Storage effects on indigenous soil microbial communities and PGPR efficacy

We tested the hypothesis that storage-induced changes in the composition of soil microbial communities altered the efficacy of three Bacillus and three Pseudomonas plant growth promoting rhizobacteria (PGPR) strains on spruce seedlings. Forest soils were collected from near Mackenzie, Salmon Arm and Williams Lake, B.C. Soil microbial population sizes were evaluated using dilution plating. Carbon substrate utilization patterns were determined on fresh forest soils and after 32 weeks of storage at 4 degrees C and -10 degrees C using the Biolog system. Small aliquots of fresh and stored comprising < 2% of seedling growth medium were also used in PGPR assays. The population sizes of viable soil microorganisms, particularly bacteria and mycorrhizal fungi, declined 10-1000 fold during soil storage at both temperatures. Differences in substrate utilization patterns between soils and after storage treatments were also detected by Shannon's diversity index (H'), principal component analysis and cluster analysis of BIOLOG data. The three fresh soils formed a distinct cluster when plotted on ordination axes or by Euclidean distance. Williams Lake and Salmon Arm soils maintained a greater degree of substrate utilization diversity when stored frozen, but the opposite was true for Mackenzie soil. In general, the six PGPR strains had similar effects on spruce seedling growth within soil x storage treatments. Pseudomonas root colonization did not differ significantly between strains (Bacillus colonization was not evaluated), forest soils and storage treatments, but PGPR efficacy depended on the origin and treatment of forest soil. For example, spruce growth promotion was greatest in fresh Williams Lake and Salmon Arm soil, but PGPR efficacy in Mackenzie soil was greatest after storage at -10 degrees C. Our results support the hypothesis that storage-induced changes in soil microflora contribute to variability in PGPR efficacy, but we were unable to identify specific characteristics of these forest soils that related directly to plant growth response variability.