Interactions of soil bacteria and fungi with plants during long-term grazing exclusion in semiarid grasslands

Microbial succession has been extensively investigated during the restoration of degraded environments, but the interactions of microbes with plants and soils have not been well documented. We examined changes in the plant communities, soil variables, and microbial communities of grasslands after different periods of grazing exclusion (0, 10, 25, and 35 y) on the Loess Plateau in China. The microbial communities were characterized based on their biomass, enzymatic activities, quantity of functional microbes, and composition using high-throughput sequencing. Grazing exclusion increased the plant diversity, above-and belowground biomass, organic carbon content, total nitrogen content, microbial biomass, enzymatic activities, abundance of ammonia-oxidizing microbes, and diversities of the bacterial and fungal communities; however, the highest values of these variables occurred at the 25-y exclusion site and subsequently declined, indicating that long-term exclusion could have a negative effect on this grassland. Decreases in the abundances of Alphaproteobacteria and Leotiomycetes and increases in Actdobacteria and Sordariomycetes along the chronosequence indicated different successional patterns in the microbial communities. The patterns of change in the composition and diversity of the plant, bacterial, and fungal communities suggest that plant and bacterial succession occurred in parallel and proceeded faster than fungal succession. Indicators of the bacterial and fungal communities, including their biomass, enzymatic activities, and community composition and diversity, were affected by the plant diversity and organic carbon, total nitrogen, and nitrate nitrogen contents. Fungal succession was also susceptible to changes in the soil moisture content. These results suggest that plant diversity plays an important role in shaping the microbial communities, likely by altering the levels of soil nutrients and moisture.