Free‐air Carbon Dioxide Enrichment of Wheat: Soil Carbon and Nitrogen Dynamics

The predicted positive impact of elevated atmospheric carbon dioxide (CO2) concentration on crop biomass production suggests that more C will reach the soil. An aspect of soil C sequestration that requires further study is the effect of elevated CO2on C and N dynamics; this relationship is the key to understanding potential long‐term C storage in soil. Soil samples (0–5, 5–10, and 10–20 cm increments) were collected after 2 yr of wheat (Triticum aestirumL.) production under two CO2levels [370 (ambient) and 550 µL L−1(free‐air CO2enrichment)(FACE)] and two water treatments [100% of ET replaced (wet) and 50% of ET replaced (dry)] on a Trix clay loam [fine, loamy, mixed (calcareous), hyperthermic Typic Torrifluvents] at Maricopa, AZ. Organic C, total N, potential C and N mineralization, and C turnover were determined during a 60‐d incubation study. Organic C content increased at all three soil depths under FACE and the total N content increased at the 5 to 10 and 10 to 20 cm depths. In general, increased N mineralization under dry conditions corresponded well with patterns of higher C mineralization and turnover. Nitrogen mineralization was unaffected by CO2treatment, indicating that factors other than N may limit C mineralization and turnover. Soil respiration and C turnover patterns were not affected by CO2treatment level at the 0 to 5 cm depth; however, these measures were lower under FACE at the lower depths. Soil respiration and C turnover at the 10 to 20 cm depth were increased by water stress under ambient CO2; these measures under both water levels for FACE were similar to the ambient CO2/wet treatment, suggesting that more C storage in wheat cropping systems is likely under elevated CO2regardless of water treatment.