N 2 fixation, N transfer and biomass production of soybean cv. Bragg or its supernodulating nts1007 and sorghum mixed-cropping at two rates of N fertilizer

Abstract

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, N 2 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 N 2 fixed by Bragg was greater than that by nts1007, however no significant difference between the proportion of total N derived from fixed N 2 was observed. N 2 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.