Free‐Air Carbon Dioxide Enrichment of Soybean

Elevated atmospheric CO 2 can result in larger plants returning greater amounts of residue to the soil. However, the effects of elevated CO 2 on carbon (C) and nitrogen (N) cycling for different soybean varieties have not been examined. Aboveground residue of eight soybean [ Glycine max (L.) Merr.] varieties was collected from a field study where crops had been grown under two different atmospheric CO 2 levels [370 μmol mol −1 (ambient) and 550 μmol mol −1 (free‐air carbon dioxide enrichment, FACE)]. Senesced residue material was used in a 60‐d laboratory incubation study to evaluate potential C and N mineralization. In addition to assessing the overall effects of CO 2 level and variety, a few specific variety comparisons were also made. Across varieties, overall residue N concentration was increased by FACE, but residue C concentration was only slightly increased. Overall residue C to N ratio was lower under FACE and total mineralized N was increased by FACE, suggesting that increased N 2 fixation impacted residue decomposition; total mineralized C was also slightly increased by FACE. Across CO 2 levels, varietal differences were also observed with the oldest variety having the lowest residue N concentration and highest residue C to N ratio; mineralized N was lowest in the oldest variety, illustrating the influence of high residue C to N ratio. It appears (based on our few specific varietal comparisons) that the breeding selection process may have resulted in some varietal differences in residue quality which can result in increased N or C mineralization under elevated CO 2 conditions. This limited number of varietal comparisons indicated that more work investigating varietal influences on soil C and N cycling under elevated CO 2 conditions is required.