POPULATION-DYNAMICS OF BACILLUS-MEGATERIUM STRAIN B153-2-2 IN THE RHIZOSPHERE OF SOYBEAN

Populations of Bacillus megaterium strain B153-2-2 with a rifampicin-resistant marker were studied in the rhizosphere and surrounding soil of soybean (Glycine max) plants for 2 yr in a silt loam soil in the field. B153-2-2 populations were established soon after introduction into the soil; a significantly greater population density (6 cfu/g of soil) was observed at 2 or 3 wk than at 1 or 4 wk after emergence in a root zone containing a taproot, its main branches, and adjacent soil. B153-2-2 populations increased after introduction, remained constant during the growing season, and then decreased toward the end of the year at a 10-cm depth in the root zone of a soil cylinder 0-10 cm out from the stem and 30 cm deep. Populations of the bacterium at 20- and 30-cm depths remained relatively constant throughout the season. Residual populations of B153-2-2 survived the winter at no less than 4 log cfu/g of soil and were recoverable during the second growing season without further introduction of the bacterium. Introduced bacterial populations in the rhizosphere and root zones were dispersed further vertically than horizontally from the application site and were recoverable to at least 50 cm deep and 30 cm laterally from a taproot 39 days after seedling emergence. B153-2-2 populations reached 6 log cfu/g of soil with minimum recovery of 2 log cfu/g of soil in the extended rhizosphere and root zones. The compatibility with indigenous soil microflora, the close association with soybeans, and the ability to colonize soybean roots and rhizosphere make B. megaterium B153-2-2 a potential candidate for management of microflora in the rhizosphere.