N2 fixation, N transfer and biomass production of soybean cv. Bragg or its supernodulating nts1007 and sorghum mixed-cropping at two rates of N fertilizer

Two soybean (Glycine max L Merr.) genotypes, cv. Bragg and its supernodulating mutant nts1007, were grown under mono- or mixed-cropping with sorghum (Sorghum vulgare cv. FS 103) at 20 or 90 kg N ha−1. Differences in dry matter production, N2 fixation and N transfer to the companion crop sorghum were examined using the 15N-dilution method. The total dry weights of the Bragg-sorghum mixtures per concrete frame were not significantly different from Bragg monocrops. The dry weight of the nts1007-sorghum mixture at 20 kg N ha−1 was greater, but that of the mixture at 90 kg N ha−1 was smaller than nts1007 monocrop. The dry weights of Bragg or nts1007 mixtures were consistently greater than that of sorghum monocrops. Mixed-cropping sorghum with Bragg or nts1007 was more efficient than the monocrops in dry matter production, as indicated by the land equivalent ratios (LER). The LER value for Bragg-sorghum mixed-cropping was greater than nts1007-sorghum. While the dry weight of Bragg increased by mixed-cropping at both rates, that of nts1007 increased only at 20 kg N ha−1. The dry weight of sorghum component was consistently greater than the monocrop, and was more pronounced when grown in association with Bragg. The amount of N2 fixed by Bragg was greater than that by nts1007, however no significant difference between the proportion of total N derived from fixed N2 was observed. N2 fixation was greater under 90 kg N ha−1 and mixed-cropping than under 20 kg N ha−1 or monocropping, except for nts1007 at 90 kg N ha−1. The nodule dry weight of nts1007 was consistently greater than Bragg. Total N accumulated by the sorghum component was greater than that of the monocrop and was also greater in association with Bragg than with nts1007. N transfer accounted for 20 and 55% of the N accumulated by the sorghum component with Bragg at 20 and 90 kg N ha−1, respectively. N transfer to the sorghum component in association with nts1007 accounted for 11 and 35% of the N accumulated at 20 and 90 kg N ha−1, respectively. The consistent dilution of 15N in the tissues of the sorghum component compared to the monocrops suggests that N transfer rather than “sparing” of fertilizer N accounted for the improved N nutrition. It seems that the more the roots of the companion crops were intermingled, the greater the amount of N transfer.