Tuesday, November 14, 2006

Changes in Soil Organic Matter and Inorganic C after Five Years of Doubled CO2 in a Semi-Arid Grassland.

Elise Pendall1, Jean Reeder2, Arvin Mosier2, and Jack Morgan3. (1) Univ of Wyoming, 1000 E University Ave #3165, Laramie, WY 82071, (2) USDA-ARS (retired), 862 Panorama Place, Windsor, CO 80550, (3) USDA-ARS Crops Res. Lab., 1701 Center Ave, Fort Collins, CO 80526

Elevated atmospheric CO2 may lead to increased C sequestration in terrestrial ecosystems if the C is stored in long-lived pools such as organic matter and soil minerals.  Above- and belowground primary production were stimulated by elevated CO2 in an open-top chamber (OTC) experiment on a semi-arid grassland ecosystem in northeastern Colorado. We evaluated responses of soil organic and inorganic C after five years of experimental treatment by excavating soil monoliths to 75-cm depth. Soil organic matter (SOM) was partitioned into water extractable, particulate and mineral associated portions, and organic and inorganic C content and isotopic composition were determined. We found no change in total SOM, but water soluble organic C was higher under the elevated CO2 treatment. The storage rate of new organic C, estimated from stable C isotopes, averaged 30 g C m-2 y-1 in ambient chambers and 75 g C m-2 y-1 in elevated CO2 chambers.  Inorganic C (IC) content increased substantially below 40-cm depth under elevated CO2, amounting to a doubling of the inorganic C content after 5 years. IC storage rate averaged 27 g C m-2 y-1 in ambient and 52 g C m-2 y-1 in elevated chambers, demonstrating the importance of this C pool in semi-arid systems. Total soil N content decreased by 70 g m-2 in the top 60-cm after 5 years of elevated CO2, suggesting “mining” of soil N to support increased plant growth. Soil electrical conductivity and pH also changed in response to the 5-year OTC experiment. Elevated CO2 thus affected the total soil environment, in addition to doubling C sequestration rates.