Responses Of Microbial Community Taxonomic Composition And Functional Genes To Precipitation Changes In Semi-arid Grasslands

Semi-arid grassland is one of the most important terrestrial ecosystem types, which is vulnerable to environmental disturbances due to its role of transition zone between arid and humid ecosystems. Precipitation is the leading driving factor of soil water content, and it is one of the most important limiting factors of semi-arid grasslands. Therefore, precipitation changes have vital influences on semi-arid grasslands. Previous works revealed that aboveground plant biomass increased with increasing precipitation, which interacted with other changes and resulted in changes of the soil nutrient level. Microorganisms are major decomposers in ecosystems, and play irreplaceable roles in biogeochemical cycling. However, responses of soil microbial communities to precipitation changes in semi-arid grasslands remained largely unknown. Our experimental sites located in a Mediterranean semi-arid grassland near Stanford University, California, US and a continental monsoon semi-arid grassland in Duolun, Inner Mongolia, China. We investigated responses of microbial phylogenetic composition and functional structure to precipitation changes with high-throughput MiSeq sequencing and a microarray called GeoChip. We found that(1) At the phylogenetic level, different taxa revealed different response strategies. And the same taxa revealed conserved strategy. The abundant Actinobacteria increased linearly with decreasing precipitation, showing clear drought resistance in these two grasslands under distinguished climate types, implying the conservation of particular microbial traits in the face of similar environmental disturbances under different ecosystem types.(2) At the functional gene level, different microbial functional genes responded differently to precipitation changes. The relative abundance of microbial carbon degradation genes,ammonia oxidation genes, denitrification genes and nitrogen limitation stress genes clearly increased under increased precipitation, but the relative abundance of genes related to the last step of ammonia fixation clearly decreased. These changesimpliedsoil carbon and nitrogen loss under increased precipitation.(3)Responses of soil microbial phylogenetic composition and functional structure were inconsistent, which might be attributed to the difference between their environmental driving factors. Microbial phylogenetic composition was significantly linked to environmental factors such as soil temperature and aboveground vegetation, while microbial functional genes were significantly related to soil nutrient concentration. Our study analyzed overall microbial phylogenetic composition and functional structure as well as different microbial taxa and functional genes, illustrating responses of semi-arid grasslands to precipitation changes from the view of soil microbial communities. It also discussed theories and technologies in the field of soil microbiology, revealing both opportunities and challenges in future works.