nitrogen dynamics in volcanic soils under permanent pasture
TRANSCRIPT
Geoderma 160 (2011) 384–393
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Geoderma
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Nitrogen dynamics in volcanic soils under permanent pasture
J.C. Fontes a, M.R. Cameira b,⁎, L.G. Borba a, E.D. Amado a, L.S. Pereira b
a University of Azores, 9700 Angra do Heroísmo, Portugalb Biossystems Engineering Research Center, Institute of Agronomy, Technical University of Lisbon, Tapada da Ajuda, 1349-017, Portugal
⁎ Corresponding author. Instituto Superior de Agron017, Lisbon, Portugal. Tel.: +351 21 3653399.
E-mail address: [email protected] (M.R. Cameira
0016-7061/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.geoderma.2010.10.008
a b s t r a c t
a r t i c l e i n f oArticle history:Received 8 February 2010Received in revised form 11 October 2010Accepted 12 October 2010Available online 12 November 2010
Keywords:AzoresAndosolsGrasslandsN mineralization rateMinilysimetersLaboratory incubationN losses
In Terceira Island (Azores, Portugal), 87% of the agricultural land is used for permanent pasture underperiodical grazing. The edaphoclimatic conditions promote nitrate leaching into the groundwater and runoff,which carries sediments and fertilizers into the surface waters. Of particular interest is the fact that volcanicAndosols are rarely study to analyze nitrogen dynamics. The soil capacity to provide mineral N from itsorganic matter pool throughmineralizationwas estimated using twomethods: in laboratory under controlledmoisture and temperature conditions; and in natural conditions using in situ minilysimeters. The direct Ninput through animal's excreta was determined by a field mass balance. It was observed that mineralizationand animal excreta contributed respectively with 160 and 65 kg mineral N ha−1 for the N budget. N losses byleaching and runoff amount respectively for 89±18 kg ha−1 and 0.5±0.08 kg ha−1, representing 53% of thefertilizer inputs. To control such losses the fertilizer amount should be recalculated considering the directinput through the animal excreta and the mineral N input from the net mineralization process. In addition,due to the random characteristics of the rainfall events, the fertilization should be split into two or moreapplications, hence reducing the amounts of mineral N available for leaching. Further studies may bedeveloped using a modeling approach to allow the fertilizer management optimization.
omia, Tapada da Ajuda, 1349-
).
ll rights reserved.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
In Terceira Island, Azores, 87% of the agricultural land is used forpermanent grazing pasture, mainly ryegrass (Lolium perenne) andclover (Trifolium repens). Nitrogen (N) fluxes in grazed grasslandsdiffer greatly from those of other agricultural land uses due to theperiodical presence of the grazing animals. The removal of N from thefield is often less than for other cropping systems and there issubstantial recycling of N in the soil–crop system due to animalexcreta (Hutchings et al, 2007). The presence of grazing animals alsocontributes to an increase in spatial heterogeneity in the cycling of N.This is primarily due to the deposition of excreta in patches ratherthan evenly over the field (West et al., 1989; Haynes and Williams,1999; Hutchings et al., 2007). This non uniform distribution maymodify soil chemical and physical attributes such as soil organic C andN, soluble salts and soil pH (Haynes and Williams, 1999; Franzlueb-bers et al., 2000) and creates technical and logistical problems thatmake the field investigation of N dynamics difficult (Velthof andOenema, 1995; Anger, 2002). Within cattle urine and dung patches,the N deposited in a single event is typically equivalent to valuesranging from 200 to 2000 kg N ha−1 (ten Berge et al., 2002),depending upon the grass quality. As a result, the soils in these
agricultural systems have high organic matter (OM) contents in thesurface layer, which can contribute, after mineralization, as animportant input to satisfy the crop needs in nitrogen. Also, nitrate(NO3
−) leaching and nitrous oxide (N2O) emissions are known to behigh from urine and dung patches in grazed pastures (Iyyemperumalet al., 2007).
The climatic conditions in the Terceira Island (mild oceanicclimate), are characterized by frequent rainfall events and mildtemperatures. The latter vary within a narrow annual range andpromote a high OM mineralization rate. The soils are Andosols, moreoften Ferric and Haplic Andosols (FAO, 1998) of recent volcanic origincontaining allophane and imogolite. These constituents conferunusual properties to these soils, including very low bulk density,high values of water holding capacity, total porosity and hydraulicconductivity. Allophane refers to a group of clay-size mineralscontaining silica, alumina and chemically bound water (Parfitt,1990; Tan, 2000). The high porosity and water retention are thoughtto result from the abundance of inter and intra-particle pores ofallophane (Quantin, 1985). The hydraulic properties favor deeppercolation of the excess rainfall water and, consequently, theleaching of agrochemicals below the root zone (0–40 cm) and intothe groundwater (Fontes et al., 2004a,b). Because land is very steep,the high intensity rainfall events originate runoff that carriessediments and agrochemicals into the surface waters (Fontes et al.,2004b). Therefore, this agricultural system has a considerablepotential for the contamination of surface and ground waters withagrochemicals.
385J.C. Fontes et al. / Geoderma 160 (2011) 384–393
Nutrient enrichment of surface waters (primarily nitrogen andphosphorus) is undesirable both as a result of changes in thefreshwater and related eutrophication, with negative impacts ondrinking water supplies. Groundwater contamination with nitrateshas led to the delimitation of vulnerable zones to water pollution fromnitrogen compounds. In these areas the aquifer presents nitrate(NO3
−) concentrations higher that 50 mg L−1 or nitrate-nitrogen(N-NO3) concentrations higher that 11 mg L−1. These concentrationshave been related to health issues e.g. methaemoglobinaemia (bluebaby syndrome) (Heathwaite et al., 1993).
Recently, attention has focused on nitrogen-prevention measuresas a result of the EC Nitrate Directive (91/676), which insists thatnitrogen control should be by prevention at source and givesrecommendations for changes in agricultural land use. The keychanges include the accurate determinations of the crop nitrogenrequirements, especially in those cases when other sources ofnitrogen exist, like in the direct grazing pasture systems.
Eight vulnerable zones have been already identified in the AzoresIslands. In Terceira there is not yet any vulnerable zone but there is theneed to develop an appropriate strategy to manage the permanentgrazing pastures. This strategy will prevent future problems in surfaceand ground waters used as drinking water supplies. This strategymust be based upon preventive measures and couple nitratescontamination control with the economics of dairy farming systems(Dentinho et al., 2008). In these systems the only water input israinfall. Because rainfall events are random and their impacts upondeep percolation cannot be controlled, the nitrate storage in the rootzone must be, at each time, minimized in order to reduce the leachingpotential. This requires the accurate determination of crop N needsthroughout the year, the estimation of the soil capacity to providemineral N from its organic matter pool (mineralization rate) and theestimation of the ammonium input into the system by animal excreta.
Aiming to support the development of the previously referredstrategy, the objectives of this study were:
(1) Toanalyze theNdynamics involcanic soils of Terceira Island, underpermanent pasture and periodic grazing. The following N budgetterms were addressed: i) the soil capability to provide mineral Nfrom its organic matter pool (soil net mineralization rate); ii)thedirect inputs of ammonium through the grazing animals' excreta;and iii) the losses of mineral N by leaching and runoff;
(2) To identify potential measures to reduce the mineral N lossesand to contribute to the sustainability of the system.
Table 1Summary of the experiments performed in this work.
Period Methods Objectives
Laboratory experimentMarch to October2002
Incubation method undercontrolled conditions
Estimate the net minof the soil in optimalconditions for temper
In situ experimentsMarch to October2002
Minilysimeters in naturalfield conditions (bare soil)
Estimate the net minpreserving natural sowater dynamics of th
March to August2002
Field monitoring in naturalpasture conditions with periodicaldirect grazing and a fertilization event
Estimation of ammonanimal excreta; N losN losses with runoff
This lies within the scope of the general conclusions resulting fromthe concerted research action “Soil Resources of European VolcanicSystems” implemented by the European Union COST Program(Óskarsson and Arnalds, 2004). They state that Andosols have specialproperties that make them an important and unique natural resourceand that in Europe most of the Andosol areas are subjected toincreasing pressure by humans, such as urban expansion andchemical pollution.
2. Material and methods
Three sets of experiments were designed (Table 1) in order toestimate different terms of the nitrogen budget. Two sets weredesigned with the common objective of estimating the net mineral-ization rate of the soil. The third one had the objective of providinginformation about the N dynamics in field conditions, necessary forthe quantification of the ammonium inputs through the grazinganimal's excreta and the N losses by leaching and runoff.
2.1. Experimental site
The experiments were carried out in a small watershed called Granja(Fig. 1), located in the Island of Terceira, Azores (38º 41´ 55´´ N, 27º 10´ 14´´W). The watershed outlet is 380 m above the sea level. The catchment'sarea is3500m2,withanaverage slopeof 9%.During theexperimental year(2002) rainfall was 1766 mm (average of 15 years=1510 mm), totalrunoff was 80 mm (average of 15 years=47mm), the average airtemperature was 20 °C (average of 15 years=19.4 °C) during thesummer and 12 °C during winter (average of 15 years=11.7 °C). Fig. 2presents daily maximum and minimum air temperatures and rainfall forthe experimental year.
The soil is classified as a Ferric Andosol (FAO, 1998). Andosols arevolcanic ash soils, generally rich in OM and containing allophane andimogolite (Tan, 2000). Basic soil physical and hydraulic properties arepresented in Table 2 (Fontes et al., 2004a), reflecting the characteristicsof Andosols with allophanes. The water contents at saturation (θs)(h=0 kPa), field capacity (θFC) (h=100 kPa) and wilting point (θWP)(h=1500 kPa) aremuch greater than those currently expected for soilswith similar textural characteristics. The soil has very low bulk density(0.45 to 0.78) and very high total porosity, which explains the highsaturated hydraulic conductivity (19 to 38 mm h−1). Selected chemicalproperties of the soil are presented in Table 3. The profile presentsconsiderably high OM contents, decreasing with depth. C/N values are
Sampling strategies
eralization rateand controlledature and moisture
•Soil samples collected randomly in the field and putinto 51 incubation jars;•2 soil layers sampled;•3 replications per soil layer;•17 sampling dates.
eralization rateil temperature ande field
•9 minilysimeters installed in the field;•2 soil layers covered;•3 replications per soil layer;•13 sampling dates.
ium inputs throughses by leaching and
Nitrogen in soil:•samples collected over the watershed in a zigzag pattern;•4 soil layers sampled ;•9 replications per soil layer;•5 sampling dates; soil water: (neutron probe)•measurements at 8 depths;•9 replications for each depth;•13 measurement dates; Nitrogen in runoff:•samples collected after each runoff discharge
Fig. 1. Granja Watershed with the location of some experimental equipment, the soilsampling pattern and the neutron probe access tubes for soil water monitoring.
386 J.C. Fontes et al. / Geoderma 160 (2011) 384–393
small/medium, indicating a low potential for N immobilization by thesoil microorganisms. The soil is moderately acid, with pH valuesbetween 5 and 5.8.
The plot has beenmanaged under conditions of permanent grazingnon irrigated pasture, mainly L. perenne (ryegrass) and T. repens(clover), for at least 20 years. Main characteristics of these pasturesare given by Rego and Davies (1968). The average dry matter yield isabout 2700 kg ha−1 for a two-month grazing cycle and for an input of150 units of N. During the 2002 experimental year, the pasture wasgrazed with 65 feeder animals that remained in the plot for one and ahalf day on April 15th (DoY 105), May 26th (DoY 146) and July 18th(DoY 199). On May 27th, a fertilizer treatment was performed withthe application of 40 kg NH4
+ ha−1 and 130 kg NO3− ha−1
30 daily max. temperature
25
20
15
10
5
0
air
tem
per
atu
re C
150 200
rainfall
250
day of
Fig. 2. Maximum and minimum daily air temperatures and rainfall measured in t
(30 kg ha−1 N-NH4 and 30 kg ha−1 N-NO3). This application followedthe current farming practices in the area.
2.2. Experimental procedures to estimate the net mineralization rate ofthe soil
Due to the large amounts of OM in the soil resulting from theanimal excreta, it was necessary to estimate the net mineralizationrate of the soil, which is an important input for the N budget. Twomethods were used, a laboratory incubation method and an in situmethod. Since mineralization is performed by heterotrophic micro-organisms which use OM as substrate, it occurs mainly in the surfacelayers. For this reason the sampling depthswere confined to the depthof 30 cm. The Mann–Whitney statistical test was used to compare themean values obtained from both laboratory and in situ experiments.
The laboratory anaerobic incubation method was used to deter-mine the potential net mineralization rate, as the experiment takesplace under optimum and controlled environmental conditions(Nicolardot and Molina, 1994). This way the mineral N thatpotentially can be provided by the organic pool due to microorgan-isms' activity can be estimated. The optimal environmental conditionsfor this process correspond to temperatures ranging from 25 to 30 °Cand soil moisture contents between field capacity and 40% of its value(Harmsen and Kolenbrander, 1965). In this study, a 204-dayincubation was conducted. Soil samples were collected in theexperimental field, in several places at random and at the depths of0 to 10 cm and 10 to 20 cm. Samples referring to the same depth weremixed in order to get a large composite sample. This was divided into51 incubation jars, each with a capacity of one liter. The jars were halffiled with soil presenting a θ of 0.35 cm3cm−3 (80% θFC) and placedinto the incubator at 25 °C, which was the maximum temperaturemeasured in the soil in natural conditions. The jars were then sealedto create anaerobic conditions. The soil inside the jars washomogenized every two days to even moisture and temperatureinside each sample. Sample collection was performed every 12 days,resulting in 17 collection dates. In each date, three jars for each depthwere opened (three replications), the samples were collected andanalyzed for: Total N-Kjeldhal method (wet-oxidation) (McGill andFigueiredo, 1993); N-NO3, N-NH4-ion chromatography method andN-NO2-colorimetric method (Clesceri et al., 1998) ; OM-organic Cdetermined by the Walkley–Black method (Nelson and Sommers,1982), and converted to OM bymultiplying the organic C by a factor of1.724; Organic N was calculated by subtracting mineral N to Total N.Each jar was opened once and not reutilized.
Net mineralization rates (mg N kg−1 soil) were calculated for allthe 12 day periods as the difference between the mineral N (N-NO2+
daily min. temperature
rain
fall
(mm
)
year300 350
0
30
90
120
60
he weather station located by the watershed outlet during the year of 2002.
Table 2Selected physical properties of the soil in the experimental field (Ferric Andosol).
Depth(cm)
Coarsematerial
Particle size (%) BulkDensity
ϕ(m 3 m−3)
θ (m3 m−3) h (k Pa) Ks(mm h−1)
Sand Silt Clay(%) 0 1500 100
00–10 8.8 51.3 35.3 13.4 0.78 0.70 0.67 0.25 0.44 19.110–20 2.9 70.7 21.1 8.2 0.78 0.72 0.67 0.25 0.44 19.120–40 3.9 91.3 3.3 5.4 0.71 0.71 0.62 0.27 0.41 37.540–60 0.0 91.0 3.6 5.4 0.52 0.75 0.73 0.52 0.65 33.360–85 0.9 89.2 3.6 7.2 0.55 0.81 0.78 0.60 0.72 5.485–100 0 81.7 16.0 2.3 0.45 0.78 0.7 0.17 0.35 20.8
ϕ—porosity; θ—moisture content; h—matric potential (kPa); Ks—Saturated hydraulic conductivity (Fontes et al., 2004a).
387J.C. Fontes et al. / Geoderma 160 (2011) 384–393
N-NO3+N-NH4) at the end and at the beginning of each period,divided by the period duration in days (simple mass balanceapproach). Final estimates of the net mineralization rates wereobtained by averaging these values for the 0–10 and 10–20 cm layers.
The in situ experiment was used to mimic the field naturalconditions. The preservation of the natural soil water dynamics is acritical factor when attempting to reproduce field conditions with insitu containment devices, as shown by Hanselman et al. (2004). Toachieve these conditions the in situ method was based in mini-lysimeters, thus allowing the natural inflow of rainfall and the outflowof drainage water that was collected in an appropriate recipientlocated at the bottom. The technique was applied during the periodbetween March and October 2002 (207 days). The experiment wasperformed in bare soil. The objective was to estimate for short periodsof time the amounts of mineral N originated from the organic pool byusing a mass balance approach. The method used is similar to the onedescribed by Hatch et al. (1990, 1991) in the way that N-NH4, N-NO2
and N-NO3 are all considered. The minilysimeters used in thisexperiment were PVC cylinders, 20 cm in diameter by 30 cm inheight. Nine minilysimeters were carefully installed in the groundwithout disturbing the soil inside of them. Then, a screen was put atthe bottom of the cylinder to prevent soil losses. A funnel was placedunderneath, attached to a plastic tubing to collect the leachate in areservoir. The original soil was then relocated around the cylinders.The surface was protected in order to prevent nitrogen inputs, butallowing rainfall inputs. No runoff was allowed.
Each minilysimeter provided for two collection dates and a total offour samples. Soil sampleswere collectedevery 15 days, fromMarch8th(DoY 67) to October 2nd (DoY 275), resulting in 13 sampling dates.From each minilysimeter two samples were collected at each time,representing the depths 0 to 10 cm and 10 to 20 cm, and analyzed forN-NO3, N-NH4 using the ion chromatographymethod and N-NO2 usingthe colorimetricmethod (Clesceri et al., 1998). Leachateswere collectedafter each rainfall event. The volumes were measured and the solutionwas analyzed for N-NO3, N-NO2 and N-NH4 using the laboratorymethods referred previously. Samples from rainfall were also taken andanalyzed for mineral N using the previously referred methods. Threeminilysimeters were used in each date, in order to produce threereplications at least in half of the sampling dates.
The N budget equation was simplified for the experimentconditions. This means that terms like N extraction by the crop andN inputs through the fertilizer and/or manure were not considered
Table 3Selected chemical properties of the soil in the experimental field (Ferric Andosol).
Depth (cm) OM (%) C/N pH (H2O)
0–10 10.0 21.0 5.010–20 8.9 16.0 5.520–30 5.0 17.0 5.880–90 0.6 – 5.8
OM = organic matter.
since they were inexistent. The equation was rewritten in order topresent net mineralization as the dependent variable, producingEq. (1). This equation was applied for the periods between samplecollections:
Net minði;i+1Þ = N−NH4 ði+1Þ−N−NH4ðiÞh i
+ N−NO3 ði+1Þ−N−NO3ðiÞh i
+ LNði;i+1Þ−RNði;i+1Þ
ð1Þ
whereNetmin(i, i+1) is the net mineralization in the period from time ito i+1; N−NH4(i) and N−NO3(i)are respectively the ammonium andnitrate storages in the soil at the time i,LN(i, i+1) and RN(i, i+1) arerespectively the amount of mineral N in the leachate and in therainfall during the referred period, i to i+1. All terms are expressed inmg kg−1.
Soil temperature at the depth of 15 cm was continually measuredduring the experiment. Gravimetric soil moisture was determined byoven drying at 105 °C for 48 h.
2.3. Analysis of N dynamics in natural conditions of the grazed pasture
The second in situ experiment was performed in natural nonirrigated pasture conditions with periodical direct grazing, betweenMarch and August 2002. The objective of this experiment was to (a)estimate mineral N losses by leaching; (b) estimate mineral N losseswith the runoff; and (c) estimate the amount of N-NH4 left in the rootzone by the animals after each grazing. During six months soilmoisture contents, soil mineral N concentrations, and runoff weremonitored in the field. A nitrogen fertilizer application was performedon May 27th.
N leaching is defined as the convective transport of nitrate (watersoluble and non adsorbed anion) to layers below the root zone. This Ngoes beyond the range of the roots, and potentially can reach theground water. Therefore there is the need to sample the soil layerbelow the grass root zone (40–90) in order to follow N movement.
Because animal excreta are heterogeneously distributed in thefield, the sampling points followed a zigzag pattern to ensure thatthey represented the entire field. Soil samples were collected at thedepths of 0–10 cm, 10–20 cm, 30–40 cm and 80–90 cm (with ninereplications) to determine N-NH4 and N-N03 concentrations. Thesampling frequency was every other week. An extra sampling wasperformed three days after the fertilizer application. After collection,the soil samples were stored in plastic bags and frozen until analysis.Soil water contents were measured approximately every 10 days, atthe depths of 10, 15, 20, 30, 40, 60, 80 and 100 cm, using a neutronprobe previously calibrated for this soil. At each time, nine replicatemeasurements were performed for each soil layer.
In the basin outlet (Fig. 1) a V-notch weir was installed formeasuring runoff discharges using a sensor connected to a digitalrecorder. Runoff volumes were measured in sedimentation tanks,whichwere fed after fractionating the runoff flow. The tankswere alsoused for water and sediment sampling. More details about the
388 J.C. Fontes et al. / Geoderma 160 (2011) 384–393
weather and runoff measuring facilities are given in Fontes et al.(2004b). During each runoff discharge, water samples were collectedin plastic bottles and kept refrigerated for laboratory determinationsof mineral N (N-NH4 and N-NO3).
3. Results
3.1. Laboratory incubation method
Before incubation, the soil from the first layer (0–10 cm) presentedan OM content of 9% and a C/N of 19. For the second layer (10–20 cm),OM was 5% and the C/N was 22. This results are in the range of theones obtained when the general characterization of the soil wasperformed (Table 3), being the small differences due to spatialvariability. The medium/low C/N values indicate that immobilizationwas not to be expected since microbes had enough N in the organicsubstrate. The values also indicate the potential for the release of Nright from the beginning of OM turnover (Heathwaite et al., 1993).During the 204-d experiment N-NH4 concentrations ranged from 0.5to 11.1 mg kg−1 for the 1st layer and from 0.5 to 9.2 mg kg−1 for the2nd layer. Fig. 3 shows an evolution pattern that consists ofalternating increases and decreases in N-NH4 concentration, resultingfrom the mineralization process followed by nitrification (average ofthree replications and standard deviation, SD). N-NO2 contents werealways very low and are not shown in the figure although they wereused for the calculations. N-NO3 concentrations showed significantand almost linear increases, from 73.5 mg kg−1 to 649.2 mg kg−1 inthe 1st layer and from 31.5 mg kg−1 to 121.9 mg kg−1 in the 2ndlayer. Fig. 4 shows the evolution of the netmineralization rate for bothsoil layers, calculated for the 16 incubation periods using a simplemass balance approach. The results show time fluctuation, which issmaller than the average SD of the rates calculated from measuredvalues. This means that an average rate can be used for the entireperiod. For the surface layer, the net mineralization rate wasestimated to be 3±1.3 mg Nkg−1 d−1. For the 10–20 cm layer therate was much smaller: 0.4±0.05 mg Nkg−1 d−1, likely due to thelower OM content (the remaining conditions were kept the same forall the incubation jars). An average value of 1.7 mg Nkg−1 d−1 will beconsidered for the soil layer 0–20 cm. These rates were estimated foroptimal conditions of temperature and soil moisture (25 °C and 80%
800
0 - 10 cm600
400
200
00 20 40 60 80 100
800
600
400
200
00 20 40 60 80 100
days after beginni
N-NO3
N-NH4
N-N
O3
(mg
kg
-1)
N-N
O3
(mg
kg
-1)
Fig. 3. Evolution of N-NH4 and N-NO3− during the laboratory incu
θFC), therefore they correspond to the maximum rates that can occurin the field under natural temperature and moisture conditions.
3.2. In situ minilysimeters method
At the beginning of the in situ minilysimeters experiment, soil OMwas 7.5, 4.3 and 3.1% respectively for the layers 0–10 cm, 10–20 cmand 20–30 cm. Fig. 5b shows the daily soil temperature measured atthe depth of 15 cm during the experiment and the rainfall for thesame period. Soil temperature ranged from 12 to 24 ºC, approachingthe optimum value for OM turnover nearly 170 days after thebeginning of the experiment. Total precipitation during this periodwas 746 mm, distributed in time as shown in Fig. 5, and contributedwith a mineral N input of 3.5 mg Nkg−1. These persistent precipita-tion events led to the maintenance of soil moisture contentsfrequently above θFC (Fig. 6). Volumetric θ varied from 27 to 51%(60 to 115% θFC) in the layer 10–20 cm, and from 35 to 71% (82 to167% θFC) in the layer 20–30 cm. For these high θ values the hydraulicconductivities are also very high resulting in a rapid response ofdrainage to the rainfall inputs. Fig. 5a shows the drainage leachatescollected in the lysimeters. A total amount of 251±6 mm wascollected at the lower boundary of the minilysimeters, correspondingto 34% of the rainfall inputs. The total amount of mineral N measuredin the leachate was 84±12 mg Nkg−1 soil.
Netmineralizationwas calculated by adding the difference betweeninorganic N (N-NH4+N-NO2+N-NO3) at the end and at the beginningof each period, to the inorganic N accumulated in the minilysimeterleachate during the same period (Brye et al., 2002). The N inputsthrough the precipitationwater were then subtracted to this result (seeEq. (1)). The result was then divided by the number of days in theperiod. The N mineralization rate calculated for the short periodsshowed considerable fluctuations according to temporal variations ofsoil temperature and moisture. However, a regression analysis showedthat the variation in N mineralization between periods was poorlyexplained by these two variables, probably due to the multipleinteractions between all the variables influencing the process. Thesamewas found by Trindade et al. (2001). Table 4 shows the applicationof the mass balance approach (Eq. (1)) for the entire period betweenMarch 8th (DoY 67) and October 2nd (DoY 275) and for the soil layer
10 - 20 cm
120 140 160 180 200
ng of incubation120 140 160 180 200
12
8
4
0
12
8
4
0
N-N
H4
(mg
kg
-1)
N-N
H4
(mg
kg
-1)
bation experiment (averages of three replications with SD).
0 20 40 60 80 100
days after beginning of incubation
120 140 160 180 200
0
8
4
0
8
4
0
20 40 60 80
0-10 cm
10-20 cm
average + SD
average + SD
100 120 140 160 180 200
net
min
eral
izat
ion
rat
e (m
g k
g-1
)
Fig. 4. Evolution of the mineralization rate during the laboratory incubation experiment (averages of three replications with SD).
389J.C. Fontes et al. / Geoderma 160 (2011) 384–393
0–20 cm. The average net mineralization rate estimated for the entireperiod (207 days) was 0.94±0.6 mg Nkg−1 d−1.
3.3. Monitoring N dynamics in the permanent pasture with periodicalgrazing
This experiment was performed between March 17th (DoY 76)and August 10th (DoY 231). Fig. 6 shows the average volumetric soilmoisture for the root zone (0–40 cm) and for the under laying layer(40–90 cm), as well as the rainfall distribution during the exper-imental period. Due to the persistent rainfall, the volumetric soilwater content in the surface layer was generally above θFC. For thesubjacent layer, θ was always smaller than θFC but close to it. Thesevery high volumetric θ are typical for the volcanic soils, which present
Fig. 5. (a) Leachates collected from the lysimeters; and (b) Soil temperatures measured
large porosities. θ reacts to precipitation events almost simultaneous-ly for the two soil layers, confirming the high kinetics for water whichresults from the considerable hydraulic conductivities of thisallophanic soil. Fig. 7 shows the ammonium profiles (in mg N-NH4
per kg of soil) in the soil obtained before and after the grazings. Asexpected, the N-NH4 contents in the surface layer increased after thepresence of the animals, due to their excreta. The grazing of April 15thand July 18th led to an increase of 14 and 24 kg N-NH4 ha−1
respectively, in the 0–40 cm soil layer (Fig. 7a and c). The conversionof N amounts from soil mass units to per unit area wasmade using thesoil bulk density. The profile sequence in Fig. 7b, shows, immediatelyafter the fertilization, an increase of 30 kg ha−1 in the N-NH4 stored inroot zone (0–40 cm). The storage in the 40–90 cm soil layer hasincreased by 19 kg ha−1. This low value is due to the fact that the
at the depth of 15 cm and rainfall events, during the in situ lysimeters experiment.
50
40
30
rain
fall
(mm
)vo
lum
etri
c so
il w
ater
co
nte
nt
(%)
20
10
70
60
50
40
30
20
1060 80
0-40 cm 40-90 cm
θFC 0-40 cm
θFC 40-90 cm
100 120
day of year140 160 180 200
0
Fig. 6. Volumetric soil water contents in the root zone (0–40 cm) and in the layer underneath (40–90 cm), and precipitation during the experimental period (θFC=soil moisture atfield capacity).
390 J.C. Fontes et al. / Geoderma 160 (2011) 384–393
ammonium cations are retained in the soil exchangeable complex,thus staying in the upper layers until they are up taken by the plants,nitrified or volatilized.
Fig. 8 shows the storage of nitrate-nitrogen in the 0–40 cm root zonelayer and in the layer beneath (40–90 cm). Nitrate storage in the soilprofile (0–90 cm) always increases with time until June 9th. BetweenMay 29th and May 16th there was an increase of 50 kg N-NO3 ha−1 inthe soil profile. About 80% of this increase occurred in the root zonelayer. Sinceno fertilizerwasappliedduring this period, this increasewasdue to the ammonium directly applied with animal excreta and thennitrified and the soil OMmineralization. During the followingperiod theincrease in nitrate storage was higher. Fertilization was performed byMay 27th, consisting in 30 kg N-NH4 ha−1 and 30 kg N-NO3 ha−1. Fig. 8shows that the ammonium added with the fertilizer was graduallynitrified, since there was still an increase in nitrate storage during thefollowing week. There is an increase in nitrate storage in the deepestlayer and simultaneously a decrease in the root zone. Fig. 6 shows thatduring the month of June, and until the next soil collection date (July10th), persistent precipitation events occurred and the soil moisturecontent was kept higher that θFC in the root zone. At the end of thisperiod, a residual mineral N profile of 30±10 kg N-NO3 ha−1 remainedin the root zone.
Estimates for mineral N losses with surface runoff during theperiod of observations are 0.24±0.05 kg ha−1 and 0.24±0.1 kg ha−1
for N-H4 and N-NO3 respectively.
4. Discussion
4.1. Net mineralization rates
These results show that the mineralization and nitrification processesare important in this system. Average netmineralization rate estimated in
Table 4Determination of the Nmineralization (Netmin) and the average Nmineralization rate by theDoY 275.
N-NO3(i)+N-NH4(i) N-NO3(i+1)+N-NH4(i+1) RN(i, i+1)
(mg kg−1)
8.97 122.46 3.5
i and i+1 are respectively the first and the last day of the minilysimeter experiment; RN i
situ for the soil layer 0–20 cm (0.98±0.6 mg Nkg−1 d−1) represents 58%of the one determined in the laboratory (1.7±1.1 mg N kg−1 d−1). Thetwo means were compared by the Mann–Whitney test and wereconsidered significantly different at pb0.005. Two reasons can beidentified: (a) during the field experiment, soil moisture and tempera-tures were sometimes times far from the optimal values (Figs. 5 and 6).According to Harmsen and Kolenbrander (1965), below the optimumtemperature (25 °C) both amonification and nitrification are reducedfollowing an asymptotic line. Soil water content was, in several occasions,above the field capacity, partially inhibiting the performance of themicrobes responsible for the mineralization. Precipitation during theexperimental year was 18% above the 15-year average, creatingconditions for uniformly low netmineralization rates and thus, unusuallylowvariability.Hanselmanet al. (2004)also foundvery small variability inthe net mineralization rates determination in situ for an exceptional wetclimatic year. (b) The laboratory rates were estimated for optimalconditions of temperature and soilmoisture (25 °C and80% θFC), thereforethey correspond to the maximum rates that can occur in the fields undernatural conditions.
According to the previous discussion, the laboratory values wereused as an upper limit control for the mineralization rates. The in situmean value with SD (0.98±0.6 mg kg−1 d−1) was chosen to estimatethe OM contribution to the N mineral pool. The conversion of netmineralization rates from soil mass units to per unit area was madeusing the soil bulk density. For this particular situation (soil bulkdensity=0.745, rooting depth=40 cm) a contribution of160.6 kg ha−1 of mineral N was estimated for a period of 55 days.Similar results were found by Trindade et al. (2001) and Iyyemperumalet al. (2007), for related conditions. Since this considerable mineral Namount is released in the soil during time, being gradually available tothe grass, the fertilizer inputs (60 kg Nha−1) could be reduced or eveneliminated.
mass balance approach (Eq. 1) applied to theminilysimeters data, between DoY 67 and
LN(i, i+1) Netmin(i, i+1) Average mineralization rate(mg kg−1) (mg kg−1d−1)
84 194 0.94
s the mineral N input through rainfall; and LN is the mineral N loss through leaching.
N-NH4(mg kg-1) N-NH4(mg kg-1) N-NH4(mg kg-1)
0
20
40
60Dep
th (
cm)
80
20
40
60
80
00
20
40
60
80
00100
10 20
april 16thapril 28thmay 25th
may 16thmay 19thjune 2thjune 9th
july 10thjuly 19thaug 19th
june 10th
30 0 10 20 30 0 10 20 30
a) b) c)
Fig. 7. Ammonium profiles in the soil: a) after the grazing of April 15th; b) after the grazing of May 26th and the fertilization of May 27th; and c) after the grazing of July 18th.
391J.C. Fontes et al. / Geoderma 160 (2011) 384–393
4.2. Ammonium (NH4-N) inputs through animal excreta
When the animals are in the field they contribute directly to the Nmineral pool through the ammonium in the excreta. This ammonium,as well as the nitrates resulting from the nitrifier microbes' activitywill be available for crop uptake. For the studied system, a referencevalue of 150 g NH4
+, added to the soil per animal and per day (data notpublished yet) is indicated. This means that when 65 animals arepresent in the field for three days, an input of approximately65 kg N-NH4 ha−1 is produced. The difference between the amountsobtained from the reference value and the ones obtained from themass balance calculations (14 and 24 kg N-NH4 ha−1 respectively inApril and July) is probably due to both spatial variability andammonium gaseous losses by volatilization (not quantified in thisstudy). Based upon the results obtained by other investigatorsvolatilization can be considered negligible for the studiedconditions. Saarijärvi et al. (2006) studied the dynamics of ammoniavolatilization from dung and urine patches in Finland. They found outthat over 80% of the total emissions occurred during the first 48 h, butpersistent rainfall markedly decreased volatilization. They concludealso that volatilization is highest with dry and warm soil conditionsand that the importance of pastures as a source of NH3 emission inFinland is minor and has been overestimated. Sommer et al. (1991)found similar results.
4.3. Mineral N losses through leaching and runoff
The θ values observed in the field, during the natural conditionsexperiment, indicate that the hydraulic conductivities were high,
180
150
graz
ing jun 2nd
may 29th
may 16th
graz
ing
+ fe
rt
120
90
60
30
0
N-N
O3(
kg h
a-1)
Fig. 8. N-NO3 storage in the grass root zone (0–40 cm), in the la
resulting in a rapid response of drainage to the rainfall inputs. Thismeans that the water fluxes through the root zone were significant,thus creating a potential for the rapid nitrate convective transportthrough the profile. This hydrologic behavior corresponds to the onealready observed for these soils (Fontes et al., 2004b). Nitrate storagein the soil profile (0–90 cm) always increaseswith time, except for theperiod between June 9th and July 10th. These increases were due tothe ammonium input with the animal excreta during the grazings, themineralization of the OM that resulted from the animal's presence inthe pasture, and to the mineral fertilizer applied. This ammoniumwasthen nitrified, releasing N-NO3 gradually into the soil. Besides beingup taken by the crop from the layer 0–40 cm, nitrate is gradually beingtransported to the layer below the root zone, becoming theninaccessible to be used by the crop. Considering that there were noroots in this layer, and due to the existence of downwardwater fluxes,this amount was likely leached to deeper layers. A leaching loss of70±18 kg N-NO3 ha−1 can be estimated by a simple mass balanceapproach, corresponding to the amount that passed through thebottom of the root zone, at the depth of 40 cm. This is a roughestimation (probably an underestimation) due to the following:
(a) Records of the water fluxes (using tensiometry) would benecessary to accurately assess the convective transport of N throughthe soil profile;
(b) Gaseous losses, e.g. volatilization and denitrification, are notcurrently being considered in the mass balance. Volatilization is knownto occur in pastureswith direct grazing, although it was neglected in thepresent one (justification in Section 4.2). Losses by denitrification are inthe form of nitrous oxide (N2O) (Dendooven et al., 1994, Iyyemperumalet al., 2007). This process is significant when the following abiotic
graz
ing
jun 9th
jul 10th
time
0-40 cm
40-90 cm0-90 cm
yer beneath (40–90 cm) and in the soil profile (0–90 cm).
392 J.C. Fontes et al. / Geoderma 160 (2011) 384–393
conditions are met: soil pH higher than seven, anaerobiosis conditionsassociatedwith soil saturation during large periods of time (Dendoovenet al., 1994) andhigh temperatures. pHvalues lower than six (like in thissoil—Table 3) and temperatures above 40 °C are inhibitory fordenitrification, (Harmsen and Kolenbrander, 1965). In this volcanicsoil the considerably large porosity and hydraulic conductivity main-tains, the soil moisture part of the time near field capacity. There isalways a percentage of porosity filled with air, maintaining an aerobicenvironment. Because of this, and despite of the lack of fieldobservations for this process, it was adequate to assume that losses ofN2O have a negligible impact on the N mass balance (when comparedwith the other terms). To provide an idea about the magnitude of thisprocess, the work of Vellidis et al. (2003) is referred. They conductedstudies of denitrification removal of nitrate through in a riparian zone. Inthis wet land the average annual denitrification rate based on 2480intact cores taken in the top 24 cm of soil was 68 kg Nha−1 yr−1,producing an average denitrification rate of 0.19 kg Nha−1 d−1.
In the prairie, during fall, the grass becomes less active decreasingthe uptake of water and nutrients. At the same time the frequency ofthe rainfall events increases. These factors combined create theoptimum conditions for potential nitrate leaching.
Considering the losses of mineral N (N-NH4+N-NO3) throughleaching and surface runoff, a total value of 89.5±18 kg Nha−1 isfound (19+70+0.24+0.24), which represents 53% of the fertilizerinputs. The largest amount of the N lost was leached below the rootzone and potentially reaches the groundwater, thus affecting drinkingwater sources. The smaller amount lost with the runoff contributes tosurface waters eutrophication problems. Therefore, this study showsthat the fertilization management (amount and number of applica-tions) is not contributing for the sustainability of this grazing pasturesystem. The amount of fertilizer to be applied should be determinedconsidering the direct input of ammonium through the animalexcreta, estimated for this system as 65 kg N-NH4 ha−1, and themineral N input resulting from the net mineralization process,estimated as 160.6 kg ha−1. On the other hand, due to the randomcharacteristics of the rainfall events, the fertilization should be split intwo or more applications. Then, when rainfall occurs and producesdownward fluxes through the soil profile, only small amounts ofmineral N will be available for leaching.
To support an improved fertilizer management, it is advisable toexplore an integrated soil–plant–atmosphere model such as the RootZone Water Quality Model (RZWQM), which proved appropriate forimproved N management of cereal crops (Cameira et al., 2007) andhorticultural systems (Agostinho, 2006). This model presents adetailed deterministic approach for the calculation of water andnitrogen processes, which make it attractive to better understandingvarious aspects of the nitrogen cycle and to assess alternative fertilizermanagement options.
5. Conclusions
Regarding objective (1): the N budget terms that affect most thenitrogen dynamics in this pasture system where detected andquantified. The laboratory method used to estimate OM turnoveryielded a net (potential) mineralization rate of 1.7±1.1 mg Nkg−1 d−1
for the soil layer 0–20 cm. For in situ conditions, the average netmineralization rate was smaller: 0.98±0.6 mg Nkg−1 d−1. Thesevalues were found to be statistically different. The in situ result waschosen to estimate the N inputs due to mineralization. This estimationyielded a value of 160 kg ha−1.The direct input of ammonium throughthe animal's excreta amounts for 65 kg N-NH4 ha−1.Mineral N losses byleaching and runoffwerequantified respectively in 89±18 kg ha−1 andof 0.5±0.08 kg ha−1 for the experimental period,which represents 53%of the fertilizer inputs. These results confirm the importance of revisingthe fertilization practices of these pasture systems.
Regarding objective (2): it can be concluded that to control suchlosses, the fertilizer amount should be recalculated considering thedirect input through the animal excreta and the mineral N inputresulting from the net mineralization process. In addition, due to therandom characteristics of the rainfall events, the fertilization should besplit into two or more applications, hence reducing the amounts ofmineral N available for leaching. Further studies may be developedusing a modeling approach to allow optimizing fertilizer management.
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