AUTECOLOGY OF BRADYRHIZOBIUM-JAPONICUM IN SOYBEAN-RICE ROTATIONS

The effect of rice culture on changes in the number of a strain of soy bean root-nodule bacteria, (Bradyrhizobium japonicum CB1809), already established in the soil by growing inoculated soybean crops, was inves tigated in transitional red-brown earth soils at two sites in south-we stern New South Wales. At the first site, 5.5 years elapsed between th e harvest of the last of four successive crops of soybean and the sowi ng of the next. In this period three crops of rice and one crop of tri ticale were sown and in the intervals between these crops, and after t he crop of triticale, the land was fallowed. Before sowing the first r ice crop, the number of Bradyrhizobium japonicum was 1.32 x 10(5) g(-1 ) soil. The respective numbers of bradyrhizobia after the first, secon d 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 const ant. Thus sufficient bradyrhizobia survived in soil to nodulate and al low N-2-fixation by the succeeding soybean crop. At the second site, n umbers of bradyrhizobia declined during a rice crop, but the decline w as less than when the soil was fallowed (400-fold cf. 2200-fold). Mult iplication of bradyrhizobia was rapid in the rhizosphere of soybean se edlings sown without inoculation in the rice bays. At 16 days after so wing, their numbers were not significantly different (p < 0.05) from t hose 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 indi cate that flooding soil has a deleterious effect on the survival of br adyrhizobia but, under the conditions of the experiments, sufficient B . japonicum strain CB1809 survived to provide good nodulation after th ree crops of rice covering a total period of 5.5 years between crops o f soybean.