Population dynamics of soybean root-nodule bacteria in latosol soil used for upland and lowland rice/soybean cropping systems in West Java, Indonesia

Abstract

Two experiments were established in a latosol soil near Bogor, Indonesia to examine the population dynamics of soybean rhizobia under soybean-upland and -lowland rice management systems. Rice was sown in all plots before sowing the first soybean crop which was inoculated with either the wild-type or antibiotic-resistant mutants of strains CB1809, USDA110 and LRj I1D or left uninoculated. Numbers of soybean rhizobia in the soil before sowing the rice and in rice rhizospheres after 21 and 42 days, before sowing soybeans and in the soybean rhizospheres after 28 and 42 days were estimated by MPN using Glycine soja. In the lowland treatment, numbers of soybean rhizobia in the bulk soil were low before sowing rice (log 10 1.86 g −1 soil) but they multiplied in the rhizospheres to ca log 10 4.41 g −1 soil. In the upland treatment there were log 10 4.45 initially which increased to log 10 5.15 at 42 days. Following 2 months fallow and soil preparation for sowing soybeans in the upland treatment, the numbers of soybean rhizobia in the bulk soil were only log 10 1.29 g −1 but they multiplied to log 10 4.84 g −1 soil at 28 days in the soybean rhizospheres. Numbers in the soybean rhizospheres in the lowland treatment reached log 10 5.27 g −1 at 28 days. They again declined rapidly, after the soybeans were harvested, to log 10 1.32 and 1.98 g −1 of bulk soil in upland and lowland systems respectively. Inoculation did not affect nodulation, shoot dry weight or grain yeild of soybean despite nodule occupancy by selected strains, of between 52–63% in the lowland and 18–65% in the upland system. The implications of the results are discussed in terms of inoculation and management strategies.