| Human activities such as fertilizer application, fossil fuel combustion have resulted in increased nitrogen(N) loads for terrestrial and aquatic ecosystems causing worldwild concern. As terrestrial ecosystems become saturated with N, excess N moves with surface runoff and groundwater flow to streams, lakes, rivers, and coastal oceans. Headwater streams convey water and nutrients to larger streams and despite their relatively small dimensions, play a disproportionately large role in nitrogen transformations on the landscape. Quantitative information on N cycling in streams is needed to understand how N loading from watersheds for streams transporting much of this N directly from terrestrial ecosystems.In this study, Water quality monitoring, field measurement, mechanism model, isotope tracing method and GIS technique were linked to estimate N flux of streamflow discharge and leaching. The work provides a sound understanding of the influence of surface runoff and groundwater flow on stream N, and highlights transformation and biogeochemistry behavior of dissolved nitrogen in stream.First, annual and seasonal patterns of N loss in streamflow were evaluated based on monitoring data of water quality and flow in 2004-2006. N loss in stormflow was positively related to the ratio of arable land, and varied greatly among the representative subwatersheds, reflecting the differences in precipitation, land cover, and N inputs. The annual maximum total N export was 108 kg N hm-2 of 2006, and the minimum was 37.11 kg N hm-2 of 2004. Compared to average annual N inputs, 421.4 kg N hm-2, average annual total N loss of these three years from streamflow was 68.35 kg N hm-2. Storm runoff conveyed total N 45.51 kg N hm-2, which contributed to 10.8% of total N inputs and baseflow conveyed total N 22.84 kg N hm-2, which contributed to 5.4% of total N inputs. The riverine export of dissolved total nitrogen (DTN), formed 84% of the total flux and the dissolved inorganic nitrogen (DIN) formed 89.5%of DTN. NO3-N and NH4-N were main forms of DIN, contributed to 65.5% and 34.5%respectively.Second, N leaching in Wuchuan catchment was evaluated using shallow water concentration and GLEAMS (Groundwater Loading Effects of Agricultural Management Systems) model. The variation of N concentration in shallow aquifers indicated that higher fertilization under land of vegetable and commercial crops in spring provided the larger N source. Meanwhile, low hydrologic table in winter (dry season) resulted in a concentrating effect of N concentration. The peak N export in leachate occurred between July to September when rainfall intensity was very high. N leaching was dominated by ammonium form (40%) rather than nitrate form (20%), as a result of acid soil being positively charged with electricity when pH<5. Nitrate export rate differed substantially with landuse and varied from 5.00 to 50.79 kg N hm-2 in 2004-2006, with the area-weighted value of 19.70 kg N hm-2 accounting for 4.7% of total N inputs in Wuchuan catchment.Third, This study found that NH4 and NO3 concentrations in streams were in dynamic balance, controlled by input, nitrification, biological uptake, sorption, and regeneration. The relationship between NO3-N withδN15, NO2-N, dissolved oxygen, dissolved organic carbon indicated that nitrification was the main biogeochemical process of Wuchuan stream, which played a large role in increasing nitrate levels during stream transport over a relatively short distance. With the area of arable land increment, non-point pollution discharging into the stream was also responsible for NH4-N and NO3-N concentration downstream rising, especially for NO3-N. In headwater streams, uptake and removal processes occur mainly on sediments and biofilms covering submerged surfaces. Thus, the shallow depths and high surface-to-volume ratios characteristic of the Wuchuan stream supply a relative reactive channel conduit. |
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