Effect of introduced Pseudomonas fluorescens strains on soil nematode and protozoan populations in the rhizosphere of wheat and pea

Previous studies have shown that inoculation of pea seeds with Pseudomonas fluorescens strains F113lacZY or F113G22 increased mineralization of organic nitrogen in the rhizosphere. In contrast, inoculation of the same strains onto wheat seeds reduced mineralization of N from organic residues incorporated into soil. In the present study, we report on a likely explanation of this phenomenon, which appears to be governed by the effect of plant-microbe interactions on bacterial-feeding nematodes and protozoa. In soil microcosm tests, inoculation of pea seeds with Pseudomonas fluorescens strains F113lacZY or F113G22 resulted in an increase in the number of nematodes and protozoa in the rhizosphere as compared to noninoculated controls. This trend was repeated using a model sand system into which the bacteriophagous nematode Caenorhabditis elegans was introduced. It was subsequently found that non-inoculated germinating pea seeds exerted a nematicidal effect on C. elegans, which was remedied by inoculation with either strain F113lacZY or F113G22. This suggests that nematicidal compounds released by the germinating pea seeds were metabolized by the microbial inoculants before they affected nematode populations in the spermosphere or rhizosphere of pea. In contrast, inoculation of wheat plants resulted in significantly lower nematode populations in the rhizosphere, whereas protozoan numbers were unaffected. No nematicidal effects of inoculated or noninoculated wheat seeds could be found, suggesting that microfaunal populations were affected at a later stage during plant growth. Because of their key roles in accelerating the turnover of microbially immobilized N and organic matter, plants that support a larger microfaunal population are likely to benefit from a higher availability of inorganic nitrogen. Therefore, an understanding of plant-microbe interactions and their effects on soil microfaunal populations is essential in order to assess the effects of microbial inocula on plant mineral nutrition.