Bacterial cells small enough to pass through 0.4-μm-pore-size filters made up 5 to 9% of the indigenous bacterial population in 0- to 20-cm-depth samples of Abiqua silty clay loam. Within the same soil samples, cells of a similar dimension were stained with fluorescent antibodies specific to each of four antigenically distinct indigenous serogroups of Rhizobium leguminosarum bv. trifolii and made up 22 to 34% of the soil population of the four serogroups. Despite the extensive contribution of small cells to these soil populations, no evidence of their being capable of either growth or nodulation was obtained. The density of soil bacteria which could be cultured ranged between 0.5 and 8.5% of the >0.4-μm direct count regardless of media, season of sampling, or soil depth. In the same soil samples, the viable nodulating populations of biovar trifolii determined by the plant infection soil dilution technique ranged between 1 and 10% of the >0.4-μm direct-immunofluorescence count of biovar trifolii. The <0.4-μm cell populations of both total soil bacteria and biovar trifolii changed abruptly between the 10- to 15-cm and 15- to 20-cm soil depth increments, increasing from 5 to 20% and from 20 to 50%, respectively, of their direct-count totals. The increase in density of the small-cell population corresponded to a significant increase in soil bulk density (1.07 to 1.21 g cm−3). The percent contribution of the <0.4-μm direct count to individual serogroup totals increased with soil depth by approximately 2-fold (39 to 87%) for serogroups 17 and 21 and by 12-fold (6 to 75%) for serogroups 6 and 36
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