Increasing atmospheric CO 2 concentration has led to concerns about potential effects on production agriculture as well as agriculture's role in sequestering C. In the fall of 1997, a study was initiated to compare the response of two crop management systems (conventional and conservation) to elevated CO 2. The study used a split-plot design replicated three times with two management systems as main plots and two CO 2 levels (ambient=375 L L -1 and elevated CO 2=683 L L -1) as split-plots using open-top chambers on a Decatur silt loam (clayey, kaolinitic, thermic Rhodic Paleudults). The conventional system was a grain sorghum ( Sorghum bicolor (L.) Moench.) and soybean ( Glycine max (L.) Merr.) rotation with winter fallow and spring tillage practices. In the conservation system, sorghum and soybean were rotated and three cover crops were used (crimson clover ( Trifolium incarnatum L.), sunn hemp ( Crotalaria juncea L.), and wheat ( Triticum aestivum L.)) under no-tillage practices. The effect of management on soil C and biomass responses over two cropping cycles (4 years) were evaluated. In the conservation system, cover crop residue (clover, sunn hemp, and wheat) was increased by elevated CO 2, but CO 2 effects on weed residue were variable in the conventional system. Elevated CO 2 had a greater effect on increasing soybean residue as compared with sorghum, and grain yield increases were greater for soybean followed by wheat and sorghum. Differences in sorghum and soybean residue production within the different management systems were small and variable. Cumulative residue inputs were increased by elevated CO 2 and conservation management. Greater inputs resulted in a substantial increase in soil C concentration at the 0-5 cm depth increment in the conservation system under CO 2-enriched conditions. Smaller shifts in soil C were noted at greater depths (5-10 and 15-30 cm) because of management or CO 2 level. Results suggest that with conservation management in an elevated CO 2 environment, greater residue amounts could increase soil C storage as well as increase ground cover.
