An irrigated small catchment with intensive agriculture in subtropical China was monitored to estimate loads of N and P off erosion plot, paddy field, subcatchment and whole catchment and to identify main hydrological pathways of nutrient losses and source area. Nutrient concentration in the stream waters and overland flows from the uplands and the paddy fields were elevated although nutrient concentrations was low and stable in irrigation water and rain water. Total N (TN) concentrations in the subsurface waters (9.96 mg L^-1 in the well and 1.71 mg L^-1 in the spring) were up to 10.2 times than in the stream waters (0.98 mg L^-1-1.47 mg L^-1). Inorganic N and P dominated in the streams. Sediment associated N and P losses accounted for over 30% of TN and over 50% of TP from the uplands, while the N and P losses from paddy fields were composed only by dissolved fractions. The highest nutrient concentration occurred during the period from early April to September when fertilization and tillage for sowing and harvesting were performed. Multi-scale monitoring results indicated that N loss was higher in the catchment outflow than in the overland flows from the upland erosion plots, paddy fields and subcatchment, while P loss was equivalent for all the monitoring scales. Regular weekly sampling and intensive sampling during rainstorms revealed that nutrient losses off the catchment were 45.75 kg N ha^-1 a^-1 and 1.10 kg P ha^-1 a^-1, accounting for 22.4 % and 2.0 % of chemical N and P fertilizers applied to the catchment. The nutrient losses during rainstorms accounted for 55 % total N loss and for 74 % of total P loss. The nutrient losses from the agricultural catchment were both agronomically and environmentally significant. Nutrient losses from the paddy fields were higher than from the uplands, indicating that the paddy fields were not sinks of nutrients exported from the uplands. These results suggest that control of nutrient losses with overland flow and subsurface is necessary when timing of fertilization is best designed. The measures to soil erosion control and irrigation control would be effective to reduce nutrient export.

Keywords: Non-point source pollution; Agricultural catchment; Nutrient losses; Nitrogen; Phosphorus; Overland flow; Subsurface flow; Paddy fields