Nitrogen and phosphorus retention budgets of a semiarid plain basin under different human activity intensity.

Excessive nitrogen (N) and phosphorus (P) runoff from human activities results in degraded water quality. It is, therefore, crucial to quantitatively assess nutrient inputs over time and their impact on riverine nutrient exports. In this study, we estimated the long-term (1995-2015) nutrient inputs at the county scale by integrating Net Anthropogenic Nitrogen Input (NANI) and Net Anthropogenic Phosphorus Input (NAPI) methods, and nutrient exports into rivers by the Export Coefficient Model (ECM) for a semiarid plain basin, the Baiyangdian (BYD) Basin, China. The results showed that N and P input intensities in the year 2015 reached 18852 kg N km2 yr-1 and 2073 kg P km-2 yr-1, showing a 35% and 11% increase in comparison with 1995, respectively. About 60% of these nutrients were derived from fertilizer application. The multi-year averaged N and P exported to rivers was 548 kg N km-2 yr-1 and 79 kg P km-2 yr-1, respectively. Hotspots for nutrient budgets were found in the southeastern counties. Hotspots covered about 12% of the total land, but contributed by 38-52% of total nutrient budgets. The nutrient export ratios showed no significantly temporal variations, and only about 2.15-2.89% of NANI and NAPI were exported into rivers. The low nutrient export ratio was due to the low water discharge that limited the nutrient transportation in the semi-arid plain basin. As most of anthropogenic nutrient inputs were retained in the basin, their impacts on the pollution of soils and aquifers need to be considered and adequately addressed in the future. This study constructs the spatial quantitative nutrient budgets, which can provide effective information for region policy formulation.