Stage-specific responses and shifting regulatory mechanisms of ecosystem multifunctionality along a grassland degradation gradient

• EMF peaks at light degradation, then declines with increasing severity. • Dominant EMF drivers change with degradation: microbial diversity early, soil nutrients later. • Severe degradation limits EMF mainly via soil compaction. • EMF regulation shifts from biotic control to resource coupling and physical limits. • Grassland degradation alters biotic and abiotic regulatory mechanisms. Worldwide grassland degradation undermines ecosystem functioning, thereby affecting the sustainable utilisation of grassland ecosystems. However, the response patterns and underlying regulatory mechanisms of the ecosystem multifunctionality (EMF) in grasslands along degradation gradients remain poorly understood. This study systematically evaluated the variation trends of EMF and its driving factors across four typical degradation stages in a temperate grassland ecosystem. The results indicate a typical hump-shaped pattern of EMF along the degradation gradient, peaking at light degradation and declining markedly with increasing severity. This variation is jointly regulated by the direct and indirect effects of multiple biotic and abiotic factors. Specifically, fungal richness was the strongest predictor in non-degraded grasslands but showed a negative correlation with EMF; during light to moderate degradation stages, microbial diversity combined with soil carbon and nitrogen content jointly promoted EMF; whereas in heavily degraded stages, soil bulk density emerged as the dominant limiting factor, reflecting pronounced physical stress. EMF regulation shifts along the degradation gradient, from biotic to resource to physical control, showing complex nonlinearity and context dependence. This study reveals stage-specific responses and alternative regulatory mechanisms of grassland multifunctionality at fine degradation scales, providing a basis for targeted grassland restoration.