Water availability regulates negative effects of species mixture on soil microbial biomass in boreal forests

Soil microorganisms are critical for the maintenance of terrestrial biodiversity and ecosystem functions. Both plant diversity and water availability are individually known to influence soil microorganisms; however, their interactive effects remain largely unknown. Here, we investigated whether the effects of tree species mixtures on microbial biomass and composition were altered by water availability. This was accomplished by sampling soils in the growing season from stands that were dominated by Populus tremuloides and Pinus banksiana, respectively, and their relatively even mixtures under reduced (−25% throughfall), ambient, and added (+25% throughfall) water. Microbial community biomass and composition were determined by phospholipid fatty acid analysis. We found that water addition increased soil total microbial biomass and by individual groups, whereas water reduction had no effect. Under ambient water conditions, soil total microbial biomass, arbuscular mycorrhizal fungal, bacterial, gram-positive (GP) bacterial, and gram-negative (GN) bacterial biomass were significantly lower in mixtures than from those of constituent monocultures, but saprotrophic fungal biomass and the ratios of fungal/bacterial and GP/GN bacteria were not significantly affected by tree species mixtures. Water reduction increased species mixture effects on total and individual group microbial biomass from negative to neutral, while water addition only increased mixture effects on arbuscular mycorrhizal fungal and GP bacterial biomass. Across all water treatments, soil total and individual group microbial biomass significantly increased with the abundance of broadleaved trees, but only weakly with species richness. Further, microbial community compositions differed significantly with both overstory type and water treatment. Microbial community compositions exhibited strong associations with tree species richness, soil moisture, soil pH, and litterfall production, whereas microbial biomass did not. Our results suggest that higher species diversity is not always of benefit for soil microorganisms; however, mixed tree species have the potential to regulate ecosystem responses to climate change.