Soybean nitrogen contribution to corn and sorghum in western corn belt rotations

Increased N availability is often associated with the beneficial effects of rotating grain and legume crops. Our objective was to utilize yield response data from two long-term studies to determine the amount of N supplied by soybean [Glycine mar (L.) Merr.] for subsequent nonlegume crops in 2-yr rotations. The experiments were located in eastern Nebraska (Mead) (rainfed) and central Nebraska (Shelton) (irrigated). Continuous corn (Zea mays L.) and soybeancorn cropping systems were present in both experiments while continuous sorghum [Sorghum bicolor (L.) Moench] and soybean-sorghum cropping systems were present at Mead only. Three N fertilizer rates were used for the study at Mead and five at Shelton. Nitrogen fertilizer replacement values were estimated using graphical and regression techniques. Results from both techniques indicated that corn in the rainfed experiment at Mead (20 yr) and irrigated experiment at Shelton (10 yr) obtained approximately 65 kg N ha(-1) yr(-1) from soybean in a 2-yr rotation with soybean at both locations. Sorghum in the rainfed experiment at Mead (20 yr) obtained approximately 80 kg N ha(-1) yr(-1) from soybean in a soybean-sorghum rotation. Current fertilizer N applications based on fall or early-spring soil tests used extensively in this area need to be reduced by these amounts for corn and sorghum grown in 2-yr rotations with soybean when grown on medium- to fine-textured soils in the western Corn Belt to reduce excess N available for loss and to reduce unnecessary input costs.