ECOLOGY OF INDIGENOUS SOIL RHIZOBIA - SELECTIVE RESPONSE OF BRADYRHIZOBIUM-JAPONICUM TO A SOYBEAN-MEAL

The population dynamics of indigenous Bradyrhizobium japonicum were st udied following the addition of a ground soybean plant material, desig nated soybean meal (SBM), to their soil habitat. Three serologically i dentifiable groups, serocluster 123, serogroup 110, and serogroup 138 were enumerated by immunofluorescent (FA) assay. The total B. japonicu m population was estimated by Most Probable Number (MPN) plant assay. Initial populations were nearly identical at about 9 x 10(3) cells g(- 1) soil, and remained virtually unchanged in the untreated soil. In th e soil amended at the 1.0% rate the three serogroups responded quickly with 50- to 100-fold increases in populations in the first 12 days. F A counts remained high for 5-6 weeks before declining slowly. Similar but less pronounced effects were observed at lower rates of amendment. Serocluster 123 and serogroup 138 responded similarly throughout with peak populations at about 10(1.7) above controls from day 12 through day 36. Serogroup 110, however, was selectively enhanced by the soybea n meal reaching a peak population 1000-fold above controls at day 36 w ith the 1.0% amendment. Elevated levels of serogroup 110 persisted at more than two logs above controls after 10 weeks, Further experiments were designed to determine if the amendment-induced numerical superior ity of serogroup 110 could be exploited to enhance nodule occupancy of serogroup 110 at the expense of the usual dominant nodule occupant, s erocluster 123, One approach was to select SBM treated soil with peak populations of indigenous serogroup 110 and use that soil as an inocul ant for soybean plants, Whether applied in growth pouches, in soil pot s, or in field soil rows the inoculant resulted in only slight and inc onsistent increases in nodule occupancy by serogroup 110 strains. Simi lar failures to alter nodule occupancy in favor of serogroup 110 occur red with the second approach in which soil in greenhouse pots and in f ield plots was treated with SBM and then planted to soybeans when 110 populations peaked.