Soil CO
2
¯ux in a tallgrass prairie
P.C. Mielnick, William A. Dugas*
Blackland Research Center, Texas Agricultural Experiment Station, 808 East Blackland Road, Temple, TX 76502, USA
Accepted 16 August 1999
Abstract
Soils are an important component of the global carbon budget due to their large C storage capacity and the ability to replenish atmospheric C via soil surface CO2¯ux. Our objectives were to quantify year-round soil CO2±C ¯uxes in a tallgrass prairie and to develop an equation to predict ¯ux using soil temperature and soil water content. Soil CO2±C ¯ux, soil temperature and soil water were measured on selected days throughout the year from 1993 through 1998, n= 216, at the Blackland Research Center, Temple, Texas, USA. On any date, there was little dierence in average daily soil temperatures among years, but there were large dierences in soil water content among years that often were related to dierences in precipitation totals. Soil CO2±C ¯ux had a seasonal pattern that was more similar to soil temperature than soil water (minimum in the winter and maximum in the early summer). Average annual soil CO2±C ¯uxes, which were 1.6, 1.3, 1.2, 1.0, 2.1 and 1.5 kg CO2±C m
ÿ2
yrÿ1 in 1993 through 1998, respectively, increased with annual precipitation. Regressed separately, the exponential relationship between soil CO2±C ¯ux and soil temperature accounted for approximately 46% of ¯ux variability while a quadratic relationship between ¯ux and soil water content accounted for 26% of the variability. Both terms were combined into one equation that explained about 52% of the ¯ux variance. Predicted and measured ¯uxes showed similar patterns throughout the year, there was little bias between predicted and measured ¯uxes, averages were essentially equal and the root mean square error between measured and predicted ¯uxes was about 38% of the average ¯ux. The equation accounted for 76% of ¯ux variability of an independent data set from the Konza Prairie in Kansas. The relationship between ¯ux, soil temperature and soil water content should provide accurate predictions of soil CO2¯ux in tallgrass prairies in the midwestern US.#2000 Elsevier Science Ltd. All rights reserved.
Keywords:Soil CO2¯ux; Grassland; Carbon cycling
1. Introduction
Soils are an important component of the global car-bon budget for several reasons. Firstly, soils contain about twice the amount of C as the atmosphere, i.e. 1500 Pg of C in soils vs. 750 Pg of C in the atmos-phere (Eswaran et al., 1993), and, as a result, are an important global C reservoir. Secondly, soils contrib-ute C to the atmosphere through plant root respiration and decomposition of soil organic matter by soil microorganisms that transform organic plant-inaccess-ible C to the inorganic plant-accessplant-inaccess-ible form (CO2).
Also, on a geological time scale, dissolution of soil car-bonates is important as there are approximately 722 Pg of C in the top 1 m of soils worldwide (Batjes, 1996).
Globally, soil CO2±C ¯ux is estimated at 68±75 Pg CO2±C yrÿ1 (Raich and Schlesinger, 1992; Raich and Potter, 1995). Besides adding C to the atmosphere, soil CO2±C ¯ux can act as an index of below-ground pro-cesses that are dicult to study in situ and of the C cycling capacity of soil ecosystems (Raich and Schlesinger, 1992). Some have suggested (Jenkinson et al., 1991; Nakayama et al., 1994; Tate and Ross, 1997) that soil CO2±C ¯ux may increase as a result of increases in atmospheric CO2.
Soil CO2±C ¯ux often exhibits diel (i.e. 24 h) and seasonal variability and may be ecosystem-speci®c in magnitude. For example, Raich and Schlesinger (1992)
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* Corresponding author. Tel.: 6600; fax: +1-254-770-6561.
estimated annual ¯uxes to range from 0.4 to 0.5 kg CO2±C mÿ2 yrÿ1 for grasslands, 1.3 kg CO2±C mÿ2 yrÿ1 for tropical forests and 0.2 kg CO2±C mÿ2 yrÿ1 for deserts. Average daily ¯uxes from grasslands during the growing season have been measured from near 0 to 6 g CO2±C mÿ2 dÿ1 (Buyanovsky et al., 1987; Norman et al., 1992; Jensen et al., 1996; Bremer et al., 1998; Knapp et al., 1998).
Soil CO2±C ¯ux can be measured using both static chambers (e.g. Gupta and Singh, 1981; Beyer, 1991; Grahammer et al., 1991) and dynamic methods (often using an infrared gas analyzer) (e.g. Kucera and Kirkham, 1971; Desjardins, 1985; Rochette et al., 1991; Norman et al., 1992; Kim et al., 1992; Bremer et al., 1998). The static chamber method has been criti-cized (Nakadai et al., 1993; Nay et al., 1994; Jensen et al., 1996; Bekku et al., 1997) for introducing a measurement bias as compared to the dynamic method, which has been shown to be more accurate and less biased for a wide range of ¯ux rates (Jensen et al., 1996). Some researchers, however, have found little dierence in ¯uxes measured by either of the two methods (Rochette et al., 1997).
Several researchers have proposed models or equations to predict soil CO2±C ¯ux from more readily available biotic and abiotic measurements. Two commonly used abiotic variables are soil temperature and soil water (e.g. Kucera and Kirkham, 1971; Wildung et al., 1975; Kim et al., 1992; Norman et al., 1992; Howard and Howard, 1993; Lloyd and Taylor, 1994; Davidson et al., 1998; Knapp et al., 1998). Air temperature and precipitation also have been used in global models (Raich and Potter, 1995). In many of these eorts, however, few of the data sets exceeded 2 yr in duration and ¯ux often was often only measured for a portion of the year (usually during the growing season). In addition, models or equations were rarely validated against independent data sets. Our study is unique because we had year-round simultaneous measurements of soil CO2±C ¯ux, soil temperature and soil water and because we collected data over 6 yr. Thus, we had a large data set that re¯ected varying climatic conditions. We also compared our predictions against an independent data set from another location. The objectives of this study were to quantify year-round soil CO2±C ¯uxes in a tallgrass prairie and to develop an equation using soil temperature and soil water to predict ¯ux.
2. Materials and methods
2.1. Site description
Measurements were made from 1993 to 1998 in a tallgrass prairie (ca. 2.5 ha) at the Blackland Research
Center, Temple, TX, USA (31806'N 97820'W, el-evation 219 m).
Prior to this study, the prairie site had not been plowed, grazed, shredded or baled, fertilized, or had herbicides applied for over 20 yr and had not been burned for more than 50 yr. The predominant species are big bluestem (Andropogon gerardii Vitman), little bluestem (Schizachyrium scoparium (Michx.) Nash.), indiangrass (Sorghastrum nutans (L.) Nash), Johnsongrass (Sorghum halepense(L.) Pers.) and a var-iety of forbs (Dugas et al., 1997). The soil is a Houston Black clay (®ne, montmorillonitic, thermic Udic Pellustert) that contains more than 50% clay and has a large water holding capacity (Godfrey et al., 1960). Soil organic C content to a depth of 0.2 m was 3.3% (D. Gephart, unpublished data, 1997).
The prairie was burned on d 53 in 1995. The burn removed virtually all live and dead plant material, which had been substantial in 1993 and 1994 (Dugas et al., 1999). This resulted, temporarily, in a bare, dark soil surface. Above-average precipitation in the months immediately following the burn (Table 1) caused the surface vegetation cover to increase quickly to where, in a few months, it was visually similar to surface veg-etation observed at equivalent times in 1993 and 1994.
2.2. Measurements
Soil CO2±C ¯ux, soil temperature and soil water were measured from d 134 to 349 in 1993, d 33 to 304 in 1994, d 41 to 313 in 1995, d 100 to 313 in 1996, d 106 to 356 in 1997 and d 9 to 362 in 1998. Flux was measured via the dynamic method using a Li-Cor 6200 portable photosynthesis system (Li-Cor, Lincoln, NE, USA) ®tted with a soil chamber with a volume of 1259.2 cm3 and a diameter of 10 cm. The chamber, which has a maximum ¯ow rate of 1 mmol sÿ1, was
Table 1
Total monthly and annual precipitation (mm) from 1993 through 1998 and average precipitation at Blackland Research Center, Temple, TX
Month 1993 1994 1995 1996 1997 1998 84 yr average
January 96 20 18 4 32 95 55
placed on circular collars (115 mm i.d.45 mm deep) that had been inserted about 20 mm into the soil at the beginning of each calendar year. During very dry periods, we did not observe large cracks in soil either inside or directly outside the collars in this heavily vegetated area of the measurement site although soil within the collars did shrink away from the inside col-lar walls slightly. Colcol-lars were spaced 1 m apart in an arc-shaped pattern. Surface litter was contained within the collars, but not live or standing dead vegetation. When necessary, green vegetation was physically removed from inside the collars the day before a scheduled measurement period. Measurement locations were the same in a year, but all collars were moved a few meters between years to minimize the eects of foot trac on ¯ux.
Measurements were made over two time intervals.
1. 12 h: between 1993 and 1998, soil CO2±C ¯ux was measured continuously from 0700 to 1900 h Central Standard Time (CST) about every 2±4 weeks at nine locations. This resulted in approximately 125 measurements on each day that were used for a daily average.
2. Midday: in 1997 and 1998, soil CO2±C ¯ux was also measured from 1300 to 1500 h CST about 3 d per week at 10 locations in the prairie. This resulted in approximately 20 measurements on each day that were used for a daily average.
Measurement locations for the 12 h and midday ¯uxes were about 15 m apart. Daily averages calcu-lated from all measurements taken at both the 12 h and midday time intervals were pooled (see below).
A measurement took approximately 30 s, depending upon the ¯ux rate. To bracket the ambient air CO2 concentration during a measurement, the CO2 concen-tration of the chamber air was reduced by brie¯y diverting it through soda lime before initiating logging. Flux was calculated from the rate of increase of CO2 inside the chamber.
20 min averages of soil temperature at 5 cm depth were continuously measured using three soil thermo-couples and a data logger. Soil temperatures on the day of a soil ¯ux measurement were averaged for the period of ¯ux measurements (i.e. 12 h or midday).
Volumetric soil water content was measured in two ways. From 1993 to 1996, soil water was measured weekly using a Model 200 capacitance probe (Troxler Electronic Laboratories, Research Triangle Park, NC) that had been calibrated against soil water measure-ments from soil cores in the same ®eld. A single sen-sor, buried at 10 cm, measured a soil water content from 5 to 15 cm. The soil water content closest in time to the ¯ux measurement was used.
In 1997 and 1998, soil water was measured by tak-ing three 15 mm diameter soil cores to a depth of 12
cm at least twice a week. Cores were oven-dried and volumetric water content was calculated using soil bulk density measurements. Measurements were made within 2 d of ¯ux measurements. Daily precipitation was measured at a meteorological station approxi-mately 500 m from the prairie.
2.3. Analytical procedures
We used a multiple regression procedure (SAS Institute, 1988) to develop an equation to predict soil CO2±C ¯ux from soil temperature and soil water. To predict ¯ux from soil temperature, we used an expo-nential equation as suggested by others (Kucera and Kirkham, 1971; Norman et al., 1992; Lloyd and Taylor, 1994; Raich and Potter, 1995; Davidson et al., 1998). For soil water, we used a quadratic relationship between ¯ux and water (e.g. Bunnell et al., 1977; Linn and Doran, 1984).
3. Results and discussion
3.1. 24 h versus 12 h ¯ux
Dugas et al. (1999) showed that overnight soil CO2±
Fig. 1. 20 min average soil CO2±C ¯ux for 12 h measurements on d
C ¯uxes were about equal to those measured near dawn and dusk. On d 154 and 155 in 1993, when measurements were made continuously for 24 h, the 24 h average was 59 mg CO2±C mÿ2 sÿ1, while the aver-age ¯ux for the period from 0700 to 1900 h CST was 65 mg CO2±C mÿ2 sÿ1. Thus, on this day, the 12 h average ¯ux was only about 10% greater than the 24 h average ¯ux. Therefore, the 12 h average ¯ux was used as an estimate of the 24 h average ¯ux on all other days.
3.2. 12 h versus midday ¯ux
Average ¯uxes calculated from 12 h and midday measurements were within 8% of each other on two dierent days in 1997 (Fig. 1). Standard deviations of 20 min and midday averages were similar to each other on each day, but varied considerably between days. On days in 1997 and 1998 when 12 h and mid-day ¯uxes were measured within 1 or 2 d of each other, average daily ¯uxes for the two measurement periods were essentially equal over a wide range of ¯uxes (Fig. 2). The overall average of all midday ¯uxes was less than 20% greater than the overall average of 12 h ¯uxes.
Thus average ¯uxes, calculated either from 12 h or midday measurements, were assumed to be representa-tive of 24 h averages and 12 h and midday averages were pooled into one data set to develop the equation to predict average daily ¯ux using soil temperature and soil water.
3.3. Seasonal soil CO2¯ux, soil temperature and soil
water
Estimated average annual soil CO2±C ¯uxes were 1.6, 1.3, 1.2, 1.0, 2.1 and 1.5 kg CO2±C mÿ2 yrÿ1 in 1993 through 1998, respectively. (Note: to calculate a yearly C ¯ux we used the average daily ¯ux calculated between d 91 and 304, which de®ned a period common to ¯ux measurements in all years (Fig. 3). This average was multiplied by 365 d to arrive at the average annual ¯ux. In some years, soil CO2¯ux measurements began earlier than those of soil temperature and water.) In 1998, when measurements were made for the entire year (Fig. 3), the average annual ¯ux over the entire period of record was 1.4 kg CO2±C mÿ2yrÿ1, which is only 5% less than the average for the shorter period as de®ned above (i.e. 1.5 kg CO2±C mÿ2yrÿ1). Thus, we estimate annual averages for 1993 through 1998, as shown above, are biased too large only by about 5% because they were averaged for slightly less than a full year.
Fig. 2. Average midday vs. average 12 h soil CO2±C ¯ux for selected
days in 1997 and 1998. The 1:1 and the linear regression lines are shown (slope=1.01,r2=0.77). Average midday and 12 h soil ¯uxes were 4.7 and 4.0 g CO2±C mÿ2dÿ1, respectively.
The greatest average ¯ux was in 1997, when precipi-tation was highest and most evenly distributed throughout the year (Table 1). Similarly, the smallest average ¯uxes were in 1995 and 1996, which had the least annual precipitation totals (Table 1). In all years, ¯ux was at a minimum during the winter and was maximum in early summer, although the time of maxi-mum varied slightly, likely due to soil water dierences (Fig. 3). In 1997, maximum ¯ux occurred considerably later, i.e. near d 200 (late July), while it was around d 160 (early June) in other years. The six very large ¯uxes around d 195 in 1997, which were unusually high relative to measurements immediately before and after in 1997 and in other years at this time of year, were thoroughly examined. They may have been due to increased root respiration resulting from active root growth associated with above-average precipitation (ca. 500 mm) from April through July 1997 (Table 1), warm soil temperatures (Fig. 3) and a peak in plant growth. Winter ¯uxes diered little among years, likely
because they were limited by low soil temperatures that were consistent between years.
Soil CO2±C ¯ux had a seasonal pattern that more closely resembled that of soil temperature than soil water (Fig. 3). On any date, there was little dierence in average daily soil temperatures for the 6 yr of measurement (Fig. 3). Contrary to ®ndings by Knapp et al. (1998), we did not observe a short-term increase in soil temperature following burning in February 1995, likely because vegetative regrowth was rapid due to high amounts of precipitation following the event.
In contrast to soil temperature, there were large dierences in soil water between years (Fig. 3), which were generally related to precipitation totals (Table 1). For example, water contents were much greater from about d 120 through 180 (May through June) in 1997 than in other years because precipitation was much above average in April through June 1997 (Table 1). Similarly, soil water content was high for the ®rst 90 d of 1998 (Fig. 3), associated with above average precipi-tation in late 1997 and early 1998 (Table 1).
3.4. Soil CO2¯ux predictions
The exponential relationship between ¯ux and soil temperature accounted for approximately 46% of ¯ux variability (Fig. 4). The apparent Q10, which is an esti-mate because factors other than soil temperature may also aect the activity of soil microorganisms and plant roots, was 2.4, which is similar to those calcu-lated by others (Kucera and Kirkham, 1971; Norman et al., 1992; Raich and Schlesinger, 1992; Howard and Howard, 1993; Raich and Potter, 1995).
As expected (Grahammer et al., 1991; Davidson et al., 1998; Knapp et al., 1998), there was greater scatter in the relationship between ¯ux and soil water (Fig. 4) than between ¯ux and soil temperature. A maximum ¯ux of about 7 g CO2±C mÿ2 dÿ1 was predicted at a volumetric soil water of about 0.25 (water-®lled pore space of 51%) and was near zero under extremely dry (water content=0.1) and wet (water content=0.7) con-ditions. Soil water accounted for 26% of ¯ux variabil-ity.
One reason for the greater amount of scatter between ¯ux and soil water (Fig. 4) may be because we measured soil water to a depth of only 15 cm, a shallow depth compared to the potential deep-rooting capacity of the prairie vegetation. Wetter soil con-ditions at greater depths may buer the eects of near-surface soil water de®cits on CO2 ¯ux (Singh et al., 1998).
3.5. Model calibration and validation
We combined the exponential soil temperature and quadratic soil water equations (Fig. 4) as a product in
Fig. 4. Average daily soil CO2±C ¯ux vs. soil temperature and
volu-metric soil water content (yv). For temperature, the equation of the line shown is:flux0:691e
0:078temp (r2=0.46, n= 216). For soil
water, the equation of the line is flux 383:63 yv ÿ 0:10 0:7ÿyv
2:66
(r2=0.26,n= 208). The numbers 0.10 and 0.70
a multiple, nonlinear regression to predict soil CO2±C ¯uxes. The equation is flux 6:42e
0:087temp 2:12 yv ÿ0:10 0:7ÿyv
1:46
, where ¯ux is soil CO2±C ¯ux (g CO2±C mÿ2dÿ1), temp is soil tempera-ture (8C) andyvis volumetric water content (m3mÿ3). Measured and predicted ¯uxes showed similar patterns through the year and were essentially equal (Fig. 5 and
Table 2), while about 52% of ¯ux variability was explained by the equation. The root mean square error (RMSE) between measured and predicted soil CO2 ¯uxes was only about 39% of the average ¯ux.
The average annual soil ¯ux of 1.7 kg CO2±C m ÿ2
yrÿ1from this study is about 3-fold greater than grass-land ¯uxes estimated by Raich and Schlesinger (1992), but similar to annual ¯uxes of 1.3±2.1 kg CO2±C mÿ2 yrÿ1 measured at the Konza Prairie in Kansas by Knapp et al. (1998) and annual ¯uxes of 1.1±1.3 kg CO2±C mÿ2yrÿ1measured at the same site by Bremer et al. (1998). Fluxes from our study site are likely to be among the greatest for grasslands given the high annual precipitation and even distribution of rainfall within the year (Table 1), long growing season and large soil organic C contents.
Maximum above-canopy ¯ux at this site, measured using micrometeorological techniques in 1993 and 1994, was about 5.4 g CO2±C mÿ2 dÿ1 (Dugas et al., 1999). Maximum above-canopy ¯ux CO2±C uptake occurred in May and June and is similar to the maxi-mum net ¯ux of 4.8 g CO2±C mÿ2 dÿ1 measured by Bremer et al. (1998) in early August, 1996, at the Konza Prairie. This above-canopy CO2±C ¯ux at this site is about equal to the soil CO2-¯ux measured at this time of year (Fig. 3) and suggests up to 50% of C ®xed by leaves originates from the soil (Monteith et al., 1964). Dugas et al. (1999) showed that much of this soil-based C was from root respiration.
We applied the above equation using soil tempera-ture and soil water to an independent data set col-lected at the Konza Prairie (Bremer et al., 1998). Surface soil CO2±C ¯ux and soil temperature and soil water (at 10 cm) were measured from June 1996 to
Fig. 5. Measured and predicted average daily soil CO2±C ¯ux. The
slope of the linear regression is 0.52.
Table 2
Average measured and predicted daily soil CO2±C ¯ux (g mÿ2dÿ1)
from our study site (n= 216) and from an independent data set from the Konza Prairie (n= 31, Bremer et al., 1998); the r2 and
root mean square error (RMSE) between measured and predicted ¯uxes are shown
Data set Measured Predicted r2 RMSE
This study 4.6 4.6 0.52 1.8
Konza 5.6 5.1 0.76 1.9
Fig. 6. Measured and predicted average daily soil CO2±C ¯ux for
the Konza Prairie. Measured ¯ux is from Bremer et al. (1998),
June 1997. This equation accurately predicted measured ¯uxes at the Konza (Fig. 6 and Table 2). Again, scatter was greater at larger ¯uxes, but the means were essentially equal (Table 2). The average ¯ux for the Konza (Table 2) was biased high because Bremer et al. (1998) made more measurements in the summer than in the winter and we used all their measurements to calculate the average. The r2 (0.76) was greater for the Konza data set (Table 2). These results suggest the equation developed from our data should accurately predict soil CO2±C ¯ux for other tallgrass prairies in the midwestern US.
Soil CO2±C ¯ux is a product of diverse abiotic and biotic environmental factors. The relationship we developed between ¯ux, soil temperature and soil water, while excluding plant-related eects such as bio-mass, growth stage, or morphology, still provided an accurate prediction of soil ¯ux for two locations.
Acknowledgements
We acknowledge the ®eld technical assistance of M. Heuer, R. Hicks, C. Miller, J.R. Rodriguez and C. Speed. This study was supported by a grant from the US Department of Agriculture, Agricultural Research Service.
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