Effects of soybean cultivation on the bacterial population dynamics in cerrado soils

The effect of soybean cultivation on the population dynamics of the bacteri al community was evaluated in two "Cerrado" soils of Sao Paulo State, Brazi l. The experimental areas, in the vicinities of the cities of Sao Carlos an d Barretos, were previously cultivated, respectively, with Paspalum notatum and Brachiaria decumbens. The bacterial population densities in these soil s varied from 398.1 x 10(3) to 467.7 x 10(3) cfu (colony forming units) and from 123 x 10(3) to 218.8 x 10(3) cfu/g of dried soil, respectively, in Sa o Carlos and Barretos soils. Soybean cultivation in both soils resulted in increments in the total bacterial population density, in the actinomycetes population, and in the bacterial population resistant to the antibiotics st reptomycin and chloramphenicol. Actinomycetes were present in these soils m ainly as spores. Soybean cultivation did not alter the actinomycetes spores /hyphae ratio when comparing rhizospheric and non-rhizospheric soils. The r esults show that soybean cultivation affected the bacterial population of t hese soils.