The global effects of grazing on grassland soil nitrogen retention

Soil nitrogen (N) in grassland ecosystem is highly sensitive to grazing, thus understanding the responses of soil N retention to grazing is critical to maintaining grassland ecosystem function. Here, we quantified the effects of grazing on total and available soil N retention under different management regimes (in terms of grazing intensity and duration) and environmental factors (temperature, precipitation, and ecosystem N abundance) using 2785 paired observations from 179 publications, and found that grazing (particularly moderate-to-heavy grazing) reduced total and available N retention worldwide. Additionally, grazing is associated with more soil N loss from long-term grazing (> 4 years), warmer (mean annual temperature ≥ 5°C), drier (mean annual precipitation ≤ 300 mm), or relatively N-sufficient (initial N content ≥2 g kg⁻¹ soil) grasslands, implying the importance of climatic and environmental factors in regulating N loss. Similarly, interactive effects of grazing with mean annual temperature, mean annual precipitation, duration and initial N content on soil total N were observed. More importantly, our results emphasize the grazing effect on soil N was best explained by the response ratios of plant belowground biomass. Our study provides global insights that will aid sustainable grazing management, and more research is needed to explore the regulatory role of grazing intensity and nutrient supplementation on global grassland N retention.