Exploring the optimal nitrogen threshold for global grassland restoration

• Global experiments on gradient nitrogen application in grassland show similarities. • Plant productivity exhibits a threshold response model to nitrogen application. • Nitrogen threshold effect affects the entire vegetation-soil-microbial system. • Ecological threshold is affected by the degree of degradation and site conditions. • Threshold effect reveals a conceptual framework of active ecological restoration. Amid accelerating global land degradation, establishing high-efficiency ecological restoration principles and frameworks is crucial. Here, we explore the application of threshold effects in the ecological restoration process based on field experiments and globally available experimental data from 173 sites. Combining data integration analysis and meta-analysis, we collectively verified the universality of threshold effects in grasslands. The global grasslands’ average nitrogen application threshold is 3.78 g·m −2 ·yr −1 , while the threshold value of degraded grassland (3.65 g·m −2 ·yr −1 ) is lower than that of nondegraded grassland (5.90 g·m −2 ·yr −1 ). The low nitrogen-driven thresholds are affected by degradation status, climate (precipitation and temperature), and other site conditions, but not fertilization forms. Independent experiments further demonstrated that an increase in soil moisture content can lead to the disappearance of nitrogen threshold effects, revealing that ecological threshold effects are influenced by ecosystem stress factors. Following the significant increase in plant biomass triggered by the nitrogen threshold, the ecosystem undergoes systemic improvement. Soil organic carbon, urease activity, soil microbial diversity, and other soil properties are significantly enhanced. Soil nitrogen cycle-related microbial communities and soil physicochemical attributes are significantly activated. The results indicate that a threshold response pattern may develop before nitrogen saturation is reached, and low nitrogen input can boost productivity and improve the plant-soil-microbe system. Our findings reveal a nonprogressive path of restoration in degraded ecosystems, and thus, restoration based on threshold effects can offer an efficient and safe solution to combat ecological degradation.