Carbon dioxide-enriched agroecosystems: Influence of tillage on short-term soil carbon dioxide efflux

Increasing atmospheric carbon dioxide (CO2) concentration can increase biomass production that may influence carbon (C) dynamics in terrestrial ecosystems. Soil CO2 efflux as affected by crop residues from high CO2 environments managed under different tillage systems has not been explored. This study examined the effects of tillage systems in a legume {soybean [Glycine max (L.) Merr.]} and nonlegume {grain sorghum [Sorghum bicolor] (L.) Moench.} CO2-enriched agroecosystem on the rates of short-term CO2 evolution from a Blanton loamy sand (loamy siliceous, thermic Grossarenic Paleudults). In the spring of 1993, CO2 efflux observations initiated within 5 s after a tillage event were compared to no-tillage conditions for 8 d in plots where both crop species had been grown in open top field chambers under two CO2 conditions (ambient and twice ambient) for two seasons (1992 and 1993). Added CO2 increased yields, residue, and root biomass; higher percent ground cover was also observed in CO2-enriched plots prior to the tillage treatment. Differences in C/N ratio of the residue may have influenced CO2 efflux rates; C/N ratio was highest for sorghum and was increased by elevated CO2. Efflux patterns were characterized by flushes of CO2 following initial tillage and rainfall events. Species x tillage and CO2 x species interactions were noted on several days and for total CO2 efflux values. Our results suggest that short-term CO2 fluxes may be greater for tilled soybean and for soybean grown under elevated CO2; however, short-term flux rates in the sorghum crop were affected by tillage, but not by CO2 level. These short-term results should be viewed with caution when predicting long-term C turnover in agroecosystems.