FOREST SOIL COMMUNITY RESPONSES TO PLANT GROWTH-PROMOTING RHIZOBACTERIA AND SPRUCE SEEDLINGS

The influence of plant-growth-promoting rhizobacteria (PGPR) and spruc e seedlings on the composition and activity of forest soil microbial c ommunities was studied in a microcosm experiment in which sterile, san d-filled 25 mmx150 mm glass tubes were treated with a forest soil susp ension containing Bacillus or Pseudomonas PGPR and 2-week-old spruce s eedlings. Eighteen weeks after treatments were established, bacterial, actinomycete and fungal population sizes were determined by dilution plating, as were seedling dry weights and soil carbon substrate utiliz ation profiles using Biolog plates. PGPR inoculation had little influe nce on the population sizes of actinomycetes or fungi. However, signif icant effects were detected on the total bacterial population size, pr imarily in microcosms without seedlings. Euclidean distances between t reatments plotted on two dimensions by multidimensional scaling showed that the introduction of PGPR strains changed the type of microbial c ommunity, particularly when inoculated into soil without seedlings. Si gnificant changes were also detected in one soil type in the presence of seedlings. Our results suggest that the type of soil community and the presence of seedlings are significant factors influencing the resp onses of soil communities to bacterial inoculation, and that for some soil communities, the presence of seedlings may mitigate perturbations caused by the introduction of PGPR.