AUTECOLOGY OF BRADYRHIZOBIUM-JAPONICUM IN SOYBEAN-RICE ROTATIONS

The effect of rice culture on changes in the number of a strain of soybean root-nodule bacteria, (Bradyrhizobium japonicum CB1809), already established in the soil by growing inoculated soybean crops, was investigated in transitional red-brown earth soils at two sites in south-western New South Wales. At the first site, 5.5 years elapsed between the harvest of the last of four successive crops of soybean and the sowing of the next. In this period three crops of rice and one crop of triticale were sown and in the intervals between these crops, and after the crop of triticale, the land was fallowed. Before sowing the first rice crop, the number of Bradyrhizobium japonicum was 1.32 x 10(5) g(-1) soil. The respective numbers of bradyrhizobia after the first, second and third rice crops were 4.52 x 10(4), 1.26 x 10(4) and 6.40 x 10(2) g(-1) soil. In the following two years the population remained constant. Thus sufficient bradyrhizobia survived in soil to nodulate and allow N-2-fixation by the succeeding soybean crop. At the second site, numbers of bradyrhizobia declined during a rice crop, but the decline was less than when the soil was fallowed (400-fold cf. 2200-fold). Multiplication of bradyrhizobia was rapid in the rhizosphere of soybean seedlings sown without inoculation in the rice bays. At 16 days after sowing, their numbers were not significantly different (p < 0.05) from those in plots where rice had not been sown. Nodulation of soybeans was greatest in plots where rice had not been grown, but yield and grain nitrogen were not significantly different (p < 0.05). Our results indicate that flooding soil has a deleterious effect on the survival of bradyrhizobia but, under the conditions of the experiments, sufficient B. japonicum strain CB1809 survived to provide good nodulation after three crops of rice covering a total period of 5.5 years between crops of soybean.