CROP RESIDUE DECOMPOSITION AS AFFECTED BY GROWTH UNDER ELEVATED ATMOSPHERIC CO2

Increasing atmospheric CO2 level has led to concerns about process cha nges in the biosphere. Elevated atmospheric CO2 concentration has been Shown to increase plant biomass, resulting in greater amounts of resi due returned to soil. However, the effects on long-term storage of C i n soil are highly debated. Changes in both quantity and quality of pla nt residue, as well as residue management, may alter soil C and N dyna mics that will, in turn, affect the ability of soil to store C. Plant residues were collected from an experiment using open top chambers to increase CO2 levels under field conditions. A soil incubation study wa s conducted with a Blanton loamy sand (loamy siliceous, thermic, Gross arenic Paleudults) to examine the effect of residue additions to two c rop species (soybean, Glycine max (L.) Merr. and grain sorghum, Sorghu m bicolor (L.) Moench), grown at two CO2 concentrations (ambient and t wice ambient), and two incorporation treatments (incorporated or surfa ce placement) on potential C and N mineralization. The difference in b iomass inputs between plants grown in ambient and elevated atmospheric CO2 was also considered. Simulated residue incorporation reduced inor ganic N concentration but had no effect on C mineralization. Both inor ganic N content and C mineralization were higher with soybean than wit h grain sorghum. Although changes to both plant residue quality and qu antity caused by elevated CO2 concentration affected C cycling in soil , residue quality may be more important for determining C storage. Nit rogen cycling in soil may be a controlling factor for C storage in ter restrial ecosystems.