Superior inorganic phosphate solubilization is linked to phylogeny within the Pseudomonas fluorescens complex

Modern agriculture relies on the addition of chemical fertilizers, with little attention paid to the role of soil microbes in biogeochemical cycles. Phosphate fertilization is a particular problem because heavy use leads to groundwater contamination and waterway eutrophication. Sustainable agriculture could be promoted by harnessing soil microbes, in particular fluorescent pseudomonads, to mobilize soil inorganic phosphate and increase bioavailability for plants. In this study, a long term field site was used to assess the effects of conventional or no phosphate addition on the phosphate solubilizing fluorescent pseudomonad community. 752 fluorescent pseudomonad isolates were recovered from the rhizospheres of wheat and barley and classified as strong, weak or non-phosphate solubilizers on the basis of clearing zones formed on medium containing insoluble Ca₃(PO₄)₂. Surprisingly, no differences in the percentages of strong, weak and non-phosphate solubilizing isolates were observed when comparing isolate collections from the different crop types or different input regimes. Amplified ribosomal DNA restriction analysis (ARDRA) profiling and DNA sequence analysis of the 16S rDNA and gyrB genes was applied to a subset of the isolates to determine whether an effect of fertilizer input could be detected at the taxonomic level. No effect of phosphate fertilizer was seen, but the data did reveal that 82% of the strong-solubilizing isolates were clustered into one taxonomic group that completely lacked non-solubilizing isolates. These findings suggest that one phylogenetic lineage within the P. fluorescens complex has superior phosphate solubilizing potential and supports the view that it may be possible to select a Pseudomonas community with an enhanced capacity to mobilize inorganic phosphate in agricultural soils.