The Diurnal Variation of Biotic and Abiotic Factors of Autotrophic and Heterotrophic Respiration in a Tall Grass Prairie

Ryan BourgartBiosciences Division

Roser MatamalaNuria Gomez-Casanovas

Department of Educational Programs, Science Undergraduate Laboratory Internship Program

Argonne National LaboratoryLemont, IL

July 15, 2009

Prepared in partial fulfillment of the requirement of the Department of Educational Programs’ Science Undergraduate Laboratory Internship under the direction of Roser Matamala, Biosciences division at Argonne National Laboratory. and Nuria Gomez-Casanovas, University of Illinois at Chicago

Participant: _________________________Signature

Research Advisor: _________________________Signature

Table of Contents

Abstract: p. 3

Introduction: p. 4

Materials & Methods: p. 5

Results: p. 7

Discussion & Conclusion: p. 9

Acknowledgements: p. 10

Bibliography: p. 10

ABSTRACT

Diurnal regulation of Total Soil, Auto- and Heterotrophic Respiration in a restored Tall Grass Prairie. RYAN C. BOURGART (Valparaiso University, Valparaiso, IN, 46383) ROSER MATAMALA (Argonne National Laboratory, Argonne, IL 60439), NURIA GOMEZ-CASANOVAS (University of Illinois at Chicago, Chicago, IL, 60607)

Soil respiration is a major component of ecosystem respiration and small changes in soil

respiration can have a major impact in the amount of atmospheric CO2 . Despite the

importance of soil respiration in global carbon dynamics, little is known about the factors

controlling soil respiration and its partitioning. This study concentrated on studying the

biotic (C input) and abiotic (soil temperature and moisture) factors that control soil

respiration and its partitioning on a diurnal time scale during the months of June and

September 2008. The area studied was a tall grass prairie located at Fermi National

Laboratory in Batavia, IL. At this site, daily net ecosystem exchange (NEE) of CO2 was

measured using the eddy correlation technique. In addition, soil moisture and temperature

measurements were taken using soil sensors. Soil respiration was continuously measured

using an automated infrared gas analyzer, attached to four soil respiration chambers. The

partitioning of soil respiration was obtained using the ratio of the stable C isotope,

13C/12C, of the respired CO2 and constructing keeling plots from gas samples collected

across the tallgrass prairie. Soil respiration increased during the day in June due to

increased photosynthetic activity and in September due to warmer soil temperatures,

implying the prevalent influence of autotrophic respiration in June and heterotrophic

respiration September. More research is needed to increase understanding of the

regulation and partitioning of soil respiration.

Introduction:

Human activities are altering the composition of the atmosphere by the emission

of greenhouse gases such as carbon dioxide (CO2). The increase of CO2 in the atmosphere

is causing global climate change, threatening ecosystem functioning and integrity. Global

carbon cycle is controlled by relative differences between gross primary production

(GPP) and ecosystem respiration that includes plant and soil respiration. At a global

scale, terrestrial ecosystems can sequester vast amounts of C in soils and biomass. This C

may eventually be released back to the atmosphere via oxidation of soil organic matter.

Terrestrial ecosystems also release vast amounts of C through soil respiration.

Schlesinger and Andrews (2000) estimated total global soil respiration at 75 x 1015 gC/yr.

Thus it is obvious that soil respiration is an essential component of the global carbon

cycle. Despite the importance of soil respirationin determining the amount of carbon

released back into the atmosphere, the regulation of soil respiration is poorly understood.

Studying the process in greater depth will help ecological restoration efforts, which,

according to Keith Bowers, chair of the Society for Ecological Restoration International

(SER), “is a critical tool in addressing global climate change, enhancing the extent and

functioning of carbon sinks as well as reducing greenhouse gas emissions.”

Contributions to total soil respiration come both from free-living soil

microorganisms decomposing soil organic matter (heterotrophic respiration) and from

roots and rhizosphere microorganisms (autotrophic respiration). It is necessary to

partition soil respiration into heterotrophic and autotrophic respiration because the as the

variables change, heterotrophic and autotrophic respiration change differently. The many

variables that influence soil respiration contribute to the lack of understanding of the

process. Soil respiration and its components vary spatially and temporally. This

variation is influenced by air and soil moisture and temperature, land use history, soil

mineralogy, nutrient availability, litter quality, plant phenology, and others (Trumbore,

2005). Understanding the factors that affect CO2 exchange between terrestrial ecosystems

and the atmosphere is imperative to predict changes in atmospheric CO2 and its effects on

climate. Understanding the processes affecting soil respiration will also help us in our

efforts to slow and alleviate the effects of climate change. Carbon sequestration in soil is

a realistic strategy if a better understanding of the natural processes affecting soil

respiration might be acquired.

Materials and Methods:

This study was conducted at a 20-year-old restored tall grass prairie at Fermi

National Accelerator Laboratory in Batavia, IL. Previous to restoration, the area was

used for corn and soybean cultivation for over 100 years. In 1990, the area was restored

to a 30 ha tall grass prairie using native vegetation. Restoration of the vegetation was

done by plowing and seeding the field with a mix of prairie plants, including C3 and C4

grasses, and forbs. Soils at this site are predominantly silt loams and silty clay. The site is

burned biannually, with burns administered in early to late spring depending on the

weather conditions.

This study was conducted during the months of June and September of 2008. At

this site, daily net ecosystem exchange (NEE) of CO2 was obtained using the eddy

covariance technique. NEE is the net flux of C over an ecosystem at a given time, and it

is the integration of carbon uptake minus respiration by plants and soil. NEE, measured

on a daily basis, is the sum of half-hour values over the course of a day. Soil moisture

and temperature were continuously measured also at 30-minute intervals using a REBS

STP-1 Soil Temperature Probe and a REBS SMP-1 Soil Moisture Probe at a depth of 2.5

cm. In addition, long-term Total Soil Respiration rates (TSR) were continuously

measured (with averages calculated every 30 minutes) using an automated soil CO2 flux

system attached to four soil respiration chambers (Li-Cor 8100-8150), which measures

CO2 concentration emitted from the soil using an infrared gas analyzer. Each soil chamber

is automatically placed on top of 20-cm diameter soil collars, which are placed 7.5 cm

into the soil.

To partition total soil CO2 efflux into autotrophic and heterotrophic respiration, an

stable-isotope mass balance equation was used (Eq. 1). This equation is based on the

difference between the ratio of 13C to 12C (δ13C) of the CO2 respired from the soil versus

the CO2 respired from plants:

δ13C soil= f (δ13C root) + (1-f) (δ13C bulk soil) (Eq.1)

where the δ13C of soil was determined from gas samples collected at 18 PVC collars,

placed across the prairie. These samples were obtained using a manual infrared gas

analyzer (Li-Cor 6400) attached to a soil chamber. The chamber was placed over the

collars and the CO2 concentration inside was allowed to accumulate. When the CO2

inside the chamber reached four different CO2 concentrations (450, 550, 650 and 750

ppm), a gas sample was collected using glass flaks. The CO2 collected in the flaks was

then analyzed in a mass spectrometer, dual inlet VG-ISOTECH SIRA II equipped with

on-line preparation systems for gas analyses. Soil respiration rates were also measured

for all the collars. These collections were done every three weeks during both night and

day times. The δ13C soil was determined using the keeling plot method. To construct a

keeling plot, the δ13C of the gas samples coming from collars with similar respiration rate

values were grouped together and plotted with inverse CO2 concentrations. The δ13C soil

was determined as the intercept with the Y axes.

The δ13C of roots and bulk soil (soil free of roots) were determined from

incubations, which were done after soil core collection at the prairie. Because soil

respiration varies greatly over time, bulk soil and root incubations were also conducted

every three weeks during the day and at night. The 13C of CO2 respired from the roots

and bulk soil differ in the restored prairie. As mentioned above, previous to restoration,

the area was used for corn and soybean cultivation for over 100 years. Corn is C4 plant

whereas soybean is a C3 plant. Because C3 and C4 plants were present before the

restoration, CO2 respired from bulk soil respiration will have a δ13C signature between –

29 and –14 ‰ (common signatures of C3 and C4 plants, respectively). In contrast,

presently most of the plants in the prairie are C3, the δ13C from roots will be similar to a

C3 signature. The mass balance equation was then solved for f to determine the

proportion of CO2 respired by the heterotrophs versus roots at each experimental time.

Results:

Diurnal variation of soil respiration and its partitioning:

TSR differed greatly between June and September. Autotrophic respiration was

prominent in June (73.55% during the day and 74.55% at night) and heterotrophic

respiration was prominent in September (75.44% during the day and 79.15% at night).

Data on Figure 1 shows that total soil respiration rates in June gradually decreased from 0

until around 8:30 and increased from 8:30 to12:00. Soil CO2 efflux remained relatively

stable until 16:00 and then decreased until 23:30 (Fig.1). Ra and Rh followed a similar

pattern (Fig.1). Data on figure 2 shows that total soil respiration rates in September

gradually decreased from 0 until around 7:30 and increased from 7:30 to around 17:00.

Soil CO2 efflux remained relatively stable until 19:30 and then decreased until 23:30

(Fig. 2). Ra and Rh followed a similar pattern (Fig. 2).

Diurnal variation of soil temperature:

The month of September (Fig. 3), had the lowest temperature value recorded

between June and Sep, 18.1˚C, that occurred at 7:30. However, it also had a greater

variability (2.8˚C) than June (2.1˚C), resulting in warmer daytime temperatures.

Abiotic controls on Soil Respiration and its partitioning:

Figure 4 shows that soil temperature at 2.5 cm had a strong correlation with total

soil respiration (R2 = .81) and autotrophic respiration (.82), but not with heterotrophic

respiration (R2 = .17) in June. September (Fig. 5) showed significant correlations

between soil respiration and its partitioning with R2 values of .86, .76, and .7432 for total

soil respiration, heterotrophic, and autotrophic respiration, respectively. Diurnal

variability on soil water content (SWC) (Fig. 6) was negligible in September. SWC

showed its maximum variability in June (1.42%). Soil water content at 2.5 cm in June

(Fig. 7) had weak correlations with TSR, Rh, and Ra with R2 values of .4314, .4515,

and .3040 respectively. In September (Fig. 8) SWC had a strong correlation with Ra (R2

= .6697) and weak correlations with TSR (R2 = .4743) and Rh (R2 = .383).

Diurnal variation of Carbon Flux (net photosynthesis):

Figure 9 shows that June had the highest (5.9 mol m-2 s-1) and lowest (-19.1

mol m-2 s-1) values as well as the greatest change (25.0 mol m-2 s-1) in carbon flux (CF).

CF was relatively stable until about 5:00, then it decreased until about 12:30, increased

until about 20:00, and remained relatively stable until 23:30. In September, CF remained

relatively stable until 7:00, decreased until around 11:30, increased until around 18:30,

then remained stable until 23:30.

Biotic controls on soil respiration and its partitioning:

CF in June (Fig. 10) had strong correlations with TSR (R2 = .85),Ra (R2 = ..84),

and with Rh (R2 = .84). In September (Fig. 11), CF correlated weakly with all three

respirations with R2 values of .32, .13, and .44 for TSR, Rh, and Ra respectively. Time

lags of 7 and 10 hours were found for the FC data in June and September respectively.

Time lag calculations were done because soil respiration takes time to respond to changes

in environmental conditions.

Discussion and Conclusion:

Autotrophic respiration was the major contributing component (73.6% during the

day and 74.6% at night) to total soil respiration in June. In contrast, during September

heterotrophic was the major contributing component (75.4% during the day and 79.2% at

night) to total soil respiration. During June and September, total soil respiration peaked

during the course of the day (around 15:00) and gradually decreased at night.

It has been suggested that soil respiration is mostly driven by abiotic factors such

as soil temperature and moisture ( Hanson et al. , 2000 ; Giardina & Ryan, 2000; Fang &

Moncrieff, 2001; Janssens et al. , 2003 ; Tang et al., 2005). In this study, because soil

moisture was relatively constant during the course of the day and was weakly correlated

with total soil respiration, it didn’t have a strong effect on soil respiration. Soil moisture

did have an effect on autotrophic respiration, but not heterotrophic respiration in

September. On the other hand, soil temperature was strongly correlated to TSR and

autotrophic respiration in June and September and to heterotrophic respiration in

September. We suggest that as the sun heated up the soil during the day in September,

warmer soil temperatures and large organic matter accumulations in the soil at the end of

the season contributed to higher heterotrophic activities in September compared to June.

Carbon flux had the most impact on respiration in June. in June photosynthesis

was at its maximum and in coincidence with the peak of plant growth. Several studies

have found that photosynthesis is a major contributor to soil respiration (Ekblad &

Högberg, 2001; Bowling et al. , 2002, Bhupinderpal-Singh et al. , 2003; Ekblad et al. ,

2005; Tang et al., 2005). Soil temperature and photosynthesis have strong influences on

soil respiration and are difficult to separate. The correlation between soil respiration and

photosynthesis is often confounded with soil temperature. This is because photosynthesis

is also influenced by soil temperature and the correlation between soil temperature and

soil respiration may be indirectly affected by photosynthesis.

Through comprehensive ecosystem restoration methods, the carbon sequestration

capabilities of ecosystems can be harnessed. An improved understanding of the

partitioning and regulation of soil respiration is necessary in order to more properly

restore and manage ecosystems. More research needs to be conducted to better

understand how soil respiration varies in different ecosystems and on larger time scales.

Seasonal differences between soil respiration and how its partitioned and regulated

should be studied if a strategy for alleviating climate change effects is to be developed.

Acknowledgements:

I would like to thank my coworker and mentor, Nuria Gomez who contributed a

lot to this project including but not limited to gathering data in the field, sample

preparation, and editing this paper. I would like to thank my supervisor, Roser Matamala

for her contribution to this project including her guidance and editing this report. I would

also like to thank Stephan Vandenbroucke, who helped me with taking measurements.

Bibliography:

Bhupinderpal-Singh, Nordgren A, Löfvenius MO et al. (2003) Tree root and soil heterotrophic respiration as revealed by girdling of boreal Scots pine forest: extending observations beyond the first year. Plant, Cell and Environment, 26, 1287–1296.

Bowling DR, McDowell NG, Bond BJ et al. (2002) 13C content of ecosystem respiration is linked to precipitation and vapour pressure deficit. Oecologia, 131, 113–124.

Ekblad A, Boström B, Holm A (2005) Forest soil respiration rate ad δ13 C is regulated by recent above ground weather conditions. Oecologia, 143, 136–142.

Ekblad A, Högberg P (2001) Natural abundance of 13C in CO2 respired from forest soils revels speed of link between photosynthesis and root respiration. Oecologia, 127, 305–308.

Fang C, Moncrieff JB (2001) The dependence of soil CO2 efflux on temperature. Soil Biology and Biochemistry, 33, 155–165.

Giardina CP, Ryan MG (2000) Evidence that decomposition rates of organic matter in mineral soil do not vary with temperature. Nature, 404, 858–861.

Hanson PJ, Edwards NT, Garten CT, Andrews JA (2000) Separating root and soil microbial contributions to soil respiration: a review of methods and observations. Biogeochemical, 48, 115–146.

Janssens IA, Dore S, Epron D et al. (2003) Climatic influences on seasonal and spatial differences in soil CO2 efflux. In: Canopy Fluxes of Energy, Water and Carbon Dioxide of European Forests (ed. Valentini R), pp. 235–256. Springer-Verlag, Berlin, Germany.

Schlesinger WH, Andrews JA (2000) Soil Respiration and the Global Carbon Cycle. Biogeochemistry, Vol. 48, No. 1, Controls on Soil Respiration: Implications for Climate Change (Jan., 2000), pp. 7-20

Society for Ecological Restoration International (2007, August 21). Ecological Restoration: A Global Strategy For Mitigating Climate Change. ScienceDaily. Retrieved July 22, 2009, from http://www.sciencedaily.com /releases/2007/08/070817165031.htm

Tang JW, Baldocchi DD, Xu LK (2005) Tree photosynthesis modulates soil respiration on a diurnal time scale. Global Change Biology 11, 1298-1304.

Trumbore S (2006) Carbon respired by terrestrial ecosystems – recent progress and challenges. Global Change Biology 12, 141-153.

Figure 1: Average diurnal total soil respiration and components during the month of June 2008

Figure 2: Average diurnal total soil respiration, autotrophic respiration, and heterotrophic respiration during the month of September 2008

Figure 3: Average diurnal soil temperature at a depth of 2.5 cm during the months of June and September 2008

Figure 4: Correlation between average diurnal soil respiration, its autotrophic and heterotrophic components, and average diurnal soil temperature at a depth of 2.5 cm during the month of June 2008.

R2 = 0.8575

R2 = 0.7432

R2 = 0.7564

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

17.5 18 18.5 19 19.5 20 20.5 21 21.5

Soil Temperature (Degrees Celsius)

Soil

Res

pira

tion

(um

ol/m

^2*s

)

Total Respiration

Auto Respiration

Hetero Respiration

Expon. (Total Respiration)

Expon. (Auto Respiration)

Expon. (Hetero Respiration)

Figure 5: Correlation between average diurnal soil respiration, its autotrophic and heterotrophic components, and average diurnal soil temperature at a depth of 2.5 cm during the month of September 2008.

33

35

37

39

41

43

45

0 500 1000 1500 2000 2500

Time (Hour in Military Time)

Soil

Wat

er C

onte

nt (%

)

JuneSeptember

Figure 6: Average diurnal soil volumetric water content at a depth of 2.5 cm during the months of June and September 2008.

R2 = 0.4314

R2 = 0.4515

R2 = 0.304

0

1

2

3

4

5

6

7

8

9

34.4 34.6 34.8 35 35.2 35.4 35.6 35.8 36 36.2

Soil Volumetric Water Content (%)

Soil

Res

pira

tion

(um

ol/m

^2*s

)

Total RespirationAuto RespirationHetero RespirationPoly. (Total Respiration)Poly. (Auto Respiration)Poly. (Hetero Respiration)

Figure 7: Correlation between average diurnal soil respiration, its autotrophic and heterotrophic components, and average diurnal soil volumetric water content at a depth of 2.5 cm during the month of June 2008.

R2 = 0.4743

R2 = 0.6697

R2 = 0.383

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

43.42 43.44 43.46 43.48 43.5 43.52 43.54 43.56 43.58 43.6 43.62

Soil Volumetric Water Content (%)

Soil

Res

pira

tion

(um

ol/m

^2*s

)

Total RespirationAuto RespirationHetero RespirationPoly. (Total Respiration)Poly. (Auto Respiration)Poly. (Hetero Respiration)

Figure 8: Correlation between average diurnal soil respiration, its autotrophic and heterotrophic components, and average diurnal soil volumetric water content at a depth of 2.5 cm during the month of September 2008.

-25

-20

-15

-10

-5

0

5

10

0 500 1000 1500 2000 2500

Time (Hour in Military Time)

Car

bon

Flux

(um

ol/m

^2*s

)

JuneSeptember

Figure 9: Average diurnal carbon flux during the months of June and September 2008.

R2 = 0.8469

R2 = 0.8366

R2 = 0.8445

0

1

2

3

4

5

6

7

8

9

-25 -20 -15 -10 -5 0

Carbon Flux (umol/m^2*s)

Soil

Resp

iratio

n (u

mol

/m^2

*s)

Total Respiration

Auto RespirationHetero Respiration

Expon. (Total Respiration)Expon. (Auto Respiration)

Expon. (Hetero Respiration)

Figure 10: Average diurnal soil respiration and its components vs. average diurnal carbon flux during the month of June 2008.

R2 = 0.3245

R2 = 0.1263

R2 = 0.4371

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

-5 -4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0

Carbon Flux (mol m-2 s-1)

Soil

Resp

iratio

n (

mol

m-2

s-1)

Total RespirationAuto Respiration

Hetero RespirationExpon. (Total Respiration)

Expon. (Hetero Respiration)

Expon. (Auto Respiration)

Figure 11: Average diurnal soil respiration and its components vs. average diurnal carbon flux during the month of September 2008.

-10

-8

-6

-4

-2

0

2

4

0 50 100 150 200 250 300

Time (Julian Day)

NEE

(g C

/m^2

/day

)

JuneSeptember

Figure 12: Daily NEE values during the months of June and September 2008