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 organi c nitrogen in the rhizosphere. In contrast, inoculation of the same strains onto wheat seeds reduced mineralization of N from organic residues incorpo rated 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-micro be interactions on bacterial-feeding nematodes and protozoa. In soil microc osm tests, inoculation of pea seeds with Pseudomonas fluorescens strains F1 13lacZY or F113G22 resulted in an increase in the number of nematodes and p rotozoa in the rhizosphere as compared to noninoculated controls. This tren d was repeated using a model sand system into which the bacteriophagous nem atode Caenorhabditis elegans was introduced. It was subsequently found that non-inoculated germinating pea seeds exerted a nematicidal effect on C. el egans, which was remedied by inoculation with either strain F113lacZY or F1 13G22. This suggests that nematicidal compounds released by the germinating pea seeds were metabolized by the microbial inoculants before they affecte d nematode populations in the spermosphere or rhizosphere of pea. In contra st, inoculation of wheat plants resulted in significantly lower nematode po pulations in the rhizosphere, whereas protozoan numbers were unaffected. No nematicidal effects of inoculated or noninoculated wheat seeds could be fo und, suggesting that microfaunal populations were affected at a later stage during plant growth. Because of their key roles in accelerating the turnov er of microbially immobilized N and organic matter, plants that support a l arger microfaunal population are likely to benefit from a higher availabili ty of inorganic nitrogen. Therefore, an understanding of plant-microbe inte ractions and their effects on soil microfaunal populations is essential in order to assess the effects of microbial inocula on plant mineral nutrition .