Grazing intensity induced alternations of soil microbial community composition in aggregates drive soil organic carbon turnover in a desert steppe

Grazing is the primary land-use strategy for livestock production in grasslands, which regulate the health of grassland ecosystems. However, our understanding of grazing-induced changes in soil microbial community structure and their links to organic carbon (C) in soil aggregates is limited. By using a long-term enclosed grazing desert steppe, we examined impacts of different grazing intensity on the diversity and composition of soil microbial community in different aggregates and their contribution to soil organic C (SOC) turnover. The results showed that 15-year grazing induced a notable increase in the SOC content of the large macroaggregate (2 mm) and silt and clay (SC: < 0.053 mm) fractions, but did not change bulk soil organic C content. Soil prokaryotic communities were significantly impacted by grazing intensity and aggregates fractions (p < 0.01). Microaggregate (0.053-0.25 mm) and the SC fraction hosted relative more sequences of Acidobacteria, Gemmatimonadetes, and Rokubacteria than macroaggregates (0.25-2 mm), but relatively less sequences of Actinobacteria and Bacteroidetes. Co-occurrence network analysis revealed that microaggregates hosted a much more complex network and harbored more keystone taxa than macroaggregates and the SC fraction. Gemmatimonadetes was the dominant phylum (relative abundance similar to 9%) and keystone taxa largely belong to this phylum, and its abundance was positively correlated with the level of SOC. Overall, grazing practice altered the soil prokaryotic community, which led to increase in the organic C level in the large macroaggregate and SC fractions, the early indicators of change in SOC dynamics. Together, our findings suggest that removing livestock from these lands would reduce SOC with probable impact on the productivity of the desert steppe and that proper grazing is an import factor of long-term sustainability of the desert steppe.