White, John (ARS, Beltsville Agricultural Research Center,10300 Baltimore Avenue, BLDG 306, BARC-East, Beltsville, MD, 20705; Phone: 301-504-8101; Fax: 301-504-8162; Email: whitej@ba.ars.usda.gov)

Soil Carbon Dioxide Flux in Conventional and Organic Cropping Systems: Comparison of Measurement Methods and Relationship with Soil Moisture

J.W. White *, M.A. Cavigelli, L.J. Sikora

Accurate measurement of soil carbon dioxide (CO₂) flux is necessary to evaluate the effects of cropping systems on global warming potential and to provide accurate estimates of carbon (C) budgets. Soil CO₂ fluxes, soil temperature and volumetric water content (VWC) were measured in no till, chisel till and organic cropping systems in order to compare two flux measurement methods and to investigate the effects of soil moisture on CO₂ flux under different management regimes. Static versus dynamic closed chamber methods, both using infrared gas analysis to measure CO₂ concentrations, were compared in the no till and organic systems. Flux measurements from the chisel till system were made with static chambers only. Cumulative CO₂ flux by the static method for the period April to December was highest in the organic system (12.88 ± 1.10 g CO₂ (m²*hr)^-1), intermediate in the chisel till system (10.38 ± 0.63 g CO₂ (m²*hr)^-1), and lowest in the no till system (8.62 ± 0.55 g CO₂ (m²*hr)^-1). In comparison, cumulative CO₂ flux by the dynamic method for the period April to October was 13.31 ± 0.62 g CO₂ (m²*hr)^-1 in the organic system and 7.68 ± 0.35 g CO₂ (m²*hr)^-1 in the no-till system. While both methods described the same overall patterns of CO₂ flux over time, the dynamic chambers gave almost consistently higher readings than the static chambers (static method = 0.4583(dynamic method) + 0.1278). The discrepancy between methods may be due to a greater reduction in the diffusion gradient of CO₂ using the static versus the dynamic method since chambers were covered for a slightly longer period of time using the static method (12 min) than using the dynamic method (less than or equal to 2 min). We investigated the influence of soil moisture on CO₂ flux standardized to 25^oC using data collected by the dynamic method and found that no till system soils responded differently than organic system soils to changes in soil moisture. Maximum CO₂ flux occurred at 20.0% VWC (~38.4% water filled pore space (WFPS)) in the organic system while it occurred at 27.6% VWC (~52.5% WFPS) in the no till system. Also, at soil moisture concentrations lower than 29.1% VWC, CO₂ flux was significantly greater (P less than 0.05) in the organic system than in the no till system. There were no differences in CO₂ flux between these two systems at moisture contents between 29.2% and 39.7% VWC. At soil moisture contents of 39.8% VWC and above CO₂ flux was significantly greater (P less than 0.05) in the no till system. Greater CO₂ flux under organic management is likely due to the greater availability of labile C. We found significantly greater (P less than 0.05) concentrations of dissolved organic C (DOC) in the organic system soil (62.2 ± 6.23 mg DOC kg^-1) than in the no till system soil (41.0 ± 3.43 mg DOC kg^-1). The different soil moisture levels at which maximum CO₂ flux occurred in the two systems is likely due to significantly greater (P less than 0.001) soil porosity, possibly attributable to an increase in the proportion of macropore space, in the no till system soil (0.48 ± 0.0039) than in the tilled organic system soil (0.42 ± 0.0085). Thus, as soil moisture increases beyond 20.0% VWC, gas exchange is limited in the organic system soil. In contrast, gas exchange is not limited in the no till soil until moisture reaches 27.6% VWC.