CROP-ROTATION EFFECTS ON POPULATIONS OF BRADYRHIZOBIUM-JAPONICUM AND RHIZOBIUM-MELILOTI

Population sizes of Bradyrhizobium japonicum and Rhizohium meliloti we re determined in two long-term crop rotation studies made in Wisconsin . The B. japonicum study was on a long-term rotation of soybean and co rn. The R. meliloti study involved an ongoing 22 yr study of a corn so ybean oats-alfalfa rotation. In both experiments, we examined the popu lation sizes of root nodule bacteria once in October 1989. Populations of B. japonicum did not differ significantly unless the field had bee n planted continuously to com. There was no significant difference in the population size of B. japonicum for those plots which had not been planted to soybean for up to 4 yr. In contrast, population sizes of R . meliloti were signficantly influenced by the number of years since a lfalfa was present in the plot. Various soil characteristics such as o rganic matter, pH, nitrate, phosphorous and potassium were not affecte d by crop rotation. These soil characteristics did not influence the R . meliloti populations. In summary, populations of B. japonicum are fa r more persistent during crop rotation than are R. meliloti under the conditions of these experiments.