Impact of long-term grazing exclusion on soil microbial community composition and nutrient availability

While grazing exclusion is thought to drive soil nutrient transport and cycling, and reduce soil compaction, its direct impact on microbial community composition remains unclear. In this study, we examined the impact of grazing exclusion on abundance and composition of soil microbial (bacterial, archaeal, and fungal) communities, especially those associated with nutrient cycling. We surveyed soil physicochemical properties and litter mass, at sites undergoing varying durations of grazing exclusion (0–34 years) in a semiarid grassland. Using next-generation amplicon sequencing, we further characterized variations in the composition and diversity of soil microbial communities associated with grazing exclusion and soil depths, as well as subsequent changes in physicochemical properties. Most soil physicochemical parameter values significantly increased as the result of long-term grazing exclusion, and these properties were associated with variation in composition and diversity of microbial communities. Notably, the relative abundances of microbial families associated with C cycling (e.g., Chitinophagaceae) increased with an increase in nutrient availability following grazing exclusion. The abundance of the archaeal ammonia-oxidizing Nitrososphaerae increased with decreasing concentration of ammonium among samples. Likewise, fungal communities were also associated with the shifts in nutrient concentrations, although the majority of fungi could not be classified to the species level. Nitrate concentration also played a critical role in shaping bacterial, archaeal, or fungal communities. Moreover, bacterial and archaeal communities had a greater mean Shannon index in 0–10-cm than those in 10–20-cm soil layer. Results of this study provide novel insights regarding how the length of grazing exclusion and soil depth influence nutrient gradients and microbial community composition associated with nutrient cycling.