ARS | Publication request: DYNAMICS OF SOIL ORGANIC MATTER TURNOVER UNDER ELEVATED CO2

Submitted to: Ecological Society of America Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: June 2, 2003
Publication Date: July 21, 2003
Citation: PENDALL, E., KING, J., MOSIER, A.R. DYNAMICS OF SOIL ORGANIC MATTER TURNOVER UNDER ELEVATED CO2. ECOLOGICAL SOCIETY OF AMERICA PROCEEDINGS. 2003.

Technical Abstract: Some grassland ecosystems have been shown to store carbon as a result of exposure to elevated CO2 , but dynamics of soil organic matter (SOM) turnover rates are expected to vary with time and contribute variability to net ecosystem production estimates. A 5-year-long elevated CO2 experiment was conducted on the shortgrass steppe in Colorado using open-top chambers (OTCs). Above- and below-ground biomass increased by 15 to 35% under doubled atmospheric CO2 concentrations. Decomposition rates also increased, as indicated by stable isotope partitioning of soil respiration rates. We collected soil samples from the OTC experiment after 1, 2, and 3 years of exposure to elevated CO2 to investigate the early response of SOM pool sizes and turnover rates. Long-term laboratory incubations (up to 1 year) were used to investigate changes in active and slow pool sizes and turnover rates, and stable C isotopes were measured to evaluate changes in turnover rates of recently fixed carbon. After just one year, we found significant increases in active and slow pool sizes and turnover rates of both SOM pools. However, the increases diminished somewhat over the next two years. The turnover rate of new active pool SOM (determined by isotope labeling) was significantly lower under elevated than ambient CO2, suggesting that increasing litter C:N ratios under elevated CO2 will eventually slow decomposition rates, and potentially limit N availability. Microbially respired CO2 had significantly lower d13C values than did bulk soil C, owing to an increase in C3 biomass on all treatments. The use of fossil fuel CO2 in the elevated CO2 treatment added an additional 13C depleted tracer to the ecosystem. After 3 years of exposure to elevated CO2, d13 C of bulk soil C decreased by 2 permil, while d13C of microbial C decreased by 10 permil. These findings suggest that net ecosystem C storage is likely to change over time, possibly increasing with continued exposure to elevated CO2.