| Nitrogen loss was one of the important reasons causing agricultural non-point source pollution. Nitrogen loss of the Three Gorges reservoir area each year was not only caused great fertilizer waste, but also caused the reservoir environment great damage. To make clear the nitrogen loss load and the influence to water environment, ease the pressure on the environment, reduce non-point source pollution, protect the ecological environment of the Three Gorges reservoir area, through using grid method for sampling, and laying different slope experimental plots and engineering measures, it was studied on nitrogen distribution and loss in a small Three Gorges reservoir watershed. The results showed:①Soil total nitrogen and available nitrogen contents were 1.20±0.26g kg-1 and 96.34±25.14mg kg-1, coefficients of variation were 21.67% and 26.09%, content distribution obeyed the normal distribution, distribution trends were time distribution, concentrated in the 0.90g kg-1-1.30g kg-1 and 80.00mg kg-1-120.00mg kg-1, soil total nitrogen and available nitrogen contents was highly significant positive correlation (r=0.774, P<0.01). The semi-variance model were spherical model and index model, which had moderate spatial correlation of available nitrogen and faintish spatial correlation of total nitrogen, and the determination coefficient were 0.792 and 0.891, the maximum sample spacing was 539.71m of study area. The contents distribution trends of soil total nitrogen and available nitrogen was high in the middle, but low at surrounding, and the south was higher than north. Under different land use, soil total nitrogen and available nitrogen content were highest in paddy fields, which were 1.29 g kg-1和103.19 mg kg-1, lowest in dry land, woodland and garden between them. There were significant differences of soil total nitrogen and available nitrogen contents between paddy field and dry land, garden, but woodland. There were no significant differences of soil total nitrogen and available nitrogen contents between dry land and woodland, garden (P=0.05).②There were significant positive correlation between rainfall and runoff, erosion sediment (P=0.05). The correlation coefficient of rainfall and runoff showed that the slope was increasing, the coefficient was higher, which was converse between rainfall and erosion sediment. When the rainfall reached a certain value, it can produce runoff and erosion, and which was easier with the slope increasing. Under the same rainfall intensity, the slope greater, the runoff and erosion sediment greater. Nitrogen concentration of runoff changed large, especially ammonium, but the erosion sediment was not. On the single rainfall, the nitrogen concentration differences of the same form were not large under different slope, but the content of runoff and erosion sediment also showed 4°<9°<17°. The nitrogen concentration of runoff and erosion sediment showed a rapid change at the beginning, and gradual decrease in the middle and at late. There was enrichment phenomenon of total nitrogen in the erosion sediment loss.③On the single rainfall, the rainfall and erosion sediment were not large. The nitrogen loss of rainfall and erosion sediment showed 4°<9°<17°. There was a positive correlation between rainfall and the loss of ammonium, nitrate and total nitrogen of runoff and erosion sediment. Rainfall was greater, the amount of nitrogen loss was greater, and the differences of nitrogen loss were more significant under different slope. The loss of ammonium and total nitrogen were main sedimentary facies under different slope, especially total nitrogen, the proportion between 88.43% and 94.34%. The main form of nitrate nitrogen loss changed from water facies into sedimentary facies with slope increasing. The loss proportion of ammonium, nitrate and total nitrogen showed that with slope increasing, the loss proportion of sedimentary facies decrease, and the loss proportion of water facies increase. In one season, Ammonium lost 8.33 kg, nitrate nitrogen lost 20.71 kg, total nitrogen lost 490.90 kg, which was larger.④With each slope gradient increasing, the nitrogen load of runoff increased with multiply. In runoff, the load of ammonium was 0.21 kg hm-2 a-1, the load of nitrate nitrogen was 1.00 kg hm-2 a-1, the load of total nitrogen was 3.88 kg hm-2 a-1. Water eutrophication was serious in study area, whose pressure was large to the surrounding water. Nitrogen concentration of surface runoff over the whole class V standards (Surface Water Quality Standards-GB 3838-2002), Surface runoff water had poor quality, which can only apply to general agricultural water.⑤Engineering measures on the closing stock of nitrogen runoff intensity was 3.43 kg hm-2 a-1, and engineering measures on the closing stock of nitrogen sediment intensity was 6.73 kg hm-2 a-1, which was very significant on the role of nitrogen sequestration. The sequestration of nitrogen was 231.22 kg a-1 with engineering measures in small watershed. The average sequestration strength was 10.15kg hm-2 a-1. |
