Benefits of CO ₂ Fertilization on Global Grassland Nitrogen Cycles

Grasslands are vital for providing wildlife habitats, essential ecosystem services, and food to humanity. Elevated atmospheric CO₂ concentration (eCO₂), as a key driver of climate change, has substantial effects on grasslands. Nonetheless, a holistic quantification on the response of the nitrogen cycle to eCO₂ in global grasslands is lacking. Here, we show that solely eCO₂ could increase the net primary productivity (NPP) in grasslands by 10% (95% confidence interval, 8-12%), yet simultaneously decrease leaf nitrogen content by 8% (95% confidence interval, 5-10%). When considering the effects of eCO₂ on both NPP and leaf nitrogen content, we estimate a projected increase of 2.1 million tonnes per year (Tg yr^-1) in nitrogen harvest by global grasslands under the eCO₂ SSP2-4.5 scenario in 2050. The increase in nitrogen use efficiency by 29% (95% confidence interval, 18-40%) and biological nitrogen fixation by 66% (95% confidence interval, 18-138%) would lead to declines in nitrogen surplus (-28.3 Tg yr^-1). These changes in the grassland nitrogen budget due to eCO₂ would generate benefits to 158 billion US dollars by avoiding human and ecosystem health damages. Priority should be accorded to sustainable nitrogen management strategies to leverage the advantages of CO₂ fertilization for ensuring food security and mitigating nitrogen pollution.