| To understand the hydrological pathways of nitrogen loss in hilly area of purple soil, water flows, concentrations and fluxes of N through rainfall-runoff in three natural storm rain events were monitored at a small watershed in the central Sichuan Province, China in 2007. The results showed that the N concentrations responded with discharge flow. Total nitrogen (TN) concentration increased sharply up to the peak, then decreased slowly, and followed a rising trend at the end of stage of the discharge flow; while nitrate-nitrogen (NN) increased during the observation period. The concentrations of particulate nitrogen (PN) peaked sharply and then followed by a rapid decline. The low concentrations and small variations were observed in ammonium-nitrogen (AN). PN transported by surface flow was dominated in the early periods of storm runoff, while NN became dominant in the late period, suggesting the importance of subsurface flow transporting of nitrate. The loss loads of N in storm runoff were high, with mean loads of TN was 4.8 g·m-3. Based on a (MV) curve, the loads of PN mostly distributed in the early portion of storm-runoff, showing first flush effects was important and particulate N were transported with overland flow. By contrast, NN loads mainly were in the later portion of rainfall-runoff, implying NN was transported with subsurface flow.The nitrogen transportation in watershed-ditch was studied based on the investigation of water quality at Jieliucun watershed during the period of 2005-2007. Results showed that the mean concentrations of nitrogen was falling from upstream drain to the outlet, of which TN, NN, PN, AN, and NO2-N decreased from 12.58,5.30,2.77,1.79, and 0.54mg·L-1 to 2.28,1.29,0.15,0.37 and 0.11mg·L-1, respectively (P<0.01). NN and PN which accounted for 52% and 30% of TN were the main forms of nitrogen in the upper reaches of the drain, while then NN dominated the rest part of the drain. The total retention of TN in the ditch was 239.65kg·a-1, implying the retention of drain was significant. NN and PN were the major retention forms of nitrogen (accounted for 52% and 28%). 81% of retention occurred in the upper reaches of the drain, suggesting this section was the main "sink" of nitrogen pollution in the drain. The total release of TN was 117.18kg·a-1 and NN was dominating which accounted for 56%. So NN was the endogenous pollutant of the drain. Drain had the function as channel for nitrogen transportation and the major transportation form was NN. Temporal variations of nitrogen were significant. In total, the concentrations were high in rainy seasons. The concentrations and loads were lower in drought year than that in normal years; however the load of NN was higher in drought year.To further undertand the impact of N export on water environment, the study also investigated the nitrate concentrations in shallow groundwater at three adjacent watersheds (Jieliucun, Chenjiawan, and Daxing) in hilly area of purple soil during 6 monitoring years from 2002 to 2007. The range of groundwater nitrate-N (NO3--N) concentration was from 2.44 to 31.16 mg/L with mean value of 7.67 mg/L.19% of wells with high (≥10mg/L) NO3--N concentrations were over the recommendations (US EPA) for drinking water and 46% in medium (≥5 and<10mg/L) were approaching the standard. Temporal trends showed that NO3--N concentrations were higher in dry seasons and after rain events associated with fertilization. In addition,77% of wells were stable compared with 15% of rising,4% of falling and another 4% of variable during the period of study. The stable wells were almost adjacent to woodlands. NO3--N concentrations were higher in Jieliucun watershed and lower in Chenjiawan watershed, that is wells with higher nitrate concentration almost located on the toe slope and adjoined slopelands and residential areas, while the groundwater under the woodlands on the top and shoulder slope were lower. Spatio-temporal variation of groundwater NO3--N concentrations was controlled by an interaction of rainfall, groundwater level, fertilization, topography, and land use. However, the local hydrologic condition and land use were the most important factors which affected on it directly. A large amount of subsurface runoff as the main pathway for nitrate leaching in slopelands played an important role in high NO3--N concentration of groundwater in purple soil area. Surplus N from N-fertilization and animal manure accumulated in soil was the main non-point source for groundwater nitrate contamination. Whilst, well 17 closed to septic tanks associated with livestock showed the highest NO3--N concentration, implying septic tanks were the main point source. |
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