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DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION

Erosion Effects on Carbon Dioxide Concentration and Carbon Flux from an Ohio Alfisol

^a School of Natural Resour., The Ohio State Univ., 2021 Coffey Rd., Columbus, OH 43210 USA
^b USDA–NRCS Natl. Soil Survey Center, Federal Bldg. Rm. 152, 100 Centennial Mall North, Lincoln, NE 68508 USA

lal.1{at}osu.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Carbon dioxide concentrations in soil both reflect and influence soil biological activity and C flux to the atmosphere. We hypothesized that erosion affects CO₂ concentration and C flux from the soil surface because of its effects on soil temperature and water regimes. The soil air concentrations of CO₂ and corresponding temperature and water contents were monitored on slight (SLI), moderate (MOD), severe (SEV), and depositional (DEP) phases at 2-wk intervals between May 1996 and June 1997. The ambient soil CO₂ concentration and CO₂–C flux were determined using gas chromatograph analyses. Seasonal patterns in soil air CO₂ concentrations (ranging from a winter low of 0.56 mL L^-1 to a summer high of 20.90 mL L^-1) predominated over more subtle differences (20–80% variation) due to erosion phase effects. Significantly greater (by 12–37%) CO₂ concentrations for SEV and MOD phases over SLI and DEP were observed mainly during the summer. The effects of the erosion phase on soil CO₂ concentrations appeared to be indirect through its impacts primarily on soil temperature and, presumably, soil biological activity. Soil air CO₂ concentrations were significantly correlated with soil temperature (R² = 0.61) and CO₂–C (R² = 0.65) flux from the soil surface but not with soil water content. Both linear and second-order polynomial regression equations using soil temperature predicted soil CO₂ concentration.

Abbreviations: DEP, depositional • GC, gas chromatography • MOD, moderate • SEV, severe • SLI, slight • SOC, soil organic carbon

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
CARBON DIOXIDE CONCENTRATIONS IN SOIL AIR are a function of interactions involving biological (macro and micro) activity, aeration porosity, and soil temperature and water regimes. This dynamic component of soil varies spatially (vertically and horizontally), and temporally (diurnal and seasonal fluctuations), especially with regard to specific anthropogenic (plowing, residue management, irrigation) or climatic (rainfall, wind) events (Fernandez et al., 1993; Burton and Beauchamp, 1994; Osozawa and Hasegawa, 1995; and Yavitt et al., 1995). Soil CO₂ concentration influences the type and level of biological activity, gaseous flux to the atmosphere, and carbonate equilibria with soil solution moving downward or laterally through the profile to the aquatic ecosystem. The exchange of CO₂ between the pedosphere and the atmosphere is of particular interest because of the potential impact on greenhouse effect and global climate change (Duxbury et al., 1993; Lal et al., 1998a). Also, C sequestration in soil and reduction of CO₂ emission to the atmosphere by enhancing soil organic carbon (SOC) content is important to enhancing soil quality, improving agricultural sustainability, and mitigating the greenhouse effect (Bouwman, 1990; Lal et al., 1995a, 1995b; Lal et al., 1998b).

Soil air normally contains elevated CO₂ levels relative to the atmosphere (Brady and Weil, 1999). However, at any given time, CO₂ fluctuates depending primarily upon plant root and microbial respiration, which is influenced largely by soil temperature and moisture content (Lal et al., 1995b). Water table depth and its fluctuation also influence CO₂ concentration in soil air (Lal and Taylor, 1969). Yavitt et al. (1995) observed a wide range of CO₂ concentrations (from 350–19000 µL L^-1) at a 0.2-m depth for a northern hardwood ecosystem during mid-summer. Fernandez et al. (1993) noted a threefold increase in CO₂ concentration with depth, from 1023 µL L^-1 in the O horizon to nearly 3300 µL L^-1 in the C horizon of forest soil in Maine. Large variations in CO₂ concentrations, from 11000 to 26000 µL L^-1 (1.1–2.6%) at shallow depths of 0.1 to 0.3 m, were reported by Dudziak and Halas (1996) under wheat (Triticum aestivum), grass, and forest. The CO₂ concentration in soil air can be as great as 10 to 17% under extreme anaerobic conditions (Yamaguchi et al., 1967; Reicosky and Lindstrom, 1995). Reicosky (1998) reported that tillage methods had a significant effect on CO₂ flux, and that the magnitude of the flux was related to the tillage intensity. Moldboard plowing resulted in a maximum CO₂ flux in the fall and in the spring for a Minnesota soil.

Soil erosion adversely affects SOC content by direct removal of soil and reduction of the top soil depth; it also impacts plant growth and soil biological activity (Chengere and Lal, 1995; Izaurralde et al., 1998). The displaced sediment tends to be enriched in SOC and sedimentation may lead to increased SOC content of depositional soils (Frye et al., 1982; Fahnestock et al., 1995). Because of its varied effects on SOC and on soil biological and microbiological activity, erosion may have an important influence upon soil respiration, C flux to the atmosphere, and C sequestration in soil (Lal, 1995; Buyanovsky and Wagner, 1995; Bajracharya et al., 1998).

Based on the literature, CO₂ is a highly labile and variable component of soil air; however, there is a lack of information regarding soil erosion and deposition impacts on its behavior. Thus, the primary objectives of this study were to evaluate erosion phase effects on soil air CO₂ concentrations and CO₂–C flux from a Miamian soil in central Ohio, and to relate seasonal soil CO₂ concentrations to soil temperature and moisture regimes.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
The experimental site was located at the Horticulture and Crop Science Farm of the Ohio State University, Columbus (40°00' N, 83°02' W, 265-m elev.). The soil studied was a Miamian silt loam soil (fine, mixed, active, mesic Oxyaquic Hapludalf). Four areas along one transect of the field, which had been planted to corn (Zea mays L.) for about 20 yr, representing slightly (SLI) eroded, moderately (MOD) eroded, severely (SEV) eroded, and depositional (DEP) phases, were identified on the basis of Ap horizon thickness and profile characteristics (Soil Survey Staff, 1993). Soil air sampling tubes and static gas sampling chambers (Rolston, 1986) were installed in triplicate on each erosion phase. The soil air sampling tubes consisted of 0.2-m segments of 12.5-mm diam. PVC tubing sealed at the upper end with rubber septa. The open lower end of the tubes were inserted into the ground to a 0.1-m depth above a layer of fine gravel (to provide a porous surface for equilibration of soil atmosphere), and edges of the tubes were sealed with bentonite. The static gas sampling chambers were constructed from 0.15-m diam. PVC. Thermocouples and gypsum blocks were installed at a 50-mm depth adjacent to each gas sampling chamber and soil air tube to monitor temperature (Taylor and Jackson, 1986) and soil water (Gardner, 1986).

Soil air CO₂ concentrations and C flux from the soil surface were determined using gas chromatograph (GC) analysis (Anderson, 1982). Gas samples for soil air CO₂ concentrations were collected at mid-day on 2-wk intervals (except during winter months) between May 1996 and July 1997. Seasons were identified based on air temperature: summer (June through August), autumn (September–November), winter (December–February), and spring (March–May). Soil air samples and CO₂–C flux samples were drawn by piercing the rubber septa with a gas-tight syringe. Gas samples for C flux determinations were collected four times daily (at 0900, 1200, 1500, and 1800 h) 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, Palo Alto, CA). Simultaneous soil temperature and moisture were measured, while air temperature and precipitation data were obtained from the research farm weather station located a few hundred meters from the experimental site.

The gas samples, along with ambient atmospheric samples, were collected in Vacutainer (Becton Dickinson, Franklin Lakes, NJ) vials flushed with He carrier gas and analyzed in the laboratory using a Varian gas chromatograph (Model 3700). The peak area percentages for the CO₂ analysis on the GC output chart were used to calculate soil air CO₂ concentration and CO₂–C flux rates relative to the 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 µL L^-1. Daily values of C flux were calculated by averaging the hourly flux values and extrapolating over a 24-h period. Analysis of variance (ANOVA) was performed on the data using a repeated measures design in erosion phase and time of measurement for individual measurement dates (Littell, 1989). Seasonal trends in soil air CO₂ concentration and CO₂–C flux, soil temperature, and soil moisture were plotted against time (d from the beginning of measurement). The soil air CO₂ data were also correlated with CO₂–C fluxes from soil, soil temperature, and soil water content; and regression analyses were performed for soil temperature vs. CO₂ concentration (Gomez and Gomez, 1984). Statistical computations were performed using the Minitab Release 11 (Minitab, State College, PA) and MSTAT-C software packages (Michigan State Univ., East Lansing, MI).

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Variation of Soil Carbon Dioxide Concentration
Factors affecting soil biological activity govern the CO₂ concentration of the soil atmosphere. Among these, soil temperature, water content, substrate availability, and aeration appear to be most important, with the first two being influenced largely by season and climate. The results of this study confirmed the overriding seasonal trend in soil CO₂ concentrations but also revealed subtle, less distinctive, soil erosion phase effects (Fig. 1 and 2) . The concentration of CO₂ in soil air ranged from a low of 0.56 mL L^-1 during the winter and early spring to more than 20 mL L^-1 in the summer (June 1996) (Table 1) . Low CO₂ concentrations in soil air during the winter were attributed to low temperatures (Table 2) and frozen or partially frozen surface soil conditions, which inhibited microbial and plant root activity. The results were consistent with those of others who also noted significant seasonal trends in soil CO₂ concentrations (Burton and Beauchamp, 1994; Lal et al., 1995a; Osozawa and Hasegawa, 1995). By contrast, high soil air CO₂ concentrations during the summer months were attributed to stimulated soil biological activity, that is, microbial and root respiration, since high soil temperatures (Table 2) lead to CO₂ production that exceeds the flux from soil (Anderson, 1995; Osozawa and Hasegawa, 1995; Dudziak and Halas, 1996).

View larger version (34K): [in this window] [in a new window]   Fig. 1 Mean soil air CO2 concentrations for four erosion phases during different seasons of the year (summer: Jun.–Aug.; autumn: Sep.–Nov.; winter: Dec.–Feb.; spring: Mar.–May). Bars indicate standard deviation about the mean. Fisher's LSD values are same for comparison of erosion phase (small letters) and season (capital letters) means

View larger version (31K): [in this window] [in a new window]   Fig. 2 Variation of soil CO2 concentrations, mean diurnal CO2–C flux from the soil surface, soil temperature and soil moisture, averaged across all erosion phases during the study period from May 1996 to June 1997

Apart from temperature and moisture influences, soil CO₂ concentration is also likely to be determined, to some extent, by the availability of substrate and other external factors such as tillage, which increases aeration and incorporation of residue into the plow layer. Thus, CO₂ concentrations would be expected to increase following fall plowing or once temperatures increase in the spring with ample accumulated surface and incorporated residues. These trends were generally observed during our study as seen in Fig. 2. Other factors being equal, we hypothesized that less eroded (SLI) and depositional areas would have greater soil biological activity, and hence, higher CO₂ concentrations. A longer study duration and greater frequency of monitoring is needed to address this hypothesis.

Soil Temperature and Water Content
Soil temperatures at a 50-mm depth ranged from below 0°C during the winter to near 30°C in the summer (Table 2), exhibiting the characteristic lag effect and smaller amplitude of fluctuation compared with air temperature (data not included). The relative magnitudes of changes in soil temperature were generally similar for each erosion phase from one sampling date to the next. However, SEV and DEP phases had consistently higher temperatures than SLI and MOD during the warmer months (May through September). The high soil temperatures for SEV phases were attributed, in part, to the slope aspect and lower extent of vegetative cover (qualitative observation only), compared with SLI or MOD. In the case of DEP, however, differences in the soil temperature were thought to be due to the dark color of the soil and greater soil water contents compared with the other phases. Significant differences in soil temperature among the erosion phases were observed mainly during the spring and summer as seen from Fisher's LSD values (Table 2). Thus, the effect of the erosion phase appeared to be greatest when fluctuations were high, and insignificant during the winter because of near-freezing conditions during both the day and night.

Soil moisture showed no significant variation over sampling times except for a dry spell in late August 1996 and low values due to frozen soil conditions in January 1997. At all other times, the soil water contents ranged from about 17 to 19% by weight for SLI, MOD, and SEV phases (Table 2). The DEP phase, however, had consistently greater soil water contents by about 30 to 40%, relative to the other phases because of differences in landscape position and seasonally high water table.

It should be noted that while measurements could not be made due to logistic difficulties, nighttime differences in temperature and soil respiration may be important for overall diurnal fluctuations in CO₂ concentration and C flux. Furthermore, the lack of moisture effect on soil CO₂ was likely due to the fact that the temperate climate of Ohio did not have prolonged periods of soil water limitation but did have times (winter) when it was too cold for maximum respiration.

Regression Analysis of CO₂ Concentration with Soil Temperature and Water Content
The concentration of CO₂ in soil air closely followed the trends in soil temperature and C flux from the soil surface, but not that of soil moisture content (Fig. 2, Table 4) . A significant correlation was observed between CO₂ concentration and soil temperature for all erosion phases except for SEV, which was probably because of greater variability of CO₂ data and soil temperatures for this erosion phase than for SLI, MOD, and DEP. The results indicated a close relation between CO₂–C flux from soil and soil CO₂ concentrations, both, however, being largely dependent on soil temperature and not on soil moisture. This is consistent with the findings of other researchers (Anderson, 1995; Osozawa and Hasegawa, 1995; Lal et al., 1995b; Dugas et al., 1997; Hamada et al., 1996) Soil surface CO₂ flux may be a good indicator of soil respiration rate (Jensen et al., 1996) and of soil physical quality.

View larger version (18K): [in this window] [in a new window]   Fig. 3 First-order (bottom) and second-order (top) regression fit relationships for soil temperature vs. CO2 concentration across all erosion phases

	Conclusions

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
This study suggests that for central Ohio, seasonal patterns in soil air CO₂ concentrations predominate over more subtle differences because of erosion phase effects, which appeared to be indirect through alterations mainly of the soil temperature (and less so of soil moisture) regime, thus presumably affecting soil biological activity. More specifically, the following conclusions were derived:

1. Overriding seasonal fluctuations in soil air CO₂ concentration (ranging from 0.56 mL L^-1 in the winter to 20.90 mL L^-1 in the summer) existed, which were largely dependent on soil temperature.
   
2. Significant differences in soil air CO₂ concentration due to the erosion phase (means ranging from 0.77 mL L^-1 for DEP to 8.98 mL L^-1 for MOD) occurred mainly at times of high CO₂ concentrations, that is, during the summer.
   
3. Erosion phase significantly influenced soil temperature (typically 5–20% greater for SEV and DEP than other phases) and water contents (generally 7–45% greater for DEP than other phases).
   
4. Soil air CO₂ concentrations were significantly correlated with soil temperature (R² = 0.61) and CO₂–C flux from the soil surface (R² = 0.65) but not with soil moisture.
   
5. Linear equations, as well as quadratic regression equations, using soil temperature predicted soil CO₂ concentration, both giving similarly low R² values.
   

From a practical standpoint, the results of this study suggest that erosion phase effects on soil CO₂ concentration, and hence, C flux, may be more important under some conditions (i.e., depending on slope, aspect, climate, and vegetation) and at certain times of the year (e.g., spring and summer) compared with others. This could have implications for the management effect on C sequestration in soils.

Received for publication January 29, 1998.

	REFERENCES

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