Ecological strategies of soil microbes along climatic gradients: contrasting patterns in grassland and forest ecosystems

The forest–grassland transect in the Greater Khingan Mountains, located in the southern edge of the permafrost region in Eurasia, is more vulnerable to climatic changes than other terrestrial ecosystems. The impacts of climate-induced vegetation conversion on soil microbial ecological strategies are still under debate, and the underlying mechanisms are not known. Soil microbial community composition was investigated using 16SrRNA gene amplicon sequencing. The activities of soil enzymes responsible for organic matter mineralization, along with soil physicochemical properties and vegetation characteristics were examined in parallel. The dominance of microbial r-strategy was predicted by a variety of physiological and phylogenetic traits, including the r-/K-strategists ratio, the ribosomal RNA (rrn) operon copy number of bacterial community, saprotrophic/ectomycorrhizal fungi ratio, and the stoichiometric ratio between enzymes hydrolyzing simple (cellobiose and oligosaccharide) and complex (cellulose and protein) organic compounds. Overall, microbial r-strategy relevant traits were higher in grasslands than in forests. Within forests, when vegetation changed from conifers to broadleaf forests from northeast to southwest, the labile carbon fraction of soil organic matter increased, stimulating the prevalence of soil microbial community r-strategy. Across grassland sites, the r-strategy relevant traits decreased towards the warm, dry site, due to the declined C and N availability. This study implied that, under future warm conditions, forest ecosystems would be associated with an r-shifted soil microbial community and thus face a potential risk of carbon loss; whereas in grassland ecosystem, soil microbial community would be shifted towards a K-spectrum and might reduce the risk of carbon loss.