DISTRIBUTION OF SOIL FRACTIONS AND LOCATION OF SOIL BACTERIA IN A VERTISOL UNDER CULTIVATION AND PERENNIAL GRASS

Effects of soil management on soil characteristics were investigated o n the rhizosphere (RPP) and the non-rhizosphere (NRPP) soil of a re-gr ass vertisol under Digitaria decumbens and in the soil under continuou s cultivation (CC). A low energy technique allowed to separate eight s ize and density fractions, including macro- and microaggregates while preserving soil bacteria. Organic C and N, microbial biomass C and the number of total bacteria (AODC) and of Azospirillum brasilense and th eir distribution were determined in soil fractions isolated from the C C, NRPP and RPP soils. Soil macroaggregates (>2000 mu m) were similarl y predominant in the NRPP and RPP soils when the dispersible clay size fraction (< 2 mu m) represented more than 25% of the CC soil mass. Th e main increase of C content in RPP originated from the macroaggregate s (> 2000 mu m) and from the root fraction, not from the finer separat es. The proportion of organic C as microbial biomass C revealed the lo w 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 fin er separates from planted soils (CC and RPP) revealed the influence of living plants on the distribution of soil bacteria. The relative abun dance of A. brasilense showed the presence of the active roots of Digi taria in the macroaggregates and their contact with the dispersible cl ay size fraction of the rhizosphere soil.