167 (R2=0.56). When WSN concentrations were between 1 and 70
mg kg-1 N fertilizer recommendations could be successfully calculated using the
equations MERN =-2.02(WSN)+250 (R2=0.60) and MYRN=-1.65(WSN)+197 (R2=0.48).
Further research is required test the validity and suitability for each of the proposed linear
models over across years and sites.
64
3.1 Introduction
Nitrogen (N) fertilizer recommendations for corn (Zea Mays) at planting in Ontario are
based on the pre-plant nitrate (NO3) test (PPNT) or expected yield with the consideration
soil texture, previous crop, and market prices for corn and N fertilizer. The PPNT for
Ontario was introduced in 1992 and general fertilizer N recommendations for corn were
last updated in 2006. The major downfall to the PPNT is the lack of inclusion of the
mineralizable soil N, a significant portion of the soil N supply for corn in humid
temperate climates (Zebarth et al., 2009; Wu et al., 2008; Whalen et al., 2013). Soil
mineralizable N varies among fields and years and more accurate recommendations are
required that capture this variation and reduce over or under fertilization of N at the
expense of the producer and the environment.
Recent developments for predicting the contribution of mineralizable soil N to corn have
focused on measuring readily mineralizable pools of N such as Pool I (Sharifi et al.,
2007), water extractable carbon (C) and N (WEOC and WEON; Chantigny et al., 2008)
and particulate organic matter C and N (POMC and POMN; St. Luce et al., 2014). The
potential for use of these tests for optimizing fertilizer N recommendations have not been
fully explored for grain in Ontario.
The soil N supply in humid temperate climates includes carryover N from the previous
growing season plus the amount of N that will mineralize over the growing season from
organic sources (Zebarth et al., 2009). Soil N mineralization under field conditions is
difficult to predict due to the dynamic nature of soil moisture and temperature, the
65
primary controls on mineralization. Potentially soil mineralizable N parameters in the
laboratory can be measured using the long-term aerobic incubation (Standford and Smith,
1972). This method involves incubation of a soil and sand mixture at optimal moisture
and temperature conditions for mineralization for 20+ weeks with periodic leaching
events. This method is time consuming and laborious and does not necessarily reflect the
amount of N available over one growing season. A short-term incubation (Pool I) is a
common practice derives from the long-term aerobic incubation (Stanford and Smith,
1972). Pool I is the amount of N released in the first two weeks of the incubation and is
known as a flush of N from rewetting of the soil (Sharifi et al., 2007). In arable soils,
Pool I can represent between 7 and 45% of the total N mineralized during the long-term
incubation (Sharifi et al., 2007; Dessureault-Rompré et al., 2010 and 2011; Nyrianeza et
al., 2012; Villar et al., 2014) and has been significantly correlated with the soil N supply
(r=0.41) for grain corn in Ontario, Quebec, British Columbia and Atlantic Canada
(Nyiraneza et al., 2012).
The WEOC and WEON have also shown promise as indicators of potentially available N
(Chantigny, 2003). This soluble portion of soil organic matter (SOM) is an important
substrate for microorganisms and its decomposition into plant available N is dependent
on the biochemical transformation performed by the microbial biomass (Stevenson, 1994;
Haynes, 2000; Chantigny, 2003; Gregorich et al., 2003). The organic N in this pool
represents on average 0.75% of the total soil N and it is hypothesized that this pool is
composed of a complex mixture of molecules that are a direct reflection of the
composition of the SOM (Chantigny, 2003; Haney et al., 2012). Biodegradability studies
66
have shown that 60% is biodegradable over the course of 40 days; however, this is
dependent on the type of amendment added to the soil (Gregorich et al., 2003). The C: N
ratio of this fraction is also an important indicator of N availability (Haney et al., 2012).
This labile pool of organic N has been suggested as an early indicator of the effects of
soil management and cropping systems on SOM quality (Gregorich et al., 1994; Haynes
and Beare, 1996) and therefore can provide useful information on the availability of soil
N to the growing field crop.
Particulate organic matter C and N is composed of partially decomposed plant residues
and organic amendments, a transient pool between fresh and humified organic matter
(Gregorich and Janzen, 1996) and the POMN can represent up to 18% of total soil N
(St.Luce et al., 2011). The size and decomposition rate of the POM is directly influenced
by soil texture and management practices, and is the most responsive of the labile organic
matter fractions to management changes (Biederbeck et al., 1998; Franzleubbers et al.,
2000; Wander, 2004; Spargo et al., 2011). In Western Canada, POMN successfully
predicted the canola yield and N uptake (R2=0.56 and 0.69) and in Maryland, USA, a
strong correlation between corn grain yield and N uptake (r=0.72 and 0.63) was
observed. The POM fraction is a promising indicator of N availability to corn as it can
detect management changes and actively contributes to the plant available pool of N
(St.Luce et al., 2014).
For the 2013 growing season, I concluded that the PPNT was not a good indicator of
plant available N and labile pools of soil N have a significant relationship with field-
67
based indices of soil N supply and have potential to be used for fertilizer N
recommendations in grain corn. In this chapter corn N response trials were conducted at
various field sites across Ontario. The interpretation and fertilizer N recommendation
tables were developed for selected N tests. Calibration studies provide meaning of the
soil test and allow the establishment of soil test categories to calculate fertilizer
recommendations (Dahnke and Olson, 1990). I hypothesize that readily mineralizable
pools of N can be used to predict crop N availability for corn in southwestern Ontario.
The objectives of this study are to evaluate the ability of mineral N at planting (including
the PPNT), Pool I, WEOM, and POM to i) correlate with grain corn N response
measurements and ii) enable calibration of fertilizer N with maximum economic rate of N
(MERN) and nitrogen rate at 95% maximum yield (MYRN) corn across Ontario. The
mineral N was chosen as it is the most commonly used indicator of plant available N in
Ontario but can vary considerably from year to year. Pool I was chosen because it
showed a promising relationship with relative yield in chapter 2. Finally, WEOM and
POM were chosen as they are becoming recognized as important sources of substrates for
N mineralization in agricultural soils.
3.2 Materials and methods
3.2.1 Field site description and plot setup
Seven sites were selected for establishment of corn N response trials in 2014 across
southwestern Ontario. These sites included previously established trials at the Elora
Research Station, at Agriculture and Agri-Food Canada (AAFC) Woodslee and Ottawa
68
locations, and at the University of Guelph Ridgetown Campus. New trials were also
established on growers fields in Pinkerton and Teeswater, ON and at the Trent
Experimental Farm in Peterborough, ON (Figure 3.1, Table 3.1). Soils were classified as
Teeswater silt loam (Grey Brown Podzolic; Pinkerton and Teeswater), Woolwhich silt
loam (Grey Brown Podzolic; Elora Research Station), Brandon Loam (Orthic Humic
Gleysol; AAFC-Ottawa), Otonabee loam (Orthic Melanic Brunisol; Trent Experimental
Farm) and Brookston clay loam (Orthic Humic Gleysol; U of G-Ridgetown and AAFC-
Woodslee). Soil texture varied between 155 and 440 g kg-1 clay and 258-709 g kg-1
sand, the total C was between 14 and 35 g kg-1, total N between 1.3 and 2.8 g kg-1, C: N
of 8.6 to 13 and pH in the range of 6.8-8.1 (Table 3.1). Previous crops included soybean
(Glycine max) at Pinkerton, Teeswater and U of G Ridgetown, corn at AAFC Woodslee,
AAFC-Ottawa, and Elora Research Station, and buckwheat (Fagopyrum esculentum) at
the Trent Experimental Farm. All sites were under conventional management receiving
mineral N fertilizers as their N source except for Trent Experimental Farm, which was
under organic management prior to site establishment.
The growing season rainfall and average temperature in 2014 ranged from 381-585 mm
and 14-17oC, respectively. The crop heat units (CHU) were between 2500 (Trent
Experimental Farm) and 3560 (AAFC-Woodslee).
At each of the experimental field sites, corn N response trials were setup in a randomized
complete block design to include 4 to 5 rates of fertilizer application ranging from 0-200
69
kg N ha-1 (Table 3.3) in replication of four. Field trials were planted in grain corn and
fertilizer was applied at planting as 28% urea ammonium nitrate (UAN 28-0-0).
Figure 3.1 Map of locations of experimental field sites in Southwestern Ontario in 2014. n=7.
70
Table 3.1 Summary of site characteristics for 2014 corn N response trials in Ontario (n=4).
Site Name Latitude and Longitude
Growing season rainfall
Mean growing
season air temperature
Crop heat unit
(CHU) Soil Texture Soil classification pH Total
C Total
N C:N Ratio
Clay Silt Sand
oN oW mm oC
g kg-1
g kg-1 Pinkerton 44o 13' 45.6564"N,
81o17'49.6032"W 395 17 2700 155 257 588 Teeswater silt loam
(Grey-brown Podzolic) 7.4 29 2.6 11.5 Teeswater 44o 1' 42.1932"N,
81o 22' 30.0108"W 395 17 2700 168 353 479 Teeswater silt loam
(Grey-brown Podzolic) 7.9 35 2.8 12.5 Elora Research Station 43o 38' 0.2544" N,
80o 23' 19.2480" W 410 14 2680 200 480 320 Woolwich silt loam
(Grey Brown Luvisol) 7.8 24 2.1 11.5 AAFC- Woodslee 42o 12' 52.4340" N,
82o 44' 54.0888"W 525 16.5 3560 406 336 258 Brookston clay loam
(Orthic Humic Gleysol) 6.6 19 2.2 8.6 U of G-Ridgetwon 42o 27' 3.6972" N,
81o 53' 22.9488" W 525 16.5 3340 440 250 310 Brookston clay loam
(Orthic Humic Gleysol) 7.8 22 1.9 11.3 Trent Experimental Farm 44o 21' 42.0588"N,
78o16'42.6000"W 463 15 2500 155 136 709 Otonabee loam (Orthic
Melanic Brunisol) 8.1 29 2.3 13 AAFC-Ottawa 45o22' 29.1612" N,
75o43' 26.9625"W 381 16 2900 350 270 380 Brandon clay loam
(Orthic Humic Gleysol) 6.8 14 1.3 10.3
71
3.2.2 Soil sampling and analysis
Soil samples were collected in composite from each zero N fertilizer rate treatment at
each field trial at a depth of 0-30 cm 5-10 days prior to planting and fertilizer application.
A subsample was kept moist and stored at 4oC until analysis. On the remaining soil, soil
moisture content was determined by drying soil at 105 oC for 24 hours and the remaining
was air-dried and sieved (<2mm) before laboratory analysis. On air dried soils, soil pH
was determined in a 1:2 soil: deionized water suspension (Hendershot et al., 1993).
Particle size analysis was determined using the pipette method following organic matter
removal (Gee and Bauder, 1986). Total soil C and N was measured by the dry
combustion method using a CNS analyzer (VarioMAX cube, Elementar
Analysensysteme GmbH, Hanau, Germany).
3.2.3 Soil N test parameters
The KCl extractable NH4 and NO3 (KCl-NH4 and KCl-NO3) were extracted on moist soil
using 2M KCl at a soil to extractant ratio of 1:5 and a shaking time of 30 min.
Concentrations of NH4 and NO3 were determined colorimetrically using the modified
indophenol blue method (Sims et al., 2005) and an Epoch microplate spectrophotometer
(BioTek Instruments Inc., Winooski, VT, USA). The KCl-NO3 is hereafter referred to as
PPNT and the sum of KCl-NH4 and KCl-NO3 is hereafter referred to as SMNp, soil
mineral N at planting. Pool I was determined using a modified method of the long-term
aerobic incubation procedure as described by Curtin and Campbell (2008). Thirty grams
of soils were mixed with an equal amount of sand for coarse-textured soil and with the
twice the amount of sand for fine-textured soils and packed into 5cm plastic Buchner
72
funnels. The soil to sand mixture was then re-wetted to 55% water filled pore space
(WFPS) by adding 175 mL 0.01M CaCl2 and applying vacuum. The soil and sand
mixture was incubated at 25oC for 14 days and leached on day 0 and 14 with 125 mL
0.01M CaCl2 followed by 25mL of zero-N nutrient solution (Curtin and Campbell, 2008).
Leachates were analyzed for NH4 and NO3 using a modified indophenol blue method
(Sims et al., 2005) and an Epoch microplate spectrophotometer (BioTek Instruments Inc.,
Winooski, VT, USA).
Water-extractable organic N (WEON) and C (WEOC) was determined as per Curtin et
al., 2006 and Chantigny et al., 2009. Briefly, 4 g of air-dried soil was shaken with 20 mL
room temperature water for 60 min. Extracts were then centrifuged at 4500xg for 20 min
and the supernatant was decanted and analyzed for organic C (WEOC) using an
Schimadzu TOC-VCPH (Schimadzu Scientific Instruments, Columbia, MD,USA). The
mineral N in the water extracts (WEMN) was determined using the modified indophenol
blue method as described above and the water soluble N (WSN) was determined using
the persulfate oxidation method as described by Cabrera and Beare (1993). The water
extractable organic N (WEON) was then calculated by subtracting the WEMN from the
WSN.
Particulate organic matter C and N (POMN and POMC) was determined by shaking 25g
field-moist soil overnight in a 5g/L sodium hexametaphosphate solution. Soil was then
passed through a 53-𝜇m sieve (Gregorich and Ellert, 1993). Retained sand and macro-
organic matter were dried and weighed and total C and N concentrations was determined
73
using a CNS analyzer (VarioMAX cube, Elementar Analysensysteme GmbH, Hanau,
Germany).
3.2.4 Field-based indicators of corn N availability
Corn plants were harvested at maturity in October 2014. Eight to 10 plants were
randomly harvested within a 16 m2 subplot in each of the N rate treatments. Corn plants
were separated into their kernel, cob and stover portions. Grains were threshed and
weighed, and yields (GY) were adjusted to 15% moisture content. Stovers and grains
from each N rate treatment were dried in a drying oven at 60 oC and a subsample was
weighed to determine DM yield. The grain and stover were ground for determination of
total N concentration by dry combustion using a CNS analyzer (VarioMAX cube,
Elementar Analysensysteme GmbH, Hanau, Germany). The plant N uptake in zero N rate
treatment (PNU0N) was calculated from the grain and stover tissue N concentrations from
zero N rate treatments, corrected for any starter fertilizer N applied with the seeder at
planting. Relative yield (RY) and delta yield (∆Y) were then calculated as follows:
RY =
× 100
∆Y = GY optimal N − GY zero N
In addition, composite soil samples were collected from zero N rate treatments at a depth
of 0 to 30 cm at corn harvest and stored at -20˚C until analysis. The soils were extracted
for mineral N (NH4 and NO3) using 2M KCl. Soil mineral N at harvest was calculated as
the sum of KCl-NH4 and KCl-NO3 and is hereafter referred to as SMNh. Finally, the soil
N supply was calculated as PNU0N plus SMNh and was used as an indicator of plant
available N.
74
Maximum Economic Rate of Nitrogen (MERN)
Maximum economic rate of N (MERN) for each field site was determined using the
following quadratic regression equation (McGongile et al., 1996 and Rashid et al., 2004):
Y=a + bN – cN2 (1)
Where: Y= corn grain yield (kg ha-1); N=fertilizer N applied (kg N ha-1); and a, b and c
are coefficients of quadratic response. The derivative of the quadratic equation (1) was
then taken to determine MERN:
dY/dN=b-2cN
Where dY/dN was set to the price rate of 1 kg fertilizer to the price of 1 kg of grain corn
or R to solve for N rate:
R=b-2cN
or N=(b-R)/2c
and therefore MERN=(b-R)/2c
R used in this study was determined using the 2014 corn fertilizer price: Corn price =
$0.18 kg-1, N fertilizer price= $1.38 kg-1.
The maximum economic yield was then calculated using the MERN for each site:
MEY=a+bMERN-cMERN2
Nitrogen Rate at 95% Maximum Yield (MYRN)
The N rate at 95% maximum yield (MYRN) for each site was calculated using the
quadratic regression equation given when plotting grain yield against N rates. The
75
quadratic equation for each site was derived and then solved to determine the N fertilizer
rate required to achieve 95% of the maximum yield.
Corn N calculator
Nitrogen fertilizer rates were also calculated based on the current corn N Calculator
developed by OMAFRA (2010) (Figure 3.2) for comparison to MERN and MYRN.
76
OMAFRA General Recommended Nitrogen Rates for Corn: Corn N Calculator
Western Ontario Cells Needing Input = (click to enter) Imperial Metric
A. Base N Requirement: (lb/ac) (kg/ha) Select soil type Loam 28 32
B. Yield Adjustment:
Enter proven yield (bu/ac) 150 116 129
C. Heat Unit Adjustment:
Enter CHU for your area 2800 0 0
D. Previous Crop Adjustment:
Select previous crop Soybeans -27 -30 Price Ratio Calculations
Enter expected corn price $2.80
Select fertilizer product:
Anhydrous Ammonia
Enter price per tonne of product:
$450
Nitrogen Price ($/lb actual N)
$0.25
Net Corn Price $2.80 E. Price Ratio Adjustment:
(lb/ac) (kg/ha)
Price Ratio ($N:$corn) 5.0 0 0 F. Total N Recommendation 117 132 G. Enter Starter N (lb/ac)
0 0
H. Enter Manure Credit (lb/ac)
0
I. Preplant Additional N 117 132 OR
if applying N as sidedress:
J. SideDress Additional N 93 105
Figure 3.2. A scheme of corn N calculator spreadsheet developed by OMAFRA for general recommendation of N rates for corn in Ontario.
77
3.2.5 Statistical Analysis
Statistical analyses were done using SAS Enterprise Guide 5.3 (SAS Institute, Inc. 1996).
Data was first tested normality using the Kolmogorov–Smirnov test. Additional sites
from the first year of this study (Year 1-2013) that received variable rates of N fertilizer
were added to the Year 2- 2014 data to increase the size of our data set. These sites
included OMAFRA-Elora, Hart, Rutherford, Ilderton, Moorefield and Bornholm.
Recommended rates were calculated using the procedure for MERN and MYRN. Once
the recommended rates were calculated, the experimental sites were separated into
categories based on their maximum economic yields (MEY), determined using the
derived quadratic equation between GY and N rate. To satisfy the first objective,
correlation, the soil N test parameters were correlated to RY for the whole data set and
for each soil texture group using PROC CORR. Soil texture groups were formed based
on clay content using PROC CLUSTER. Calibration was attempted on soil N test
parameters that met the following criteria, i) significant pearson correlation coefficient
with RY in the correlation stage and ii) a significant pearson correlation coefficient with
both MERN and MYRN using PROC CORR. An interpretation table for the successful
soil N test was developed using the linear regression equation derived from the
relationship between RY and soil N test values and included five categories based on the
probability of yield increase: Very High, High, Optimum, Low, Very Low and Low. The
calibration curves were then constructed by correlating MERN and MYRN to the soil N
test values and regressed using PROC REG. The N fertilizer recommendations for soil N
test values were derived using the linear regression equation for parameters with
78
significant relationships. Significance for correlations, regressions and clustering was
accepted at P<0.05.
3.4 Results
3.4.1 Soil N test parameters
The SMNp was highly variable across sites ranging from 8.7 to 21 mg kg-1 and
represented between 20 and 58% of the soil N supply (Table 3.2 and Table 3.3). Of the
total SMNp, 90% was in the NO3 form. Pool I was between 30.3 and 59.7 mg kg-1 and
represented 1.3 to 3% of the total soil N. Of the soil N supply, Pool I represented between
42 and 497%. The WSN ranged between 30.3 and 64.7 mg kg-1 and the proportion as
mineral N was highly variable across sites ranging from 7% at Woodslee to 30% at
Ridgetown. The WEON was less variable between sites ranging from 28.9 to 47.8 mg kg-
1 and was on average 1.9% of the total soil N. The WEOC ranged between 156-403 mg
kg-1 and the WEOC: N ratio was between 5.6 to 12, on average 1.8 times smaller than the
whole soil C: N (8.0-14.8) and a moderate linear relationship was observed between
WEOC and WEON (R2=0.49). The POMC ranged from 1247 to 6758 mg kg-1 and the
POMN was less variable (125-266 mg kg-1) and represented on average 8% of the total
soil N. The POMC: N was between 9.7-32.1, on average 1.8 times larger than the whole
soil C: N.
79
Table 3.2 Means for the proposed soil N tests (standard deviation in parenthesis) from 7 corn N response trials in Ontario in 2014 (n=4).
SMNp† PPNT
KCl-NH4 Pool I WEMN WEON WSN WEOC WEOC: N POMC POMN POMC: N
Site Location
mg kg-1
Year 2 (2014)
Pinkerton 21
(3.9) 19.7 (3.9)
0.57 (0.04)
43 (13)
12.0 (0.37)
47.8 (2.8)
64.7 (4.4)
306 (11)
6.4 (0.5)
3305 (183)
216 (13)
15.42 (1.6)
Teeswater 11.2 (1.7)
9.8 (1.5)
1.5 (0.18)
37.8 (12)
10.3 (0.24)
41.4 (2.1)
56.1 (2.7)
230 (5.5)
5.6 (0.6)
6758 (268)
266 (13)
25.64 (4.6)
Elora Research Station 10.2 (1.2)
8.9 (1.1)
1.3 (0.23)
30.6 (10.6)
2.5 (014)
26.9 (3.3)
30.3 (4.2)
209 (8.3)
7.8 (0.6)
3817 (292)
146 (17)
29.31 (14)
AFFC-Woodslee 8.7
(2.6) 7.0
(3.0) 1.7
(0.83) 59.7 (3.1)
2.7 (0.17)
34.7 (8.6)
38.6 (9.2)
403 (11)
12 (3.6)
1247 (146)
125 (8.3)
9.67 (2.1)
UofG-Ridgetown 15.7 (5.5)
15.4 (6.4)
0.26 (0.42)
41.9 (3.9)
11.4 (0.78)
29.7 (11.1)
37.5 (8.1)
156 (25.6)
6.8 (3.3)
3100 (759)
159 (43)
20.83 (4.9)
Trent Experimental Farm 13.9 (8.6)
12.3 (10)
1.6 (0.59)
30.3 (8.9)
6.5 (0.35)
28.9 (9.4)
38.1 (10.8)
171 (2.5)
6.5 (2.6)
5632 (143)
175 (2.8)
32.12 (1.4)
AFFC-Ottawa 10.9 (1.5)
10.9 (1.6) UDL
39.2 (13.3)
5.9 (0.39)
27.5 (2.8)
36.1 (4.4)
186 (9.7)
6.8 (1.4)
1363 (87)
133 (3.3)
10.44 (3.4)
Mean# (n=28) 24.3 16.7 7.6 41.4 7.0 33.6 43.6 172 7.3 2480 119 24.33
SD (n=28) 8.7 8.5 5.3 13.2 3.0 5.8 8.4 91 1.8 1715 76 8.26 † SMNp= soil mineral N at 0-30cm soil depth prior to planting; KCl-NH4= extractable NH4 with 2M KCl at 0-30cm depth prior to planting; PPNT= extractable NO3 with 2M KCl at 0-30cm soil depth prior to planting; WEMN=water extractable mineral N; WEON=water extractable organic N; WSN=water soluble N; WEOC=water extractable organic C; WEOC:N=water extractable organic C to N ratio; POMC=particulate organic matter C; POMN=particulate organic matter N; POMC:N=particulate organic matter C to N ratio. UDL= under detection limit # Grand mean and SD
80
3.3.2 Field-based indicators of corn N availability
The average GY, PNU0N and soil N supply for corn over the two years of the study are
shown in Table 3.3. For the 2014 growing season, the PNU0N was highly variable across
sites (50 to 194 kg N ha-1), the soil N supply was on average 135 kg N ha-1 and between 3
and 75 kg N ha-1 was remaining in the soil at harvest. Field parameters from the
additional sites from the first year of this study (Year 1-2013) that received variable rates
of N fertilizer are summarized in Table 3.3.
At each of the experimental sites, corn GY showed a strong quadratic response to
fertilizer N application (R2>0.93, Table 3.4, Figure 3.2). The MERN and MYRN were
successfully calculated for 5 of the 7 sites in Year 2-2014 and 6 out 8 sites in Year 1-
2013 (Table 3.4, Figure 3.3). The MERN values ranged from 106 to 212 kg N ha-1 while
MYRN values were consistently lower and ranged between 72 and 161 kg N ha-1. The
results from the corn N calculator are shown in Table 3.4. Recommended fertilizer N
rates based on the corn N calculator were on average within +/- 29% (109-212 kg N ha-1)
of the recommendations calculated using MERN but the response was inconsistent. The
corn N calculator over estimated N rate compared to MERN at Ilderton, Bornholm,
Pinkerton, Elora Research Station, AAFC-Woodslee and U of G-Ridgetown and
underestimated it at OMAFRA-Elora, Hart, Moorefield, Teeswater, Trent Experimental
Farm and AAFC-Ottawa. The largest deviations occurred at the Hart site were the corn N
calculator underestimated the MERN by 67 kg N ha-1 and at the U of G Ridgetown site
where it over estimated MERN by 50 kg N ha-1. Compared to MYRN, values calculated
using the corn N calculator were higher at all sites except for at the Hart and Trent
81
Experimental Farm. The highest deviation from MYRN was seen at the U of G-
Ridgetown where the corn N calculator over-estimated MYRN by 74 kg N ha-1 (Table.
3.4).
Experimental sites were separated into two categories based on their MEY (Table 3.4).
The majority (5 of 7) of sites were grouped into the high yielding group (>9 Mg ha-1)
while the Trent Experimental Farm and AAFC-Ottawa sites were categorized into
category II as their highest yield only reached 7.4 Mg ha-1. Grain yield for zero N plots in
Category I sites were as low as 4.49 Mg ha-1 to as high as 9.03 Mg ha-1. For the Pinkerton
site the addition of fertilizer only increased yields slightly (1.64 Mg ha-1) whereas at the
Elora Research Station, N fertilizer application almost tripled the grain yield (12 Mg ha-
1). The relative yield (RY) ranged from 42 to 112% and no sites over both years were
considered unresponsive to N fertilizer based on the MERN.
82
Table 3.3 Mean values for crop yield, crop response indicators and soil N supply (standard deviation in parenthesis) from 13 corn N response trials across Ontario (n=4) established in 2013 and 2014.
Site Rates applied Zero N
Yield Full N Yield ∆Y †
RY
PNU0N
Soil N Supply
Year 1-2013 kg N ha-1 Mg ha-1 % kg ha-1
OMAFRA-Elora 0,56,112,168,224 6.99
(0.5) 11.23 (0.8)
4.16 (0.5)
63.7 (2.2)
87 (20)
140 (21)
Hart 0,56,112,168,224 4.33
(0.5) 11.16 (1.0)
7.07 (1.1)
55.6 (5.0)
98 (24)
145 (22)
Rutherford 0,56,112,168,224 6.67
(1.1) 12.79 (0.9)
6.42 (0.5)
55.1 (6.9)
116 (29)
156 (29)
Ilderton 0,56,112,168,224 6.44
(1.6) 11.48 (1.2)
4.89 (2.8)
64.1 (15)
67 (12)
123 (12)
Moorefield 0,56,112,168,224 6.39
(0.9) 10.60 (0.8)
4.11 (1.0)
69.3 (7.0)
126 (13)
189 (14)
Bornholm 0,56,112,168,224 6.78
(0.8) 11.79 (0.2)
4.78 (0.6)
60.3 (7.2)
94 (14)
156 (17)
Year 2-2014
Pinkerton 0,94,134,202 9.03
(0.5) 10.85 (0.5)
1.64 (0.7)
101 (8)
194 (7.3)
269 (23)
Teeswater 0,94,134,202 7.19
(0.5) 11.35 (0.3)
4.10 (0.4)
69.9 (3)
177 (9.0)
202 (11)
Elora Research Station 0,28,57,115,188 4.67
(0.5) 11.98 (0.6)
6.91 (0.5)
42.1 (5)
61 (3.5)
98 (9.2)
AAFC-Woodslee 0,50,100,150,200 4.49
(0.4) 10.08 (0.8)
5.31 (1.1)
42.0 (7)
50 (6.2)
81 (13)
U of G-Ridgetown 0,50,100,150 8.23
(1.0) 12.34 (1.8)
3.05 (2.4)
54.6 (13)
92 (30)
115 (26)
Trent Experimental Farm 0,30,60,120,180 3.7
(1.1) 5.6
(0.4) 1.9
(0.8) 64.62
(17) 69
(18) 86
(15)
AFFC-Ottawa 0,50,100,150 3.5
(1.4) 7.1
(0.9) 3.5
(2.2) 51.95
(24) 55
(15) 58
(14)
Mean # (n=52) 6.46 11.31 4.78 57.61 97 135
SD (n=52) 1.46 0.99 1.58 12.82 7.3 17 †=∆Y=delta yield=(GY optimal N-GY zero N); RY=Relative Yield=[(GY zero N/GY optimal N) x100]; MERN=Maximum economic rate of N; MEY=maximum economic yield; PNU0N= plant N uptake in zero N plots; Soil N Supply=PNU0N + SMNh; ¶=Maximum yield was not reached and therefore MERN could not be accurately calculated # Grand mean and SD
83
Table 3.4. Recommended rate of N fertilizer (MERN and MYRN) for each site in 2013 and 2014 using the quadratic equation based on corn yield response to fertilizer N rates (Figure 3.3), and recommended rate based on the corn N Calculator.
† =quadratic equation based on the response of corn grain yield to fertilizer N rate (Figure 3.3) where y=grain yield and x= N fertilizer rate. MERN=maximum economic rate of N; MEY=maximum economic yield; MYRN= N rate at 95% of the maximum yield; Corn N Calculator= N fertilizer rates based on the Corn N calculator by OMAFRA
Site Category Quadratic Equation† R2 MEY MERN MYRN Corn N
Calculator Year 1 (2013)
kg N ha-1
OMAF-Elora I y = -0.0001x2 + 0.0484x + 6.9917 0.98 11.15 148 109 125 Hart I y = -0.0001x2 + 0.057x + 4.3288 0.98 11.40 212 161 145
Rutherford I y = -7E-05x2 + 0.0423x + 6.6656 ¶ 0.98 13.09 257 177 186 Ilderton I y = -0.0001x2 + 0.0539x + 6.4486 0.99 11.33 158 128 168
Moorefield I y = -0.0001x2 + 0.0427x + 6.3902 0.99 10.50 163 125 149 Bornholm I y = -0.0001x2 + 0.0497x + 6.7754 0.99 11.56 166 141 177
Year 2 (2014)
Pinkerton I y = -7E-05x2 + 0.0232x + 9.0273 0.99 10.66 106 72 121 Teeswater I y = -0.0001x2 + 0.0489x + 7.1913 0.98 11.29 145 110 128
U of G-Elora I y = -0.0004x2 + 0.1016x + 4.6653 0.97 11.57 126 119 153 AAFC-Woodslee I y = -0.0001x2 + 0.0548x + 4.4947 0.98 9.81 172 152 196
U of G-Ridgetown I y = 3E-05x2 + 0.0218x + 8.2316 ¶ 0.93 11.29 160 138 212 Trent Exp. Farm II y = -3E-05x2 + 0.0153x + 3.7596 0.98 5.3 132 138 109 AFFC-Ottawa II y = -7E-05x2 + 0.0347x + 3.5272 ¶ 0.98 7.4 184 123 134
84
Continued…
=MERN=95%RY
=MERN=95%RY
=MERN=95%RY
=MERN=95%RY
=MERN=95%RY
=MERN=95%RY
85
Figure 3.3 Corn grain yield response curves to N applied at all individual experimental corn trials that obtained maximum yield, each point is the mean of replicates (n=4).
3.3.3 Soil test correlation and calibration
Relative yield across sites explained about 70% of the variation in the soil N supply
(Figure 3.4). Correlation coefficients between potential soil N tests and RY are shown in
Table 3.5. For the whole data set, the water extractable fractions WEMN (r=0.60,
P<0.001), WEON (r=0.36, P<0.01) and WSN (r=0.58, P<0.001), and PPNT (r=0.35,
P<0.01) and NH4 (r=0.27, P<0.05) were significantly correlated to RY. The top three
with the highest correlation coefficients are shown in Figure 3.5.
=MERN=95%RY
=MERN=95%RY
=MERN=95%RY
=MERN=95%RY
86
For coarse textured soils (Cs-T; clay< 240 g kg-1), the water extractable fractions:
WEMN (r=0.80, P<0.001), WEON (r=0.50, P<0.01) and WSN (r=0.67, P<0.001), Pool I
(r=0.43, P<0.05), Pool I + SMNp (r=0.49, P<0.05), and POMC: N (r=-0.38, P<0.05)
showed significant correlations with RY. In medium textured soils (Md-T, clay >240 g
kg-1), WEMN (r=0.72, P<0.001), WEOC (r=0.55, P<0.05), WEOC: N (r=-0.55, P<0.05),
POMC (r=0.55, P<0.05) and POMC: N (r=0.60, P<0.05) showed strong correlations
with RY. The top two correlations for Cs-T soils and top three correlations for Md-T soils
are shown in Figure 3.6 and 3.7, respectively.
87
Figure 3.4 Relationship between soil N supply and relative yield (RY) for corn N trials in Ontario in 2013 and 2014 (n=49).
r= 0.69, P<0.001
0
20
40
60
80
100
120
0 50 100 150 200 250 300 350
RY (%
)
Soil N supply (kg ha-1)
88
Table 3.5. Correlation coefficients (r) between soil N tests and RY for 2013 and 2014 Category I field sites field sites for the whole dataset (DS), coarse textured soils (Cs-T soils, clay ≤ 240 g kg-1) and medium textured soils (Md-T soils, clay > 240 g kg-1) RY Parameter Whole
DS Cs-T
(n=28) Md-T (n=14)
Pool I + SMNp 0.16 0.49* -0.28 SMNp 0.11 0.18 0.44 PPNT 0.35** 0.22 -0.10 NH4 -0.27* -0.26 0.44 Pool I 0.17 0.43* -0.53 WEMN 0.60*** 0.80*** 0.72** WEON 0.36** 0.50** 0.07 WSN 0.58*** 0.67*** 0.35 WEOC -0.07 0.52* -0.55* WEOC: N -0.24 -0.11 -0.55* POMC 0.14 0.007 -0.55* POMN 0.15 0.28 -0.48 POMC: N 0.04 -0.38* 0.60*
*=P<0.05, **=P<0.01, ***=P<0.001
89
Figure 3.5. Relationship between relative yield and a PPNT, b WEMN and c WSN for the whole data set (n=49).
r=0.35, P<0.01
0
20
40
60
80
100
120
0 10 20 30
RY (%
)
PPNT (mg kg-1)
r=0.60, P<0.001
0 10 20 30WEMN (mg kg-1)
br=0.58, P<0.001
0 50 100WSN (mg kg-1)
ca
90
Figure 3.6. Relationship between RY and a Pool I + SMNp and b WEMN in Cs-T soils (n=28).
r=0.49, P<0.050
20
40
60
80
100
120
0 100 200 300
RY (%
)
Pool I+SMNp (mg kg-1)
a r=0.80, P<0.001
0 10 20 30WEMN (mg kg-1)
b
91
Figure 3.7. Relationship between RY and a WEMN, b WEOC: N and c POMC: N for Md-T soils (n=21).
r=0.72, P<0.001
0102030405060708090
0 5 10 15
RY (%
)
WEMN (mg kg-1)
a r=-0.55, P<0.05
0 10 20WEOC:N
b r=0.60, P<0.05
0 20 40POMC:N
c
92
Based on the correlation results between RY, MERN and MYRN (Table 3.5 and 3.6), we
chose to perform calibration on PPNT and WSN for the whole data set to predict the
proper rate of N fertilizer. Calibration was chosen for both MERN and MYRN because of
the dependence of MERN on market prices of both fertilizer and corn which can fluctuate
year to year depending on natural gas prices and market demand for corn. In the 2013 and
2014 growing season (Figure 3.3) it is apparent that market prices were not limiting N
fertilizer application rates as rates were consistently greater that MYRN. The PPNT was
chosen because it was moderately related to RY (r=0.35, P<0.01, Table 3.5) and had a
significant inverse linear relationship with MERN (R2=0.47, P<0.001, Table 3.6) and
MYRN (R2=0.56, P<0.00, Table 3.6). For the water extractable fractions, we found that
mineral portion showed the strongest correlation with RY(r=0.60, P<0.001, Table 3.5)
however it could not predict the MERN (r=-0.20, P>0.05, Table 3.6). The total WSN had
a similar relationship with RY as WEMN (r=0.60, P<0.001, Table 3.5) but had a
significant linear inverse relationship with MERN (R2=0.60, P<0.001, Table 3.6), when
the Elora site was removed from the correlation, and with MYRN (R2=0.48, P<0.001,
Table 3.6). The Elora site was removed from the regression with MERN because there
was an inconsistent response between MERN and MYRN, which was not seen for any
other site. This indicates that an error during analysis may have occurred. It was not
considered an outlier but removing the site did improve the relationship. The Pool I +
SMNp in Cs-T soils was also significantly correlated to RY and MERN but not MYRN
(Table 3.5) and therefore was not calibrated. Other parameters that were significantly
correlated to RY but not both MERN and MYRN included POMC and POMC:N.
93
Interpretation tables for PPNT and WSN are presented in Table 3.7. For the PPNT, soil
test levels >18 mg kg-1 were considered High and <2 mg kg-1 were considered Very low.
For WSN, values greater than 70 mg kg-1 were categorized as High and <26 mg kg-1 were
considered Very low. Once these categories were established, calibration curves were
constructed for each of the soil N test parameters in order to assign recommended
fertilizer N rates to each category. The calibration curves were established by correlating
the MERN and MYRN to corresponding soil N test values (Figure 3.8).
Using the calibration curve for PPNT with MERN and MYRN, the concentration of soil
N test varied between 0 and 18 mg kg-1 and N fertilizer rates were calculated using the
derived linear equations MERN=-5.15 (PPNT)+203 and MYRN=-4.72 (PPNT)+167.
Recommended N rates are presented in Table 3.8. Fertilizer rates based on the MYRN
were 28-36 kg N ha-1 lower than MERN depending on the PPNT concentration. The
WSN soil N test values ranged from 0 to 70 mg kg-1 and the MERN and MYRN N
fertilizer recommendations were determined based on the derived linear equation
MERN=-2.02(WSN)+250 and MYRN=-1.65(WSN)+197.1. Rates of N are presented in
Table 3.9. Fertilizer recommendations were between 28 and 57 kg N ha-1 lower for
MYRN than for MERN depending on the WSN concentration.
94
Table 3.6. Correlation coefficients (r) between soil N test parameters and MERN and MYRN for 2013 and 2014 field sites the whole dataset (DS), coarse textured soils (Cs-T soils, clay ≤ 240 g kg-1) and medium textured soils (Md-T soils, clay > 240 g kg-1). MERN MYRN Parameter Whole
DS Cs-T
(n=28) Md-T (n=14)
Whole DS
Cs-T (n=28)
Md-T (n=14)
Pool I + SMNp -0.11 -0.18 -0.12 -0.26 -0.42* 0.73** SMNp -0.45** -0.49** -0.35 -0.45** -0.46* -0.23 PPNT -0.52** -0.54** 0.34 -0.52** -0.53** -0.30 NH4 0.31 0.64*** -0.15 0.27 0.58** -0.60* Pool I 0.07 -0.17 0.16 -0.0008 -0.33 0.72** WEMN -0.30 -0.17 -0.60* -0.53*** -0.47* -0.65* WEON -0.37 -0.42* -0.10 -0.50*** -0.61*** 0.44 WSN -0.51*** -0.51** -0.44 -0.63*** -0.68*** 0.03 WEOC 0.12 -0.12 0.28 0.08 -0.48** 0.97*** WEOC: N 0.38* 0.41* 0.32 0.45** 0.35 0.82*** POMC -0.50** -0.30 0.87*** -0.32* -0.12 0.12 POMN -0.40* -0.28 0.95*** -0.38* -0.27 0.18 POMC: N -0.37* -0.15 -0.91*** -0.11 0.09 -0.35
*=P<0.05, **=P<0.01, ***=P<0.001
95
Table 3.7 Interpretation table for successful soil N tests PPNT and WSN using linear response curves derived from the relationship between RY and soil N test. For PPNT, RY= 3.24(PPNT)+34. For WSN, RY=1.37(WSN)+3.4.
Soil N Test Soil Test Category Soil Test Level
(mg kg-1) Probability of yield increase
PPNT High >18 <5% Optimum 11-18 5-30% Low 2-11 30-60% Very Low <2 >60%
WSN High >70 <5% Optimum 48-70 5-30% Low 26-48 30-60% Very Low <26 >60%
96
Figure 3.8 Relationship between a PPNT and MERN b PPNT and MYRN c WSN and MERN and d WSN and MYRN; x=Elora site.
y = -5.16x + 203R² = 0.47, P<0.01, n=35
0
50
100
150
200
250
0 10 20 30
MER
N (k
g ha
-1)
PPNT (mg kg-1)
ay = -4.72x + 167
R² = 0.56, P<0.0001, n=350
20
40
60
80
100
120
140
160
180
0 10 20 30
MY
RN
(kg
N h
a-1)
PPNT (mg kg-1)
b
y = -2.02x + 250R² = 0.60, P<0.001, n=35
0
50
100
150
200
250
0 20 40 60 80
MER
N (k
g ha
-1)
WSN (mg kg-1)
cy = -1.65x + 197
R² = 0.48, P<0.0001, n=350
20
40
60
80
100
120
140
160
180
0 20 40 60 80
MY
RN
(kg
N h
a-1)
WSN (mg kg-1)
d
97
Table 3.8 Recommended N fertilizer rates based on the linear relationship between PPNT concentration and MERN (MERN=-5.16 (PPNT)+203; R2 =0.47) and MYRN (MYRN=-4.72(PPNT)+167; R2=0.56).
Soil Test Level (mg kg-1)
MERN Fertilizer Rate
(kg N ha-1)
MYRN Fertilizer rate
(kg N ha-1) >18 110 82 17 115 87 16 120 91 15 126 96 14 131 101 13 136 106 12 141 110 11 146 116 10 151 120 9 157 124 8 162 129 7 167 134 6 172 139 5 177 143 4 182 148 3 188 153 2 193 158 1 198 162 0 203 167
98
Table 3.9. Recommended N fertilizer rate based on the linear relationship between WSN concentrations and MERN (MERN=-2.02(WSN)+250; R2=0.60) and MYRN (MYRN=-1.65(WSN)+197; R2=0.48).
Soil Test Level (mg kg-1)
MERN Fertilizer Rate
(kg N ha-1)
95% RY Fertilizer rate
(kg N ha-1) >70 110 82 65 120 90 60 130 98 55 140 106 50 150 114 45 160 123 40 170 131 35 180 139 30 190 147 25 200 156 20 210 164 15 220 172 10 230 181 5 240 189 0 250 197
99
3.4 Discussion
3.4.1 Soil N test parameters
The relatively high SMNp values (majority in NO3 form) compared with the long-term
data reported by OMAFRA (11 mg kg-1) is consistent with the warmer temperatures and
lower than normal May rainfall in 2014 (Environment Canada, 2014). These
environmental conditions in 2014 resulted in less NO3 leaching and high nitrification
rates.
The high range in percentage of the soil N supply that Pool I represented indicates that
the selected soils have contrasting sizes of the readily mineralizable N pool. The average
Pool I values observed in this year of the study were close to the average observed in
Chapter 2 of this study (42 mg kg-1) indicating that the Pool I is a relatively stable pool of
N. For the WSN fraction was found that the mineral portion represented on average 22%
of the WSN, which is similar to values found for soils under unfertilized corn
monoculture (15%) and soils under a corn soybean rotation receiving mineral fertilizer
(20%; Gregorich et al., 2003). The relationship between WEOC and WEON (R2=0.49)
was weaker than observed for soils under corn monoculture in Quebec, Canada (R2=0.69,
Curtin and Wright, 2006) which can be attributed to the uncoupling of C to N in fertilized
system, as an increase in N is not accompanied by an increase in C (McDowell, 2003),
resulting in a lower C: N ratio of WEOM. The high percentage of total N as WEON
indicates that these soils are high in soluble N compounds (Murphy et al., 2000; Wander,
2004). The WEON fraction also represented a considerably higher percentage of total N
100
compared with the literature (0.75%, Chantigny, 2003) indicating that this fraction may
be an important source of N for field crops at the selected sites in Ontario.
The wide range found in POMC and POMN may be due to the differences in
management practices and location as these factors have a direct influence on the size of
this fraction (Griffin and Porter, 2004; Haynes, 2005). The POMN values were lowest
where fields were under continuous corn (AAFC-Ottawa and Woodslee and Elora
Research Station). The proportion of TN as POMN (8%) was within the range reported in
the literature (Gregorich et al., 2006, Sharifi et al., 2007) and the high C: N ratio of this
fraction is characteristic of soils receiving plant residues with high C: N ratios as the only
source of POM (Sequeira and Alley, 2011; St.Luce et al., 2011).
3.4.2 Field Indicators of corn N availability
A wide range of soil N supply was observed across experimental sites indicating that
selected soils have varying productivity. A wide range in residual N at harvest (SMNh)
was also observed across sites but there was no consistent pattern across sites. Previous
research has attributed the amount of residual mineral N to differences in soil moisture
regime (Jokela and Randall, 1989) and management practices (Rasouli et al., 2014). The
MERN and MYRN values obtained in this study (72-212 kg N ha-1) are in the same range
reported by OMAFRA (107-237 kg N ha-1; 2013) for corn response trials in southwestern
Ontario in 2013. The high recommended N rates (>72 kg N ha-1) indicates that none of
our sites had enough N to fully supply corn demand but may also be due to the high N
requirement of high yielding corn varieties. Typically soil test calibration studies require
101
inclusion of both responsive and unresponsive soils to accurately predict critical soil test
levels (Dahnke and Olsen, 1990). Therefore, we could not accurately determine the
critical level for any of our soil N tests.
The high variability of response of sites to N application and inconsistencies between the
corn N calculator, and MERN and MYRN indicates that other variables that are not
included in the corn N calculator may affect the N rate. These include management
history (instead of previous crop, St.Luce et al., 2011), soil properties such as pH,
electrical conductivity (EC), cation exchange capacity (CEC) and total C and total N
(Dharmakeerthi et al., 2005; Subbarao et al., 2006). The pH in this study varied from 6.6
to 8.1, total C from 14 to 35 g kg-1 and total N from 1.3 to 2.8 g kg-1 (Table 3.1). The pH
has an influence on the activity of the microbes, the optimal range is between 6 and 7
(Hartel, 2005), and also plays a role in nutrient availability to plants (Havlin et al., 2005).
The higher pH (8.1) at the Trent Experimental Farm suggests that mineralization and
availability of nutrients was decreased at this site, which is reflected in the soil N supply
(Table 3.3) even though it had relatively high total C and total N. On the other hand, the
AAFC Ottawa site had optimal pH but low total C and total N, which may explain its low
soil N supply. The total C and total N in the soil is a reflection of the SOM content,
which holds the substrate for N mineralization (Ros et al., 2015). Ros et al. (2015) found
that the ability of the soil to supply N (measured as N uptake in an unfertilized mineral
grassland) was a direct reflection of the SOM levels in the soil. For the remainder of the
sites, Pinkerton and Teeswater soils had a near neutral pH (7.4-7.9), had the highest total
C and total N and had the highest soil N supply (202-269 kg N ha-1). These results
102
indicate that soil properties such as pH, total C and total N play an important role in N
availability to crops and should be taken into consideration when evaluating the
productivity of a soil.
The inconsistencies between the corn N calculator and MERN and MYRN indicates that
providing recommendations based on yield potential and ignoring the wide range of soil
N supply capacity of Ontario soils can result in substantial over or under fertilization and
is therefore not a sound indicator of crop N needs (Kachanoski et al., 1996; Davis et al.,
1996, O’Halloran et al., 2004).
3.4.3 Soil N test correlation and calibration
The WSN was more strongly correlated to RY than PPNT (r=0.58 and r=0.35,
respectively) and explained 13% more variability in MERN (Figure 3.8). We observed
that the NO3 at planting was an important contributor to corn N uptake as it represented
up to 30% of the soil N supply and was significantly related to RY, MERN, MYRN. This
result is inconsistent with the results from Chapter 2 of this study where PPNT was not a
good predictor of corn response to fertilizer addition. The PPNT has already been
calibrated for corn in Ontario (OMAFRA, 2009) but many producers have shifted
towards use of expected yields or visual observations of N deficiency/sufficiency to
predict their fertilizer N rates (O’Halloran et al., 2004)) even though this method results
in a greater degree of under or over fertilization.
103
The movement away from the PPNT may be due to the large in-field variability requiring
a high number of samples to be taken per field as O’Halloran et al. (2004) found that soil
NO3 was not a good predictor of yield or yield response of corn at two sites in
southwestern Ontario when sampling was done on in 3x10 m grids. Furthermore, soil
NO3 has shown varying success as a predictor of N availability as it is dependent on early
season climatic conditions resulting in the potential for substantial losses due to leaching
between time of sample collection and planting (Sharifi et al., 2009). In this study, the
PPNT concentrations in 2014 were on average twice as high (16.7 mg kg-1) compared
concentrations observed in 2013 (6.89 mg kg-1), which corresponds with the lower
average rainfall in 2014 (453 mm) compared to 2013 (546 mm).
The success of the WSN as a soil N test in this study indicates that both the mineral and
organic portion of the water extractable fraction are important when determining the
availability of N to crops. Literature on the WSN is rare as most studies report only the
organic portion (Curtin et al., 2006; Haney et al., 2012; St. Luce et al., 2014). As WSN is
hypothesized to be the product of microbial decomposition of crop residues and organic
amendments and is a mobile and available pool of organic N present in soil solution
(Murphy et al., 2000; Chantigny, 2003), these results suggest that the combination of
both the organic and mineral portion is important when determining availability of N
from soluble fractions. The wide range in proportion of WSN as WEMN (7-30%)
observed in this study may indicate that the mineral portion is an indicator of
mineralization conditions and can better explain the variability in yield response across
sites than WEON alone.
104
The composition and therefore biodegradability will ultimately determine the fate of
soluble organic N as this pool can also be composed of recalcitrant compounds that are
resistant to further microbial decomposition (Smolander et al., 1995; Gregorich, 2003;
Wander, 2004). Furthermore, it is apparent that the WSN is more consistent across years
compared to the PPNT and therefore a more reliable index of plant available N. In this
year of this study, the WSN was on average 43 mg kg-1 and ranged between 30 and 65 mg
kg-1, which is within the range observed in 2013 (27-89 mg kg-1) and close to the average
(42 mg kg-1). The only limitation to this procedure is the marginal increase in cost and
labour associated with determining organic N (persulfate oxidation and subsequent
mineral N analysis) compared to the 2M KCl extraction used to determine NO3.
3.5 Conclusion
The PPNT and WSN in the soil prior to planting and fertilizer application were the most
successful at predicting N fertilizer recommendations for corn in southwestern Ontario
for the 2013 and 2014 growing seasons. The soils N tests successfully detected the N
deficiency in fields under conventional corn production receiving primarily mineral
fertilizer as an N source. The corn N calculator was not successful in predicting the
MERN and MYRN and therefore is not an accurate indicator of crop N needs. For the
PPNT, the proposed model MERN=-5.16 (PPNT)+203 and MYRN (MYRN=-
4.72(PPNT)+167 could successfully determine fertilizer N rate when concentrations are
below 18 mg kg-1. Although both the PPNT and WSN showed promising results in this
chapter, they did not in Chapter 2. This supports the dependence on soil N on climatic
105
factors that result in inconsistencies as previously observed in other studies. The model
we have proposed for WSN, MERN=-2.02(WSN)+250 and MYRN (MYRN=-
1.65(WSN)+197 can easily be used for N fertilizer application rates when WSN is below
70 mg kg-1. An increase in WSN of 5 mg kg-1 decreases fertilizer N requirement for corn
by 8 kg N ha-1. None of our sites were unresponsive to N and therefore we could not
determine the critical soil N test for PPNT or WSN. Further research is required to test
the suitability for each of the proposed linear models over a greater spatial and temporal
area.
106
GENERAL CONCLUSIONS
This study attempted to improve fertilizer N recommendations for corn in Ontario by
considering the soil mineralizable N. Quantifying the potential for Ontario soils to supply
N using the long-term aerobic incubation showed that Ontario soils have a wide range in
soil N supply capacity and have the potential to meet corn N demand. It was also found
that the higher total C and N content in soils, the higher the potential of the soil to supply
N. It is suggested that management practices should be focused on increasing the organic
N pool within the soil, which will decrease the dependence on mineral N fertilizer to
supplement crop N needs. The mineral N in the soil at planting (SMNp) contributed
approx. 30% of the soil N supply throughout the growing season indicating that
mineralization of SON is an important contributor to the soil N supply in humid
temperate climates.
This study was executed over two years and separated into two growing seasons. In the
first year, the current pre-plant N test was found to be a poor predictor of crop N
availability and the readily mineralizable N, Pool I, was a more robust predictor of crop
N availability (RY). Grouping soils based on soil texture also revealed relationships
dependent on clay content indicating that this parameter should be taken into account
when determining crop available N.
When combing data from the first and second growing seasons, it was found that the
PPNT and WSN at planting were successful at predicting fertilizer N rates. The success
of both PPNT and WSN only when combing the two years of data supports the inherent
107
variability of soil N as it is dependent on early season climate. Future research should
focus on readily available pools of N (such as WSN) as indicators of N availability under
field conditions.
Finally, the large variability in soil chemical and physical properties observed in this
study, their influence on soil N mineralization and the inability of the corn N calculator to
successfully predict actual N fertilizer needs indicates that providing recommendations
based on expected yield can result in substantial over or under fertilization. Soil
properties such as pH, total C and total N should also be taken into consideration when
providing recommendation as this will result in more environmentally and economically
sound recommendations.
108
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APPENDIX
Appendix I. Relationship between RY and WEON in Cs-T soils
Appendix II. Relationship between WEON in Md-T soils
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Appendix III. Relationship between RY and WEOC:N in Cs-T soils
Appendix IV. Relationship between RY and WEOC:N for the whole dataset
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Appendix V. Relationship between RY and POMC for the whole dataset
Appendix VI. Relationship between RY and POMC in Md-T soils
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Appendix VII. Relationship between RY and POMC in Cs-T soils
Appendix VIII. Relationship between RY and POMC:N in Cs-T soils
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Appendix IX. Relationship between RY and POMC:N for the whole data set.
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