Benefits of CO2 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.