Hotspots of nitrogen losses from anthropogenic sources in the Huang-Huai-Hai Basin, China

Poor management of nitrogen (N) can lead to serious environmental problems, such as air and water pollution. The accurate identification of priority control areas and emission sources is critical for making effective decisions regarding sustainable N management. This study aimed to identify hotspots for N losses and quantitatively analyze the relative contributions of different emission sources in the Huang-Huai-Hai Basin at the county scale. Specifically, it focused on N losses from agricultural production (crop production, livestock production, and freshwater aquaculture) and residential sources (urban sewage and rural domestic wastewater). To achieve this, we developed the Model to assess Anthropogenic Nitrogen losses to the Environment, following the mass flow analysis approach. The total N losses from agricultural production and residential sources in the Huang-Huai-Hai Basin were approximately 6.4 Tg per year in 2017, with 56% emitted to water and 44% to the air. Crop production and livestock production accounted for 46% and 45% of the total N losses, respectively. The main pathways of N loss were NH3 emissions to the air (41%) and direct nitrogen discharge to water (28%). Hotspots of total N losses were identified in 202 out of 910 counties, primarily located in the Henan and Hebei provinces. Although these hotspots cover less than 7% of the total basin area, they account for 51% of the total N losses. The main driving factors leading to these hotspots are the number of livestock, followed by the use of chemical N fertilizers and the extent of cultivated area. The findings of this study provide important insights for the targeted control of anthropogenic reactive N loss in the Huang-Huai-Hai Basin.