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Free-Air Carbon Dioxide Enrichment of Soybean

Influence of Crop Variety on Residue Decomposition

^a USDA-ARS National Soil Dynamics Laboratory, 411 South Donahue Drive, Auburn, AL 36832
^b USDA-ARS Photosynthesis Research Unit, 1201 West Gregory Drive, Urbana, IL 61801
^c USDA-ARS, 232 National Soybean Research Center, 1101 West Peabody Drive, Urbana, IL 61801

^* Corresponding author (sprior{at}acesag.auburn.edu)

Received for publication April 30, 2005. Elevated atmospheric CO₂ can result in larger plants returning greater amounts of residue to the soil. However, the effects of elevated CO₂ 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 CO₂ 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 CO₂ 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 N₂ fixation impacted residue decomposition; total mineralized C was also slightly increased by FACE. Across CO₂ 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 CO₂ conditions. This limited number of varietal comparisons indicated that more work investigating varietal influences on soil C and N cycling under elevated CO₂ conditions is required.

Abbreviations: FACE, free-air carbon dioxide enrichment

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