Influence of carriers on rhizosphere colonization by inoculant Bradyrhizobium japonicum

Organic and inorganic materials are used extensively as carriers in the preparation of commercial soybean inocula. These carriers facilitate application of inocula and promote survival of soybean rhizobia (Bradyrhizobium japonicum) during storage, but little is known of their effects on the ability of inoculant rhizobia to survive after addition to the plant rhizosphere. Laboratory and environmental growth chamber studies were performed to determine the influence of finely milled peat, granular peat, or vermiculite carriers on survival of B. japonicum An-6a (rifampicin- and streptomycin-resistant) in samples of three Louisiana soils. Laboratory studies showed that the numbers of viable cells recovered from soils incubated for 3 and 10 days (25;Experiments to assess the influence of peat carriers on predation of inoculant rhizobia in the soybean (Glycine max (L.) Merr.) rhizosphere showed that use of fine peat in inoculum preparations markedly reduced recovery of inoculant cells 10-30 days after their addition to sterilized soil moistened with a suspension of predatory protozoa. Recovery of the inoculant from root nodules of 30-day old plants was significantly greater when cells were added with vermiculite carriers than when added with peat-based or aqueous carriers to a loamy sand (Crevasse) or a silty clay loam (Sharkey) soil.;These findings suggest that granular peat carriers promote survival of inoculant B. japonicum in moist soils or soils subjected to desiccation but, vermiculite carriers enhance the ability of the cells to occupy nodules when plants are grown under favorable moisture conditions.