COMPOSITION AND DECOMPOSITION OF SOYBEAN AND SORGHUM TISSUES GROWN UNDER ELEVATED ATMOSPHERIC CARBON-DIOXIDE

It has been hypothesized that changes in both quantity and quality of plant residue inputs to soils as atmospheric carbon dioxide (CO2) conc entration increases may alter carbon (C) and nitrogen (N) turnover rat es and pool sizes, We determined the effect of elevated atmospheric CO 2 on plant tissue quality, and flow modifications in tissue quality af fect C and N mineralization. Soybean [C-3; Glycine max (L.) Merr. cv. Stonewall] and sorghum [C-4; Sorghum bicolor (L.) Moen, cv. Savanna 5] were grown under elevated (704.96 +/- 0.33 mu mol CO2 mol(-1)) and am bient (357.44 +/- 0.12 mu mol CO2 mol(-1)) atmospheric CO2 in open-top chambers, Leaf and stem tissues were separated from harvested plants and analyzed for C, N, lignin, and cellulose. Tissues were applied to Norfolk loamy sand (fine-loamy, siliceous, thermic Typic Kandiudult) a nd aerobically incubated for 70-d to determine C and N mineralization, C turnover, relative N mineralization, and C/N mineralized. Elevated CO2 had no effect on plant residue C concentration, but N concentratio n of soybean leaves and stems and sorghum stems was reduced; however, CO2 enrichment increased C/N ratio and lignin concentration for only s orghum stems and soybean leaves, respectively. Source of plant residue (i.e., produced under either elevated or ambient CO2) had no impact o n soil C turnover, relative N mineralization, cumulative C and N miner alization, and C/N mineralized, These data suggest that increasing atm ospheric CO2 will have little effect on composition or decomposition o f field crop residues. Thus, since CO2 enrichment results in increased photosynthetic C fixation, the possibility exists For increased soil C storage under field crops in an elevated CO2 world.