Nitrogen Credits in Soybean-Corn Crop Sequences on Three Soils

Economic and environmental incentives to accurately predict corn (Zea mays L.) N requirements emphasize the need to assign appropriate N credits for soybean (Glycine max L.) in soybean-corn crop sequences. This study was conducted to determine corn response to N and N credits for soybean in crop sequences. The effects of corn-corn (CC), soybean-corn (SbC), and soybean-corn-corn (SbCC) crop sequences and applied N (0 to 225 kg N ha⁻¹) on corn grain yield, N uptake, and oil NO₃ and NH₄ concentrations were determined for 4 yr at three sites differing in climatic and soil characteristics. Four-year mean yields in SbC were 1.4 and 2.2 Mg ha⁻¹ higher than in CC at two sites with nonirrigated silt loam soils, but yield effects due to crop sequence were small on an irrigated sandy soil. Corn response to applied N varied markedly among the three sites and between years at the sites on silt loams oils. Mean corn N uptake in SbC was higher (51 kg N ha⁻¹) than in CC on silt loam soils but not on the sandy soil. Soybean N credits estimated with a fertilizer replacement value (FRV) approach and from the difference in N rates at maximum yield in SbC and CC sequences (DNM) differed markedly among locations and years and ranged from − 22 to 210 kg N ha⁻¹. Soybean provided little N to subsequent crops on sandy soils due to probable loss of residue N through leaching prior to use by the following crop. On silt loam soils, crop sequence effects on yield and N uptake indicate soybean N contributions to subsequent crops; however, fixed value N credits or N credits based on N response data combined over years will seldom accurately predict actual soybean N contributions. Site-specific diagnostic tests are needed to improve crediting of N supplied by soybean in crop sequences. Research supported by the Wisconsin Fertilizer Res. Fund, the Wisconsin Soybean Marketing Board, and the College of Agricultural and Life Sciences, Univ. of Wisconsin-Madison.