Soil microbiome mediates the recovery of soil fertility following release from overgrazing in Mongolian grasslands

Grazing pressure can degrade environmental quality and ecosystem structure and functioning. We evaluate these effects by integrating broad scale surveys of structure and function across the Mongolian Steppe with metagenomic sequencing of the soil microbiome targeting paired heavily grazed sites and exclosures of short-term (2 years), medium-term (11 years) and long-term (33 years). Our results showed that long-term overgrazing decreased plant species richness, aboveground biomass and soil fertility, including soil organic carbon, total nitrogen and total phosphorus content. We found that the recovery of soil microbiome composition and function increased with increasing duration of the release from grazing. Structural Equation Models (SEMs) demonstrated that shifts in bacterial composition correlated with changes in pH in the short term exclosures, while large effects of grazing on soil NO3- content were observed in the long-term exclosures. Long-term release from overgrazing significantly increased carbon and nitrogen metabolism, but decreased carbapanem biosynthesis metabolism. Quantitative PCR further revealed that the genes copies responsible for phosphorus solubilization, nitrification and denitrification increased with time since grazing. In medium term grazing-release sites, the denitrification genes, narG and norB genes decreased while nosZ and nirK increased. However, in long-term grazing release sites, all denitrification genes increased, thereby contributed to the increase of soil NO3- content. Bacteria in the Bradyrhizobium and Opitutus genera were the primary contributors to the shifts in nitrification and denitrification in short-term exclosures, whereas bacteria in both Nitrospira and Pedomicrobium genera play an important role in changes[in what?] to nitrification and denitrification in long-term removal of grazing. Our findings indicate that release from grazing on scales of decades can initiate changes in the soil microbiome that facilitate the recovery of primary productivity and ecosystem functions in grasslands.