Effects of soil management on soil characteristics were investigated on the rhizosphere (RPP) and the non-rhizosphere (NRPP) siol of a re-grass vertisol under #Digitaria decumbens and in the soil under continuous cultivation (CC). A low energy technique allowed to separate eight size and density fractions, including macro- and micro-aggregates while preserving soil bacteria. Organic C and N, microbial biomass C and the number of total bacteria (AODC) and of #Azospirillum brasilense and their distribution were determined in soil fractions isolated from the CC, NRPP and RPP soils. Soil macroaggregates (>2000 micrometers) were similarly predomminant in the NRPP and RPP soils when the dispersible clay size fraction (2000 micrometers) and from the root fraction, not from the finer separates. The proportion of organic C as microbial biomass C revealed the low turnover of microbial C in the PP situations, especially in the clay size fraction of the NRPP soil. A common shift of AODC toward the finer separates from planted soils (CC and RPP) revealed the influence of living plants on the distribution of soil bacteria. The relative abundance of #A. brasilense showed the presence of the active roots of #Digitaria in the macroaggregates and their contact with the dispersible clay size fraction of the rhizosphere soil. (Résumé d'auteur
