Estimation and sources apportionment of non-point source nitrogen and phosphorus loads in Beishahe sub-catchment of Beijing

Beishahe sub-catchment of Beijing was studied for the source apportionment of non-point source (NPS) pollution. The NPS pollutants in this area were generalized into two types: the dissolved and the adsorbed. The dissolved and adsorbed nitrogen and phosphorus NPS pollution loads were estimated based on the precipitation runoff model (soil conservation service curve number, SCS-CN), soil erosion model (revised universal soil loss equation, RUSLE), and the pollutant output load model empirical equation. The spatial distribution characteristics of nitrogen and phosphorus loads produced by different land use types were also analyzed. The simulation results showed that the average annual load of total nitrogen (TN) in Beishahe sub-catchment was 0.625 t/(km² ·a), of which the dissolved nitrogen was 0.190 t/(km² ·a) and the adsorbed nitrogen was 0.435 t/(km² ·a); the average pollution load of total phosphorus (TP) in Beishahe sub-catchment was 0.118 t/(km² ·a), of which the dissolved phosphorus was 0.011 t/(km² ·a) and the adsorbed phosphorus was 0.107 t/(km² ·a). The rural area produced the most TN of 0.855 t/(km² ·a) among all different land use types, followed by the forest land (0.713 t/(km² ·a)) and the unused land (0.619 t/(km² ·a)). For TP, the land use types with highest annual pollution load was the grassland (0.238 t/(km² ·a)), the forest land (0.126 t/(km² ·a)) and the unused land (0.115 t/(km² ·a)). Generally speaking, the losses of nitrogen and phosphorus were mainly in the form of adsorbed in the Beishahe sub-catchment. Therefore, controlling of the soil erosion, water and soil loss should be highlighted for the reduction of the loads of adsorbed nitrogen and phosphorus in Beishahe sub-catchment.