Effects of organic amendment applications on nitrogen and phosphorus losses from sloping cropland in the upper Yangtze River

Controlling nitrogen (N) and phosphorus (P) loss from hill slope cropland is crucial to mitigate agricultural non-point source (AGNPS) pollution in the upper Yangtze River basin. However, pathways and loadings of soil N and P loss and their responses to agricultural management practices are not well documented. We conducted a three-year field experiment using the free-drain lysimeters to evaluate the long-term effects of organic amendments on N and P loss from sloping cropland. The field experiment included five experimental treatments: mineral fertilizers (NPK) as a control, fresh pig slurry as organic manure only (OM), crop residues only (CR), organic manure combined with NPK (OMNPK), and crop residues combined with NPK (CRNPK). The results show that organic amendment applications decrease discharges of overland flow and interflow compared with NPK treatment. On average, annual total N (TN) loss loadings ranges from 8.3 to 27.6 kg N ha(-1) , with 1.1 to 5.2 kg N ha(-1) of particulate N (PN), and 6.4 to 19.1 kg N ha(-1) of nitrate N (NO3-N) loss loadings, respectively for the five treatments. Thus, PN and NO3-N losses accounts for 13-19 %, and 63-78 % of TN loss loadings, respectively across the five treatments. However, the average annual total P (TP) loss loadings ranges from 0.23 to 0.84 kg P ha(-1), with 0.21 to 0.80 kg P ha(-1) of particulate P (PP) for the five treatments. Moreover, PP loss accounts for 92-96 % of TP loss loadings across the five treatments. Compared to NPK treatment, organic amendment applications significantly decrease TN loss, by 85 % via overland flow and 117 % via interflow on average. Organic amendment applications also significantly decrease TP loss, by 162 % via overland flow and, by 90 % via interflow compared with NPK treatment on average. This study shows that the combination of mineral fertilizers and organic amendments of either manure or crop residues can mitigate hydrological N and P loss from slope croplands in the upper Yangtze River basin.