Climate Change effects on Nitrogen Cycles in Global Grasslands

Grasslands, as one of Earth’s major ecosystems, are critical for sustaining biodiversity, ecosystem services, and global food security. However, their nitrogen cycles are increasingly influenced by climate change, including elevated atmospheric CO₂ (eCO₂), warming, and shifting precipitation regimes. These changes significantly affect grassland productivity and nitrogen dynamics, with substantial regional variations. Using a synthesis of over 5,000 experimental observations coupled with multiple ecosystem models, we investigated the impacts of climate drivers on nitrogen dynamics under the SSP2-4.5 scenario. Elevated CO₂ alone is projected to enhance global grassland net primary productivity (NPP) by 10% while reducing leaf nitrogen content by 8%, resulting in a net increase of 4 Tg yr^-1 increase in nitrogen harvest by 2050. Enhanced nitrogen use efficiency (+29%) and biological nitrogen fixation (+66%) under eCO₂ would reduce nitrogen surplus (-29 Tg yr^-1) and fertilizer demand (-9 Tg yr^-1), potentially mitigating nitrogen pollution and yielding economic benefits of 235 billion USD. Warming, as another driver, is projected to increase nitrogen inputs by 17 Tg yr^-1 and nitrogen harvest by 12 Tg yr^-1 but may exacerbate reactive nitrogen losses by 5 Tg yr^-1. Adaptation measures to minimize nitrogen leakage could deliver economic gains of 121 billion USD by 2050. Precipitation shifts further complicate nitrogen dynamics. Regions with increased precipitation, such as the United States and mid-to-high latitude Asia, could see nitrogen harvest rise by 16 Tg yr⁻¹, while areas facing reduced precipitation, including Sub-Saharan Africa and South Asia, risk a 9 Tg yr⁻¹ harvest decline. These imbalances could worsen global inequalities in nitrogen cycles and food security. Our findings highlight the need for improved representation of these complex interactions in ecosystem models to guide climate adaptation strategies. Timely, targeted interventions can help balance benefits and risks, safeguard ecosystem health, and support sustainable, equitable grassland management in a changing climate.