Land use impact on nitrogen discharge by stream:a case study in subtropical hilly region of China

As a crucial factor of water eutrophication, nitrogen (N) discharge by agricultural non-point sources (NPS) has become a worldwide concern, and so has its relationship to land use. This study was aimed at the quantitative relationships between N discharge by stream and land use. It was conducted in the Meicun watershed of Xuancheng County, Anhui Province, in the subtropical low hilly area of China. The study integrated dynamic monitoring of nutrient discharge by stream water and Geographic Information Systems (GIS) analysis of land use of the watershed. Results showed that NO₃⁻-N discharge ranged between 50 and 60% of the total nitrogen (T-N) and was 2.5–3.0 times as much as NH₄⁺-N. There was a significant difference between forested and mixed sub-watersheds for NH₄⁺-N, NO₃⁻-N and T-N concentrations. Significant correlations existed between NH₄⁺-N, NO₃⁻-N and T-N concentrations in stream water and the area percentages of forest and paddy fields. The study found that the discharges of NH₄⁺-N, NO₃⁻-N and T-N decreased exponentially with forest area increase, but a steep decline for NO₃⁻-N and T-N seemed to occur when the forest percentage surpassed 70% and then there was almost no change. Similarly, the discharges of NH₄⁺-N, NO₃⁻-N and T-N increased exponentially with the paddy fields increase, but a steep augmentation occurred for NO₃⁻-N and T-N when the paddy percentage surpassed 20% and then it remained at a plateau. The study showed that in the subtropical hilly region, paddy fields could increase N discharge due to farming management practices. Thus, primary measures to reduce N in the receiving water body would include a change in farming management and building an ecological interception system for paddy fields.