Effect of Soybean Genotypes on the Carry-over Nitrogen Supplied to Winter Rye Cover Crop.

Potential contributions of soybean nitrogen (N) fixation to the N supplied to subsequent crops have been claimed, and they may be associated with the N derived from soybean residues after harvest as well as the below-ground plant structures (i.e., roots and nodules) and root exudates. Of these two sources of carry-over N, the latter is the most underestimated due to assessment difficulties. Furthermore, the lack of information on this source of N can interfere with the estimation of the overall N balance. In this study, we aimed to assess the potential contribution of soybean genotypes with different capacities in N fixation to the N accumulated in the following winter rye cover crop. To this end, 25 soybean genotypes were planted in a randomized complete block design under field conditions in Fall 2016 in Carbondale, IL. After soybean harvest, the aboveground residues were removed, and the plots were sown with winter rye. In Spring 2017, the effect of the below-ground N derived from the soybean genotypes was assessed in the winter rye using data from SPAD meter readings, shoot %N, shoot dry matter, and shoot N content. The results demonstrated that all the variables were genotype dependent (p<0.01), except for the SPAD meter readings. We also identified a group of 12 soybean lines that supplied significantly more N to winter rye than the non-nodulating check. Our findings provided new insights regarding the ability of soybean genotypes to accumulate N in the below-ground plant structures that might be valuable to cover crops, and also to the selection of soybean genotypes that supply relatively more N to the following crops.