Source of the soybean N credit in maize production

Nitrogen response trials throughout the United States Corn Belt show that e conomic optimum rates of N fertilization are usually less for maize (Zea ma ys L.) following soybean (Glycine max L.) than for maize following maize; h owever, the cause of this rotation effect is not fully understood. The obje ctive of this study was to investigate the source of the apparent N contrib ution from soybean to maize (soybean N credit) by comparing soil N minerali zation rates in field plots of unfertilized maize that had either nodulated soybean, non-nodulated soybean, or maize as the previous crop. Crop yields , plant N accumulation, soil inorganic N, and net soil mineralization were measured. Both grain yield (6.3 vs. 2.8 Mg ha(-1)) and above-ground N accum ulation (97 vs. 71 kg ha(-1)) were greatly increased when maize followed no dulated soybean compared with maize following maize. A partial benefit to y ield and N accumulation was also observed for maize following non-nodulated soybean. Cumulative net soil N mineralization following nodulated soybean, non-nodulated soybean, and maize was 112, 92 and 79 kg N ha(-1), respectiv ely. Net mineralization of soil N appeared to be influenced by both quality (C:N ratio) and quantity of residue from the previous crop. In addition to an increase in plant available N from mineralization, the amount of soil i norganic N (especially in soil 5 cm from the row) was greater following nod ulated soybean than non-nodulated soybean or maize. Based on these data, th e soybean N credit appears to result from a combination of a decrease in ne t soil mineralization in continuous maize production and an increase in res idual soil N from symbiotic fixation.