Influence of soil biota on Douglas-fir (Pseudotsuga menziesii) seedling growth: the role of rhizosphere bacteria

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Abstract
The influence of soil biota on Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedling growth was investigated by sowing seed collected from two ecologically distinct zones in British Columbia, Chilliwack and Williams Lake, in soil collected from the same two areas. Seedling biomass of both Douglas-fir ecotypes was greatest in unpasteurized soil collected from the same area as was the seed, but pasteurization negated this effect. In addition, Chilliwack seedlings responded specifically to biota present only in Chilliwack soil. To determine if rhizosphere bacteria were involved in these growth responses, bacteria were isolated from the rhizosphere of wild Douglas-fir seedlings collected from both locations, and putative growth promoting bacteria were selected in a screening experiment. Selected bacteria were then tested for seedling growth response specificity using a factorial design in which seed, soil, and bacteria from both locations were evaluated in all combinations. Inoculation of Douglas-fir seed with some of the selected bacterial isolates increased seedling biomass in pasteurized soil to a level similar to that of seedlings grown in unpasteurized soil, but there was no evidence that adaptive relationships involving bacteria, Douglas-fir, and the soil in which they naturally grew were important in effecting seedling growth promotion. However, Douglas-fir shoot biomass increased 39% and root biomass 68% after bacterial inoculation in some treatment combinations involving pasteurized soil. Key words: Douglas-fir seedlings, growth, rhizosphere bacteria.

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