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 soil NO 3 and NH 4 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 soils. 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.
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Bundy et al. (1993) studied this question.