Changes in microbial biomass, community composition and diversity, and functioning with soil depth in two alpine ecosystems on the Tibetan plateau.

Background and aims: Microbial communities play an important regulatory role in soil carbon and nutrient cycling in terrestrial ecosystems. Most studies on microbial communities and biogeochemical cycling focus on surface soils (0–20 cm). However, relatively little is known about how structure and functioning of microbial communities shift with depth in a soil profile, which is crucial to understand biogeochemical cycling in deep soils. Methods: We combined a number of complementary techniques to investigate the microbial biomass, community composition and diversity, and potential functioning along soil profile (0–70 cm) in two alpine ecosystems (meadow and shrubland) on the Tibetan Plateau. Results: The microbial biomass and fungi:bacteria ratio declined significantly with depth, while the ratio of Gram-positive to Gram-negative bacteria increased with depth in both ecosystems. Microbial community composition showed significant differences among soil depths and between ecosystems. The relative abundance of some phylum of archaea, bacteria or fungi (e.g. Basidiomycota, Bacteroidetes) changed significantly with soil depth and ecosystem type. Bacteria diversity declined with depth, while archaea richness (OTU number) increased with depth and fungi diversity and richness did not show clear trend with depth. The co-occurrence network analysis further showed that surface soil microbes were more connected and interacted among each other compared to deep soil microbes. Moreover, total enzymatic activities and soil C mineralization rate declined with depth in both ecosystems. We also detected shifts with depth in some functional guilds of bacteria (based on faprotax database) in both ecosystems and fungi (based on FUNGuild database) only in shrubland. Conclusions: The biomass, community composition and diversity, and potential functioning of microbial communities all shifted significantly along soil profile in both ecosystems, and the vertical patterns of diversity varied among different microbial groups. This may have important implications for carbon and nutrient cycling along the soil profile in alpine ecosystems. [ABSTRACT FROM AUTHOR]