Soil Science Society of America Journal 64:286-293 (2000)
© 2000 Soil Science Society of America
DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION
Diurnal and Seasonal CO2–C Flux from Soil as Related to Erosion Phases in Central Ohio
R.M. Bajracharyaa,
R. Lala and
J.M. Kimbleb
a School of Natural Resources, The Ohio State Univ., 2021 Coffey Rd., Columbus, OH 43210 USA
b National Soil Survey Center, USDA-NRCS, 100 Centennial Mall North, Rm. 152., Lincoln, NE 68508-3868 USA
rmbajra{at}ccsl.com.np
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ABSTRACT
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The CO2–C flux from soil is an important part of the global C cycle, whose dependence on erosion is as yet largely unknown. We hypothesized that the magnitude of diurnal and seasonal C flux is related to soil erosion phase and is influenced by soil water and temperature regimes. The CO2–C flux from the surface of a Miamian silt loam soil (fine, mixed, active, mesic Oxyaquic Hapludalf) in central Ohio was monitored using the static chamber method. Gas samples were collected four times daily (at 0900, 1200, 1500 and 1800 h) at 
2-wk intervals between July 1995 and June 1997. The CO2 evolved from the soil surface varied significantly among seasons ranging from 0 g C m-2 d-1 in the winter to 1.6 g C m-2 d-1 during the summer. Soil erosion phase had no direct effect on C flux from the soil, although depositional areas had 20 to 25% higher water content than other phases, while severely eroded and depositional areas generally had higher soil temperatures than slightly and moderately eroded phases. Soil C flux exhibited diurnal variations with high values differing from lows by as much as 0.04 g C m-2 h-1. Peak flux rates as high as 0.082 g C m-2 h-1 occurred during the mid afternoon during the spring, summer, and autumn seasons. Soil C flux was significantly correlated with soil temperature
and air temperature
but not with soil moisture content.
Abbreviations: CT, conventional tillage • DEP, deposition phase • GC, gas chromatograph • NT, no-tillage • MOD, moderately eroded • PVC, polyvinyl chloride • SEV, severely eroded • SLI, slightly eroded • SOC, soil organic C
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INTRODUCTION
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THE FLUX of C from soil is an important part of the terrestrial C budget and the global C cycle. Carbon released from the soil, mainly as CO2 under aerobic conditions, also impacts global climate change by enhancing the greenhouse effect (Bouwman, 1990; Yaalon, 1993; Lal et al., 1995a). The soil plays an important role in sequestration of atmospheric C, as well as in the emission of radiatively active trace gases. Agricultural practices contribute an estimated 25, 65, and 90% of total anthropogenic emissions of CO2, CH4, and N2O, respectively, and could contribute between 28 and 33% of the radiative forcing in the next century if emission of these gases are doubled (Duxbury, 1994, 1995).
Land use and soil management practices significantly impact soil organic C (SOC) dynamics and C flux from the soil (Paustian et al., 1995; Bajtes, 1996), although the mechanisms and processes of C sequestration in soil are not completely understood (Lal et al., 1995b; Bajracharya et al., 1998). Many studies have focused on CO2 evolution from forestland or pasture, and numerous have involved monitoring CO2 flux from croplands. However, relatively little information is available about land use effects on C flux from soil, with particular regard to erosion's impact. Soil erosion is likely to influence C flux and dynamics through its effects on a number of processes. These include (i) removal of SOC-rich topsoil and exposure of low SOC subsoil, (ii) burial of soil by sediment deposition, (iii) texture change associated with subsoil exposure and subsequent mineral–organic matter interactions, (iv) microbial activity due to soil moisture and temperature changes, and (v) plant growth and residue amounts.
Franzluebbers et al. (1995) studied the effect of tillage method and crop rotation on CO2 evolution from a silty clay loam soil in south-entral Texas. Mean CO2 evolution rates for sorghum [Sorghum bicolor (L.) Moench]–wheat (Triticum aestivum L.)–soybean [Glycine max (L.) Merr.] and wheat–soybean rotations under conventional tillage (CT) ranged from 1.55 to 2.45 g C m-2 d-1. No-till (NT) treatment resulted in 9 to 12% higher evolution rates for sorghum–wheat–soybean rotations compared with CT. Soil temperature, soil moisture, and day of the season (reflecting residue decomposition) explained 65 to 98% of the temporal variation. The study indicated that conversion from CT to NT increased C sequestration in soil, but soil under NT released similar or greater amounts of C as evolved CO2, depending on the cropping sequence.
Lal et al. (1995b) in central Ohio observed CO2 flux during midsummer to be highest (0.80 g C m-2 h-1) for alfalfa (Medicago sativa L.), followed by corn (Zea mays L.) and woodland (0.37 g C m-2 h-1), although CO2 concentration in soil air at the 150-mm depth was in the order: woodland (1113 µL L-1) > alfalfa (1081 µL L-1) > corn (851 µL L-1). Ridge-till corn had higher CO2 emissions than plow-till corn, that is, 0.48 vs. 0.41 g C m-2 h-1 (Lal et al., 1995b). In a study to assess the impact of fall tillage on CO2 flux, Reicosky and Lindstrom (1993, 1995) reported highest flux rates and 19-d cumulative CO2 evolved from moldboard-plowed soil and the lowest from no-till treatments. High initial flush of CO2 immediately following tillage was attributed to release of CO2 in pores and from solution or rapid direct oxidation of C substrates rather than to residue incorporation. Shallow spring cultivation of fall-tilled treatments led to similar relative flux rates among tillage methods but a smaller magnitude of CO2 flux, which was attributed to the preceding season's microbial activity rather than to temperature or moisture differences (Reicosky, 1998).
Apart from CO2, agricultural systems significantly affect the emission of radiatively active gases such as CH4, N2O, and NOx, which may account for 20% of the anticipated atmospheric warming (Mosier et al., 1991; Duxbury et al., 1993). However, the influence of agricultural practices on fluxes of these gases is complex and varied, with often opposing effects depending on fertilizer, water, and residue management (Mosier et al., 1991; Lauren and Duxbury, 1993).
Our literature survey shows a lack of information concerning the effects of erosion on C flux from soil. We hypothesized that the magnitude of diurnal and seasonal C flux is related to soil erosion phase and is influenced by soil water and temperature regimes. The objectives of this study, therefore, were to quantify the magnitude of diurnal and seasonal variations in CO2–C flux from an agricultural soil in central Ohio in relation to soil erosion phases and to relate C flux with soil moisture and temperature.
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Materials and methods
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The study was conducted in a field planted to corn at the Horticulture and Crop Science Farm of The Ohio State University, Columbus, OH (40°00' N, 83°02' W, 265 m). The field had been planted to corn for 20 yr prior to this study, with autumn plowing following harvest in October and planting in late April or early May. During the study period the experimental plot areas were not planted but left fallow with occasional spraying to control weeds. On the basis of Ap horizon thickness and profile characteristics such as, soil color, texture, and depth to Bt horizon (Soil Survey Staff, 1993), four soil erosion phases—slight (SLI), moderate (MOD), severe (SEV), and deposition (DEP)—were identified on a Miamian silt loam soil across a transect of the field. Soil bulk density and organic C data are shown in Table 1
. Three plot areas of 1 m2 each, randomly selected 2 to 3 m apart, were demarcated for installation of gas samplers and temperature and moisture probes. The erosion phases ranged from 20 to 100 m apart across the field. After establishment, the plots with gas chambers installed were neither plowed nor planted to avoid uprooting or otherwise disturbing the chambers and probes. Subsequent harvesting and planting operations were conducted around the plots, leaving border areas of 1 m on all sides of the plot. Although the plots were thereafter maintained under fallow conditions with periodic spraying to minimize weed growth, there was constant weed growth and at times weeds covered the plot areas.
Static gas sampling chambers constructed from 0.15-m-diameter polyvinyl chloride (PVC) pipe (Rolston, 1986) were installed to a depth of 0.1 m in triplicate in three 1-m2 plots on each erosion phase. Adjacent to each gas sampling chamber, a thermocouple for soil temperature measurement and gypsum block for soil moisture monitoring were installed at the 50-mm depth. Each gypsum block was calibrated by measuring gravimetric soil water content for a range of different moisture conditions from 0.1 to 0.3 g g-1 (near field capacity). Standard curves for each block were constructed to calculated moisture contents on the basis of electrical resistance measurements. This technique appeared to be satisfactory for most moisture conditions except the driest and frozen soil conditions.
Static gas chamber techniques tend to underestimate gaseous flux from soil surfaces because of alteration of the microclimate, air pressure differences within and without the chamber, and reduction in turbulent air flow above the soil surface (Hutchinson and Livingston, 1993; Rochette et al., 1992). It is, nonetheless, a simple and rapid method of determining gaseous flux from soils as long as care is taken in the design, installation, sampling, and analyses of gas samples to minimize experimental and analytical biases (Hutchinson and Livingston, 1993).
Carbon flux from the soil surface, as CO2 evolved, was monitored using the static chamber method with gas chromatograph (GC) analysis (Anderson, 1982). Gas samples were collected four times daily (at 0900, 1200, 1500, and 1800 h) at 2-wk intervals (except during winter months) between July 1995 and June 1997. The samples were drawn by means of a gas-tight syringe from the chamber 30 min after PVC caps with septa were placed over the open chamber tops. The gas samples, along with ambient atmospheric samples, were collected in glass vials (flushed with He carrier gas to eliminate any prior contamination) and brought back to the laboratory for analyses using a Varian Gas Chromatograph (Model 3700, Varian, Sunnyvale, CA). Simultaneously, soil temperature and moisture were recorded for each measurement time, and air temperature and precipitation data were obtained from the research farm weather station located a few hundred meters from the experiment site.
The peak area percentages for the CO2 peaks on the GC output chart were used to calculate C flux rates relative to CO2 concentration of ambient atmospheric CO2. The ambient CO2 concentration was measured in quadruplicate at each flux measurement time and the measurements averaged for each day. This average atmospheric CO2 concentration was generally close to 350 mmol mol-1. The data were reported in grams C per square meter per hour (diurnal flux) and grams C per square meter per day (seasonal flux). Daily values of C flux were calculated by averaging the hourly flux values and extrapolating across a 24-h period. Statistical analysis for computing the analysis of variance was performed on the data as a randomized complete block design for erosion phase (Factor A) with time of measurement (Factor B) for individual measurement dates as a split plot on erosion phase (Gomez and Gomez, 1984). Seasonal trends in C flux, soil temperature, and soil moisture were plotted against time (measurement date). The C flux data were also correlated with soil temperature, soil moisture, and mean air temperatures, and regression analyses were performed (Steele and Torrie, 1980).
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Results and discussion
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Seasonal Carbon Flux
Mean daily C flux from the Miamian silt loam soil in central Ohio ranged from near zero during January and February to 1.58 g C m-2 d-1 for July 1995 (Table 2)
. Mean daily C flux during the growing season (May through September) ranged from 0.6 to 1.58 g C m-2 d-1, which was similar to those reported in other studies (Feigl et al., 1995; Franzluebbers et al., 1995; Raich and Potter, 1995). Primarily soil temperatures (Table 2) governed seasonal variations in C flux, with high values during the summer and low values during the winter when soil biological activity is minimal due to near-freezing soil temperatures. Release of CO2 from soil biological and microbial respiration is highest at moderately high soil temperatures with adequate soil moisture and substrate C (Kirschbaum, 1995; Follett, 1997). The cyclical pattern with summer highs and winter low C flux was observed for two successive years and corresponded closely with soil temperatures, despite considerable variability of the data (Fig. 1) . Buyanovsky and Wagner (1995) observed similar trends in soil respiration for ecosystems in Missouri.
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Table 2 Mean air temperature, precipitation, soil temperature, soil moisture, and C flux for each measurement date between July 1995 and June 1997
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Fig. 1 Seasonal CO2–C flux, soil temperature, and soil moisture content variation for four erosion phases
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Soil moisture content as measured by gypsum blocks did not have a primary influence on mean C flux (Fig. 1, Table 3)
. This differed from the findings of Feigl et al. (1995) who reported that for a tropical pasture during the dry season, soil moisture was a better predictor than was soil temperature. Unlike in tropical or subtropical climates, where temperatures can remain high throughout the year but soil moisture becomes limiting for biological activity, in central Ohio the soil remained moist during the fall and winter, but temperature became too low for maximum respiration. Hence, moisture was not expected to be related strongly to C flux in our case. In general, mean soil moisture contents did not vary markedly during the year, with the exception of a few dry periods, and ranged from 0.13 to 0.21 g g-1 (Table 2). There was no significant variation in soil moisture at times of flux measurement. Due to the low measured variation in soil moisture content at our site, we had little power to test its influence on CO2 evolution from soil. The occasional low soil moisture values occurred only at two or three sampling times during a dry spell or in winter (frozen surface soil). However, among different erosion phases, DEP typically had the highest soil moisture contents, ranging from 0.20 to 0.30 g g-1 (Fig. 1).
Erosion Phase Effects on Carbon Flux
Soil erosion phase did not have any clear or consistent effects on C flux (Fig. 2)
. For most measurement dates statistical analyses indicated no significant differences among the four phases, reflecting high variability of data (CVs ranged from 21.5 to 180%, data not shown). Where differences were observed, the results were inconsistent. Although erosion did not appear to have a direct impact on C flux, we hypothesized that it had an indirect impact through its effects on soil moisture, soil temperature, residue inputs, and SOC contents, which all affect soil respiration (Anderson, 1995). One-time measured SOC data (Table 1) did not show any significant or consistent relationship with C flux from the various erosion phases.
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Fig. 2 Erosion phase effects on seasonal CO2–C flux between July 1995 and June 1997. Bars indicate Fisher's LSD where significant at P < 0.10
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Differences in soil moisture contents and temperature among the erosion phases during different times of the day and sampling dates might suggest slight differences in C flux from the various phases. For instance, DEP phases almost always had higher moisture contents (Fig. 1), which would lead to a greater lag in soil temperature as opposed to air temperature, compared with the other erosion phases. On the other hand, eroded areas had greater extremes in soil temperature because of exposure of the surface and less vegetative cover than other phases. Thus, we hypothesized that SEV phases would have more variable data and perhaps lower C flux during cool periods and higher values during warm periods than other phases; however, the data did not support these hypotheses.
Diurnal Variation in Carbon Flux
Significant diurnal variations in C flux from the soil were observed in this study. There was a general trend of somewhat lower values in the early mornings and late evenings, with peak C flux occurring around midday or mid afternoon (Fig. 3)
. This trend was accentuated during the spring and autumn when nighttime air temperatures were considerably lower than daytime highs (data not shown). Evening (1800 h) measurements were frequently higher than early morning (0900 h) measurements perhaps because of a lag effect in soil temperatures relative to air temperatures. Differences in C flux were less marked during the winter because of low values (near zero) and during the late autumn and early spring when greater fluctuations in temperature led to higher variability. The data set on the whole exhibited considerable variability, making interpretations of the temperature response complicated.
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Fig. 3 Diurnal variations in seasonal CO2–C flux averaged across four erosion phases between July 1995 and June 1997. Bars indicate Fisher's LSD where significant at P < 0.10
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Relationship between Soil Temperature, Soil Moisture, and Carbon Flux
Correlation analysis revealed a good relationship between C flux and temperature, but not between C flux and moisture (Table 3). Both mean soil temperature and mean air temperature were significantly
correlated (r2 ranging from 0.80 to 0.81) with C flux regardless of whether mean hourly or mean daily rates were used. Mean air temperatures were also highly correlated with soil temperature (50-mm depth) as seen from the correlation coefficient (r2) of 0.96
in Table 3. Thus, under the conditions of this study, it was predominantly temperatures that affected soil biological activity, and hence, soil respiration. Soil moisture, however, varied relatively little, with 90% of mean observations between 0.17 and 0.21 g g-1, and was poorly correlated with C flux (r2 0.10).
Regression analyses revealed that a number of linear and nonlinear functions predicted the effect of soil temperature on CO2–C flux reasonably well for both diurnal and hourly flux rates (equations not shown). Best-fit relationships were provided by second-order polynomial (Fig. 4 and 5)
curves, giving r2 of 0.784 for mean hourly and 0.832 for average daily C flux, respectively. The data exhibited higher scatter as temperatures increased, with maximum scatter occurring in the temperature range of 20 to 28°C. This probably reflected increasing spatial variability of soil biophysical conditions and variation of microclimate in the vicinity of the soil surface (a few centimeters above and below the surface) at higher soil temperatures. Thus, square-root transformation of the C flux data was found to give the most uniform distribution of data points for regression fit of the equations, while other transformations (e.g., log, lognormal, exponential) did not. A small positive y-intercept seen for both curves (Fig. 4 and 5) indicated a small C flux (near zero) at freezing or subzero temperatures. This was in agreement with actually observed C flux values during the winter months. On the other hand, soil moisture content did not predict C flux well, and its addition in a multiple regression equation did not improve prediction capacity compared with regression with temperature alone.
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Fig. 5 Plot of curve fit line for average daily C flux vs. soil temperature data across all erosion phases
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Conclusions
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Clear seasonal variations in CO2–C flux were observed for the central Ohio soil despite high variability of the data. No distinct trends were seen in C flux from different soil erosion phases. Additional work is required to establish clear cause–effect relationships between the severity of soil erosion and C flux. The following general conclusions can be drawn from this study:- Carbon flux, as CO2 evolved from the soil surface, varied significantly among seasons, ranging from 0 g C m-2 d-1 in the winter to 1.6 g C m-2 d-1 during the summer.
- Soil erosion phase had no clear effect on CO2–C flux from the soil.
- Soil C flux exhibited diurnal variations, with peak flux rates occurring during the mid afternoon.
- Soil C flux was significantly correlated with soil (and air) temperature but not with soil moisture.
- A curvilinear relationship was observed between C flux and soil temperature with second-order polynomial equations giving the best fit.
Fernandez Son Kraske Rustad David 1993; Korner Arnone 1992; Raich Nadelhoffer 1989; Raich Schlesinger. 1992; Vose Elliott Johnson Walker Johnson Tingey 1995
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ACKNOWLEDGMENTS
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The authors thank Dr. Warren Dick and Pierre Jacinthe for providing the gas sampling chambers. Financial support for this project provided by USDA-NRCS, and the National Soil Survey Center, Lincoln, NE is gratefully acknowledged.
Received for publication November 5, 1997.
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REFERENCES
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ABSTRACT
INTRODUCTION
