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TECHNICAL REPORTS

Free-Air CO₂ Enrichment of Sorghum: Soil Carbon and Nitrogen Dynamics

^a USDA-ARS National Soil Dynamics Lab., 411 South Donahue Drive, Auburn, AL 36832
^b USDA-ARS U.S. Arid-Land Agricultural Research Center, Maricopa, AZ 85239. Names are necessary to report factually on available data; however, USDA does not guarantee or warrant the standard of the production. The use of the name by the USDA implies no approval of the product to the exclusion of others that may be suitable

^* Corresponding author (sprior{at}ars.usda.gov).

Received for publication May 25, 2007. The positive impact of elevated atmospheric CO₂ concentration on crop biomass production suggests more carbon inputs to soil. Further study on the effect of elevated CO₂ on soil carbon and nitrogen dynamics is key to understanding the potential for long-term carbon storage in soil. Soil samples (0- to 5-, 5- to 10-, and 10- to 20-cm depths) were collected after 2 yr of grain sorghum [Sorghum bicolor (L.) Moench.] production under two atmospheric CO₂ levels: (370 [ambient] and 550 µL L^–1 [free-air CO₂ enrichment; FACE]) and two water treatments (ample water and limited water) on a Trix clay loam (fine, loamy, mixed [calcareous], hyperthermic Typic Torrifluvents) at Maricopa, AZ. In addition to assessing treatment effects on soil organic C and total N, potential C and N mineralization and C turnover were determined in a 60-d laboratory incubation study. After 2 yr of FACE, soil C and N were significantly increased at all soil depths. Water regime had no effect on these measures. Increased total N in the soil was associated with reduced N mineralization under FACE. Results indicated that potential C turnover was reduced under water deficit conditions at the top soil depth. Carbon turnover was not affected under FACE, implying that the observed increase in soil C with elevated CO₂ may be stable relative to ambient CO₂ conditions. Results suggest that, over the short-term, a small increase in soil C storage could occur under elevated atmospheric CO₂ conditions in sorghum production systems with differing water regimes.

Abbreviations: FACE, free-air CO₂ enrichment