Integrated climate effects on nitrogen cycles in global grasslands

Grasslands play a crucial role in providing essential ecosystem services through biogeochemical processes. Improving grassland productivity and nitrogen use efficiency, reducing reactive nitrogen losses, and ensuring environmental sustainability represent major challenges, especially under the influence of global climate change. While previous studies have shown substantial effects of individual climate change factors on grassland nitrogen cycling, a comprehensive understanding of how grassland nitrogen cycling responds to multiple climate change remains limited. In this study, using data from 150 countries, we identified climate warming as the primary driver of increased nitrogen harvest, biological nitrogen fixation, and nitrogen surplus in global managed and undisturbed grasslands. These increases, with respective increments of 19.8, 8.8, and 28.2%, were determined by comparing scenarios with and without climate change from 1980 to 2020. Precipitation variability further amplifies these nitrogen increases, displaying notable spatial heterogeneity. Conversely, elevated atmospheric CO₂ levels mitigate nitrogen surplus by enhancing plant nitrogen uptake. Under the SSP2-RCP4.5 scenario for the year 2050, nitrogen input, harvest, and surplus in global grasslands are projected to increase annually by 22.3, 7.2, and 15.1 million tonnes, respectively, compared to baseline scenarios. These climate-induced alterations in nitrogen budgets could incur additional costs up to USD $69 billion because of associated impacts on human health and ecosystem integrity. Our findings emphasize the urgent need for robust management strategies aimed at mitigating the negative effects of climate change on grassland nitrogen cycling, thereby supporting global sustainable development objectives.