Grassland soil bulk density decreases under grazing exclusion and responds asymmetrically to altered precipitation

Soil structure is a key determinant of grassland health and functioning, with bulk density serving as a widely used proxy. Grazing exclusion is a common restoration practice that often reduces soil bulk density in grasslands by restoring plant productivity and removing livestock trampling. Plant productivity is strongly influenced by precipitation, but the interactive effects of precipitation and grazing exclusion on grassland soil bulk density remain unclear. We presented a synthesis based on 671 paired observations from 111 field studies to investigate the responses of soil bulk density in grasslands to grazing exclusion and altered precipitation, and the key drivers. Overall, grazing exclusion decreased soil bulk density by 7.88% compared to grazing, with the strongest effect occurring in tropical grasslands and in surface soils. This decrease was more strongly controlled by standardized precipitation evapotranspiration index (SPEI) than by soil factors, declining with increasing wetness and becoming negligible in severely wet regions. Under altered precipitation, increased precipitation reduced bulk density by 9.06% relative to ambient precipitation, whereas decreased precipitation increased it by 17.10%, showing an asymmetric response. In extreme events, the response per unit precipitation change was stronger for decreased than for increased precipitation. The asymmetric response was directly and negatively related to SPEI, soil organic carbon content, and the magnitude of precipitation change. Publication bias and sensitivity analysis confirmed that the results were reliable. These results quantitatively demonstrate the global-scale decrease in grassland soil bulk density under grazing exclusion and its asymmetric response to altered precipitation. The results suggest that the efficacy of grazing exclusion in reducing soil bulk density is probably modulated by precipitation regimes, particularly extreme events.