Prior, Stephen A. (USDA-ARS National Soil Dynamics Laboratory, Global Change Research, 411, South Donahue Drive, Auburn, AL, 36832; Phone: 334-844-4741 ext.143
Fax:  334-887-8597; Email: sprior@acesag.auburn.edu, sprior@ars.usda.gov)

Elevated Atmospheric CO₂ Effects on Residue Decomposition of Different Soybean Varieties

Stephen A. Prior *, H.A. Torbert, G.B. Runion, H.H. Rogers, D.R. Ort, R.L. Nelson

Elevated atmospheric CO₂ can result in larger plants returning greater amounts of residues to the soil. However, the effects of elevated CO₂ on carbon (C) and nitrogen (N) cycling for different soybean varieties has not been examined. Aboveground residue of eight soybean varieties (Glycine max [L.] Merr.) was collected from a field study where crops had been grown under two different atmospheric CO₂ levels [370 ppm (ambient) and 550 ppm (free-air carbon dioxide enrichment) (FACE)]. Senesced residue material was used in a 60-day laboratory incubation study to evaluate potential C and N mineralization. Residue N concentration was usually increased by FACE, but residue C concentration was not altered. Varietal differences were observed with the oldest variety having the lowest residue N concentration and highest residue C: N ratio. Residue C: N ratio was lower under FACE, which could be attributed to increased N fixation. Mineralized N was usually increased by FACE, except for a non-nodulating variety, suggesting that increased N fixation impacted residue decomposition. Mineralized N was lowest in the oldest variety illustrating the influence of high residue C: N ratio. Across varieties, mineralized C was increased slightly by FACE, however, differences in varieties suggest that the impact of elevated CO₂ on C mineralization could be influenced by soybean variety selection