15N-uptake, N 2-fixation and rhizobial interstrain competition in soybean and bean, intercropped with maize

Abstract

N 2-fixing activity of soybean and beans monocropped or intercropped with maize was measured by the 15N dilution method. Intercropped bean and soybean, planted in the same hole with maize or in two separate planting holes, showed a decrease in atom % 15N as compared with monocropped bean and soybean. However, compared to when monocropped, intercropped non-nodulating soybean isoline and unincoulated bean showed a highly significant decrease in atom % 15N. Whereas the total amount of 15N-label accumulated in monocrop and intercrop systems remained constant, intercropped maize accumulated more and intercropped legumes less 15N compared with monocropped maize or legume, respectively. The decrease in atom % 15N excess of intercropped N 2-fixing legumes could be explained by higher N 2-fixing activity, a different 15N-uptake pattern or a difference in competition for soil N between intercropped legumes and maize. Intercropping bean or soybeans did not alter the competition pattern of the introduced strains of rhizobia. TAL 102 (USDA 110) and TAL 182 were the best competitors for soybeans and beans, respectively, and formed the greatest number of nodules, independent of cropping systems tested.