| Nitrogen is an important biogenic element and widely distributed in nature. The cycling of nitrogen is significantly influenced by human activities. The extensive application of chemical fertilizers, combustion of fossil fuel and vehicle exhaust emissions have greatly accelerated the nitrogen accumulation in the agro-ecological system, which causes a lot of environmental problems, such as acidification of soil and water bodies, acid rains and eutrophication.From a global scale, the generation of Nr was in a slow growth from 1860 s to 1960 s, while the growth was accelerated in late 50 years. Nr content increased from 15 Tg in 1860 s to 156 Tg in 1995, and reached 187 Tg in 2005. At present, the wet deposition of atmospheric nitrogen flux is 8.85 Tg?a-1 in average, China has become the third largest N deposition area after Europe and the United States. China is located in the monsoon climate zone where changes environment climate fastest all over the world. The nitrogen pollution in the basin researches in China are mostly concentrated on Yangtze, the Yellow River, Pearl River and main lakes of Chaohu Lake, Taihu Lake and Dianchi Lake watershed. There are few researches in subtropical hilly area of agricultural watershed.In this study, Xiangxi River Basin in Qianyanzhou of JiangXi province was monitored to reveal the cycling of nitrogen in watershed and explore the southern red soil region of N wet deposition characteristics and rainfall-runoff process. In addition, the interception mechanism of forestry canopy to atmospheric wet deposition was disclosed and the source of nitrate nitrogen contamination by the means of the stable isotopes of nitrogen and oxygen was analyzed. The main results obtained are as follow:1. In the monitoring period, annual rainfall was 1330.5 mm with 273896.4 m3 runoff. The wet deposition flux of TN(total nitrogen) was 41.72 kg?hm-2 during the year 2014. Different rainfall events had different nitrogen concentrations, among which the DTN(dissolve total nitrogen) and DIN(dissolve inorganic nitrogen) in the light rain were higher than other kinds of rainfall events. The rainstorm concentrations were the lowest during the rainfall-runoff events in spring. The pH range of rain in 2014 was from 5.83 to 4.68. The incidence of acid rain was about 75%.Under the base-flow scenario of spring, the nitrogen concentrations were highest in the midstream of the river basin and lowest in the downstream, the maximum TN concentration was 12.10 mg?L-1. In contrast to the base-flow scenario, the different forms of N were higher under rainfall-runoff scenario,the concentrations of DTN, NO3--N and NH4+-N were highest in light rain and lowest in rainstorm. The reason might be torrential rain diluted the nitrogen concentrations and also related to local fertilization.2. High nitrogen output load value was concentrated in the rainy season, the more rain, the more nitrogen output. The TN output load in March to June was accounted for 92.79% of the whole year, so the rainy season controlled the annual nitrogen output. The loss of nitrogen forms: NO3--N accounted for 54.84% of the DTN output load, DTN was mainly form of nitrate nitrogen loss. PN(particulate nitrogen) runoff accounted for 54.98% of TN the output load, PN was the main form of TN output.The wet deposition in the river basin contributed about 385.2 to 511.1 kg N to rivers. Among the six rains in rainy season, the lowest nitrogen contribution to river was 3.06~4.06 kg of the light rain and the highest was 30.62~40.63 kg of heavy rain. In the spring of 2014, the contribution rate of the typical rainfall on watershed and nitrogen output of minimum was on May 17. For the light rain, the highest was May 22, while for torrential rain, the higher rainfall intensity, the higher contribution rate.3. During winter in 2014 to 2015, the main forms of wet deposition were nitrogen and sulfur deposition, and N, S, C, P deposition fluxes were 4.31, 14.57, 2.03 and 0.19 kg·hm-2, respectively. There were differences between various nitrogen concentrations of throughfall in different forests. Broad-leaf forest had stronger absorption effect with nitrogen than aciculiailvae. Forestry canopy had dilution effect in DTN and NH4+-N, for TN was absorption effect. Masson pine had great absorption effect with SO42- and NO3-, which had buffer action for acid rain.4. Research belonged to farmland ecosystem, by testing the nitrogen and oxygen isotopes, it was disclosed that the main source of contamination of NO3- were organic fertilizer and sewage in the watershed, affected by human activities.5. The pH range of Xiangxi basin was higher in summer and it had a small range in winter. pH values were over 7 in both spring and winter, which was the normal range in freshwater system. The results showed the concentration of TN and DTN under base-flow and rainfall-runoff scenario were all over 1.5 mg?L-1, which was the stream eutrophication threshold. According to the surface water environment quality standard(GB 3838-2002) for the total nitrogen and ammonia nitrogen density, water quality in the study area ranged between class III and IV. Although algal bloom had not appeared in the Xiangxi river basin yet, the local farmer should pay more attention to the N fertilizer usage. |
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