N2 fixation by soybeans grown with different levels of mineral nitrogen, and the fertilizer replacement value for a following crop

Goss, M. J., de Varennes, A., Smith, P. S. and Ferguson, J. A. 2002. N-2 fixation by soybeans grown with different levels of mineral nitrogen, and the fertilizer replacement value for a following crop, Can. J. Soil Sci. 82: 139-145. A field experiment was established to study the impact of added mineral N on the prediction of N-2 fixation by soybean, and the consequences for the nature of any N credit that might be used to modify fertilizer recommendations to a following non-fixing crop. Nodulating and non-nodulating isolines of soybean were grown with five rates of N fertilizer, and in a second year corn was grown in the same plots and its yield compared with a response curve. Yield, total N content, amount of N derived from soil, and fertilizer utilization of the nodulating isoline of soybean were not affected by fertilizer N. In contrast, mineral N inhibited nodulation and led to a decrease in the amount of N fixed. The balance of N in the soil was more negative for lower levels of applied N, but by the following spring the amount of mineral N in the soil was the same in all plots, The yield of corn was greater in the plots that had grown nodulating soybean than the non-nodulating isoline. The N fertilizer replacement value of 25+/-8 kg N ha(-1) resulted from a greater amount of root residues in the nodulating soybean, together with a C:N ratio that would favour faster mineralization than in the non-nodulating isoline.