Synergistic effects of fertilization and climate change on increasing nitrate loading in China.

Nitrate-nitrogen losses from terrestrial ecosystems to rivers, driven by human activities and climate change, pose significant threats to water quality in China. However, comprehensive long-term quantification of terrestrial nitrate loading across China and the mechanisms driving it remain unclear. Here, we quantified long-term trends in nitrate loading from 1979 to 2018 using Community Land Model version 5 and evaluated the responses to nitrogen fertilizer inputs and climate change using a random forest model. Our results indicated that nitrate loading increased significantly over the past four decades at a rate of 3.3 × 10-3 Tg N yr-2. This increase was primarily attributable to enhanced nitrate leaching (5.2 × 10-3 Tg N yr-2), despite a concurrent decline in runoff of nitrate (-1.9 × 10-3 Tg N yr-2). Attribution results further revealed a clear pathway divergence: fertilization showed the highest explanatory importance for nitrate runoff (60.8%), whereas precipitation and temperature had greater explanatory importance for nitrate leaching. Nitrogen fertilizer accounted for 38% of total nitrate loading. In contrast, precipitation contributed more to nitrate leaching than nitrogen fertilizer. Moreover, the relative importance of predictors varied substantially among China's seven exorheic basins. Temperature showed the highest importance in southern basins, whereas precipitation ranked highest in the Haihe Basin. The influence of nitrogen fertilizer was greatest in northern agricultural basins. Together, these findings clarify the interplay between anthropogenic nitrogen inputs and region-specific climate sensitivities and highlight the need for tailored, basin-scale nitrogen management strategies.