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 wil l reach the soil. An aspect of soil C sequestration that requires furt her study is the effect of elevated CO2 on C and N dynamics; this rela tionship is the keg to understanding potential longterm C storage in s oil. Soil samples (0-5, 5-10, and 10-20 cm increments) were collected after 2 yr of wheat (Triticum aestivum L.) production under two CO2 le vels [370 (ambient) and 550 mu L L-1 (free-air CO2 enrichment) (FACE)] and two water treatments [100% of ET replaced (wet) and 50% of ET rep laced (dry)] on a Trix clay loam [fine, loamy, mixed (calcareous), hyp erthermic Typic Torrifluvents] at Maricopa, AZ. Organic C, total N, po tential 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 an d 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 CO2 treatment , indicating that factors other than N may limit C mineralization and turnover. Soil respiration and C turnover patterns were not affected b y CO2 treatment level at the 0 to 5 cm depth; however, these measures were Lower under FACE at the lower depths. Soil respiration and C turn over at the 10 to 20 cm depth were increased by water stress under amb ient CO2; these measures under both water levels for FACE were similar to the ambient CO2/wet treatment, suggesting that more C storage in w heat cropping systems is likely under elevated CO2 regardless of mater treatment.