Abundance and ribotypes of phosphate-solubilizing bacteria in Argentinean agricultural soils under no-till management

Phosphate-solubilizing bacteria (PSB) abundance and ribotypes were examined in the top soil (0–10 cm) of agricultural fields under no-till management sampled in two seasons, summer (February) and late winter (September), in 2010. No-till plots under sustainable agricultural practices (intense crop rotation) or under non-sustainable practices (soybean monocropping) were sampled at four different locations as replicates in a 400-km west–east transect in the most productive agricultural region of Argentina. Natural grasslands were selected close to the cultivated fields for comparative purposes. Culturable heterotrophic bacteria (CHB) were enumerated on nutrient agar plates, and PSB were counted on NBRIP agar plates containing Ca3(PO4)2. The PSB community structure was explored by ribotyping (16S rDNA PCR-restriction fragment length polymorphism). Quantitatively, data showed that the number of CHB and PSB per gram of dry soil was not statistically different among sampled sites, soil management programs or seasons. Qualitatively, ribotyping showed that the most abundant PSB species differed in their fingerprinting patterns among geographical sites, which suggests that local soil conditions impose strong selective constraints. The comparative analysis of PSB ribotypes revealed seasonal differences among February and September isolates for all sampling sites.