Exploring the optimal nitrogen threshold for global grassland restoration

Abstract

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.

Keywords

Ecological restoration / Thresholds effect / Grassland / Nitrogen / Meta-analysis

Data availability

All materials, raw data, and codes used in the article are available upon request and without restriction, and all data that support the main findings of this study will be made publicly available in the Supplementary information upon publication.

CRediT authorship contribution statement

Qi Zhang: Writing - original draft, Visualization, Validation, Investigation, Formal analysis, Data curation. Fu Chen: Writing - review & editing, Investigation, Funding acquisition, Conceptualization. Zhanbin Luo: Writing - review & editing, Supervision, Methodology, Investigation. Jun Fan: Resources. Yanfeng Zhu: Methodology, Investigation. Jing Ma: Methodology, Investigation, Funding acquisition. Yongjun Yang: Investigation. Xi-en Long: Resources. Alejandro Gonzalez-Ollauri: Methodology. Miao Gan: Resources, Investigation. Weihong Guo: Investigation. Yuxiang Ma: Validation. Qiaoling Wang: Validation. Shenglu Zhou: Supervision. Mingan Shao: Resources.

Declaration of competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Major Special Projects of the National Natural Science Foundation of China (Grants No. 52374170 and 42377465), the Third Comprehensive Scientific Exploration in Xinjiang (Grant No. 2022xjkk1005), the Special Technology Innovation Fund of Carbon Peak and Carbon Neutrality in Jiangsu Province (Grant No. BK20231515), and the Shaanxi Shenmu Natural Field Observation and Research Station of Erosion and Environment, which provided the site and data on experimental conditions for field trials.

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.geosus.2025.100396.

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