[Identifying sources of nitrate in surface water of Jiulong River Watershed]

Two representative catchments, Wuchuang catchment and Xiandu catchment, were selected from Jiulong River Watershed as study areas to trace sources of nitrate in surface water by using 15N isotope technique. In the spring of 2005, the surface water samples were twice collected from the catchments and determined for dissolved total nitrogen (DTN), nitrate nitrogen (NO3(-)-N), ammonia nitrogen (NH4+-N) and delta 15N value of nitrate. The results showed that in the samples from Xiandu catchment, the concentration ranges of DTN, NO3(-)-N and NH4+-N were 1.47-5.31 mg/L, 0.83-4.05 mg/L and 0.21-0.36 mg/L respectively, and the delta 15N values of nitrate ranged from 2.54 per thousand to 7.92 per thousand; in the samples from Wuchuang catchment, the concentration ranges of DTN, NO3(-)-N and NH4+-N were 1.14-5.56 mg/L, 0.96-1.46 mg/L and 0.12-1.28 mg/L respectively, and the delta 15N values of nitrate were between -0.19 per thousand and 5.89 per thousand. In contrast with the typical ranges of delta 15N isotopic signature of nitrate from different sources and in consideration of the information of local crop planting circumstances and application of fertilizers, the data indicated that the first, the nitrate in the surface water of Xindu catchment mainly came from inorganic fertilizers and soil organic nitrogen, and some originated from organic fertilizers; the second, the nitrate in the surface water of Wuchuang catchment mostly flowed from inorganic fertilizers and soil organic nitrogen, and few derived from organic fertilizers; the third, the sources of nitrate varied with the sampling location and time and were dependent on the kinds of local crops and their planting seasons.