Nodulating and non-nodulating soybean rotation influence on soil nitrate-nitrogen and water, and sorghum yield

Soybean [Glycine max (L.) Merr.] rotation has been shown to enhance grain sorghum [Sorghum bicolor (L.) Moench] growth and yield due in part to N contribution. Sorghum grain and stover yield, yield components, soil water and soil NO3-N were measured in a long-term rotation study in 2003 and 2004 on a Sharpsburg silty clay loam (fine, smectitic, mask Typic Argiudoll). The objectives were to separate biologically fixed N from other rotation effects on sorghum grain and stover yields, and to relate yield to yield components, soil NO3-N and water contents. The cropping sequences were continuous grain sorghum, and sorghum rotated with non-nodulating or nodulating soybean. Soil amendment treatments consisted of control (zero), manure (17-25 Mg dry matter ha ^-1 yr^-1), and N (41 kg ha^-1 for soybean and 84 kg ha^-1 yr^_1 for sorghum). Cropping sequence x soil amendment interaction effects were found for most parameters measured. High soil NO₃-N following soybean rotation and from amendment application promoted plant growth leading to low soil water content at anthesis, and increased kernel weight, grain and stover yield. Rotation with non-nodulating soybean without soil amendment increased grain yield by 2.6 to 3.0 Mg ha ^-1 and stover yield by 1.5 to 1.8 Mg ha^-1 over continuous sorghum without soil amendment. Rotation with nodulating soybean further increased grain yield by 1.7 to 1.8 Mg ha^-1 and stover yield by 0.6 to 0.9 Mg ha^-1. Biologically fixed N effects accounted for only 35 to 41% of enhanced sorghum yield due to crop rotation with soybean. Soil NO ₃-N during vegetative growth, plant height, soil water content at anthesis and kernel weight were the most important parameters related to sorghum grain yield across cropping sequences and soil amendments.