The effects of water availability on the temperate forest soil microbiome.

The effects of drought on soil microbial communities, plant responses, and the interactions that arise in the rhizosphere, are increasingly the focus of research in agricultural and natural systems. However, that attention is usually steered towards annual systems, leaving many questions on how woody species and perennial systems alter carbon allocation in response to water stress. We sampled forest soils to understand how tree species identity and available soil water interact to shape microbial communities. The Kranzberg experimental forest (kroof.wxw.tum.de) is located in Freising, Germany, where Norway spruce (Picea abies) and European beech (Fagus sylvatica) plots have been subjected to seasonal drought since 2014. This site is one of five replicate, long-term research plots along a natural precipitation gradient in Bavaria, Germany, where the same tree species are planted in monocultures and in mixed stands. Our 16S rDNA sequencing results show that there are strong interactions between tree physiological responses, precipitation, soil horizon, and season on the assembly of bacterial communities, that suggest a response to the climatic gradient. Overall, the results of this study show that the determinants of bacterial communities are complex and often significant in interaction with other variables. More specifically, differences between seasons were related to the tree species and precipitation, moderated by soil depth. Precipitation and, by extension, soil water content, became less important in the mineral layer than in the organic layer of soil. While we know that the interactions between plant and microbial communities are critical to forest productivity, our data suggest that the interactions determining bacterial community composition are more complex than we initially considered.