Persistence of an introduced bradyrhizobium japonicum and other soybean rhizobia in a soybean/maize rotation
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Streptomycin resistant mutant of Bradyrhizobium japonicum strain IRJ 2114 was developed, tested for ability to nodulate and fix nitrogen, and introduced on seeds at the start of the field experiments. Soybean (Glycine max L.) and maize (Zea mays L.) were then grown in four cropping sequences namely soybean/soybean/soybean, soybean/soybean/maize (SSM), soybean/maize/soybean, and soybean/maize/maize. Percentage soybean nodulation by the introduced rhizobium, and seasonal changes in soybean rhizobial populations along and between crop rows were determined using the antibiotic resistance and most probable numbers methods, respectively. Results showed the number of nodules per plant and proportion of nodules due to introduced rhizobium strain varied significantly (P less than or equal to 0.01) with cropping sequences. In the first season, nodule recovery due to the introduced rhizobium was low (15%). In the subsequent seasons, maize crop adversely affected soybean nodulation. As in the third season, occupancy of mutant rhizobium was 60% for continuous soybean cropping (SSS), and 42% for the soybean/maize/soybean (SMS). Populations of soil soybean rhizobia were similarly affected by the cropping sequences. Rhizobia numbers were significantly (P less than or equal to 0.05) higher when the first two crops were soybean (SS) than when maize followed soybean (SM). Throughout the sampling period, more rhizobia occurred along the crop rows (AR) than in the inter-row spaces (BR), indicating positive effects of rhizospheres on the rhizobial population. It was concluded therefore that for successful establishment of improved strains of B.japonicum, a second soybean crop should follow the first inoculated crop.