Change in Surface Soil Carbon under Rotated Corn in Eastern South Dakota

A diversified crop rotation may reduce fertilizer N inputs for corn (Zea mays L.) and increase soil organic C (SOC). Our objectives were to determine the effects of crop rotation and fertilizer N on soil C within the surface soil (0-15-cm depth). Rotations were started in 1990 on a Barnes sandy clay loam near Brookings, SD. Measurements of SOC began in 1996. Primary tillage since 1996 was chisel plow. All crop residues were returned to the soil. Rotations were continuous corn (CC), corn-soybean [Glycine max (L.) Merr.], and corn-soybean-wheat (Triticum aestivum L.) companion seeded with alfalfa (Medicago sativa L.)-alfalfa hay (CSWA). Uncropped treatments included perennial grasses. Corn N treatments were based on the soil NO3 test and yield goal. Corn was fertilized for a grain yield of 8.5 Mg ha(-1) (high N), 5.3 Mg ha(-1) (mid N), and no N. Under grass, SOC increased 3.8 Mg C ha(-1) from 1996 to 2006. Continuous corn under high N returned 34% more aboveground plant C (PC) to the soil compared with the CSWA rotation, but this did not offset the SOC loss. Under high N, there was a loss of 2.3 Mg C ha(-1) in the surface soil from CC and a gain of 0.3 Mg C ha(-1) from CSWA (1996-2006). There was a significant effect of fertilizer N addition and rotation on SOC. A combination of greater crop diversity and fewer tillage operations on CSWA, compared with CC, probably contributed to a balance of SOC (return of PC approximate to loss of SOC).