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

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.