Carbon dynamics and microbial activity in tallgrass prairie exposed to elevated CO2 for 8 years

Alterations in microbial mineralization and nutrient cycling may control the long-term response of ecosystems to elevated CO2. Because micro-organisms constitute a labile fraction of potentially available N and are regulators of decomposition, an understanding of microbial activity and microbial biomass is crucial. Tallgrass prairie was exposed to twice ambient CO2 for 8 years beginning in 1989. Starting in 1991 and ending in 1996, soil samples from 0 to 5 and 5 to 15 cm depths were taken for measurement of microbial biomass C and N, total C and N, microbial activity, inorganic N and soil water content. Because of increased water-use-efficiency by plants, soil water content was consistently and significantly greater in elevated CO2 compared to ambient treatments. Soil microbial biomass C and N tended to be greater under elevated CO2 than ambient CO2 in the 5-15 cm depth during most years, and in the month of October, when analyzed over the entire study period. Microbial activity was significantly greater at both depths in elevated CO2 than ambient conditions for most years. During dry periods, the greater water content of the surface 5 cm soil in the elevated CO2 treatments increased microbial activity relative to the ambient CO2 conditions. The increase in microbial activity under elevated CO2 in the 5-15 cm layer was not correlated with differences in soil water contents, but may have been related to increases in soil C inputs from enhanced root growth and possibly greater root exudation. Total soil C and N in the surface 15 cm were, after 8 years, significantly greater under elevated CO2 than ambient CO2. Our results suggest that decomposition is enhanced under elevated CO2 compared with ambient CO2, but that inputs of C are greater than the decomposition rates. Soil C sequestration in tallgrass prairie and other drought-prone grassland systems is, therefore, considered plausible as atmospheric CO2 increases.