Population dynamics of Rhizobium japonicum strains used to inoculate three successive crops of soybean

Abstract

Three closely related strains of Rhizobium japonicum, equally effective in N2 fixation, were used to inoculate each of three successive crops of soybeans [Glycine rnax (L.) Merr. cv. Bragg] grown on the same block of land. The soil was a vertisol previously free of R. japonicum, and inoculant was applied at different rates by spraying a suspension of peat culture into the seed bed at time of sowing the seed. The populations of rhizobia that developed in rhizosphere and soil were counted at intervals during crop growth and in the fallow period between crops. There was usually a substantial decline in recovery of inoculant strains immediately after sowing. In soil initially devoid of R. japonicum, populations in the rhizospheres of young seedlings were related to rates of inoculation, but differences disappeared as the plants aged. Shortly after harvest, the soil contained large populations of rhizobia which increased up to 200 times during the fallow period between crops, probably due to release of bacteria from disintegrating nodules. The size of these populations was maintained up to the time of sowing the following crop. Although strains used for second- and third-year crops were dominated by already established rhizobia in rhizosphere colonization and nodule formation, the magnitude of the domination could be reduced by increased rates of inoculation. In soil already containing R. japonicum, the proportion of nodules formed by inoculant strains was greater than the relative number of inoculant rhizobia in the soil or the rhizosphere; this was ascribed to an advantage of specific placement of the inocula in that zone of the soil where infection foci first formed. The results are explicable in numerical terms and are discussed in relation to an inoculation strategy for maximum nodulation by applied inoculant in competition with rhizobia already established in soil.