| The exceeding of nitrogen and phosphorus is one of the key factors that cause the eutrophication. Agricultural non-point source pollution has become the most important source of nutrient for the reservoir and has brought huge damage to the water environment. Studies have shown that the soil nutrient loss has strong connection with its content, and the accumulation of soil nutrient can increase the loss potential. Therefore, the determination of soil nutrient content can be used to assess the loss risk and environmental impact degree.Soil nutrient background value of 20 sampling points of five different types of land use were monitored in situ in 2009, and the changing trends by time and spatial distribution of soil organic matter, soil nitrogen and soil phosphorus were studied to lay the foundation for understanding the agricultural non-point pollutant migration process and the transformation behavior. The form of soil nitrogen and phosphorus were analyzed of soil samples of different season. Rainfall runoff samples were collected in August from agricultural drainage ditches in the watershed, and the characteristic of the runoff nutrient loss were analyzed. Appropriate methods were adopted to evaluate the potential of soil nutrient loss in research area and help to understand how the loss of agricultural non-point source pollution of nitrogen, phosphorus influence the receiving water body. The thesis draws following conclusions:The content of soil organic matter of the whole watershed has no obvious difference among seasons and the changing of content of forest land, rice paddies, dry land with time are small. The soil nitrogen, phosphorus content slightly increase over time and there is some difference in different seasons.The average content of soil organic matter in research area is (14.44±3.57) g/kg, the average content of soil nitrogen in research area is (1.03±0.26) g/kg, and the average content of soil nitrogen in research area is (0.54±0.18) g/kg. The content has significant differences among different types of land use. Compared with small watershed of other different regions, soil nitrogen and soil phosphorus content of research area is much lower.Along the top of the slope to base of the slope in the watershed, the soil nutrients show different disciplines. Soil organic matter totally increased with a relatively slower decrease after the increase. Soil nitrogen is showing a trend of gradual increase, whereas soil phosphorus is totally decreasing following with a relatively slower increase.The average content of soil mineral nitrogen is (15.99±13.64) mg/kg, the ranking of the content of different land use from high to low is forest land, paddy field, grass land, dry land and riparian land. The average content of Olsen-P and water soluble phosphate is (32.14±32.63) mg/kg and (2.46±2.21) mg/kg, and the ranking of these content of different land use from high to low is forest land, dry land ,paddy field, grass land and riparian land.The total nitrogen (TN) in runoff from agricultural drainage ditches is 12 times of the third limit for surface water environment quality standards, and the dissolved nitrogen in runoff accounts for 54.7% of TN. The total phosphorus (TP) in runoff from agricultural drainage ditches is 2 times of the third limit for surface water environment quality standards, and the dissolved phosphorus in runoff accounts for 63.3%% of TP. Soil nutrient are lost mainly in the form of dissolved shape .The soil of forest land, paddy field, and grassland will have high potential of loss with runoff erosion. The forest land, dry land and riparian land has much higher content of nitrate nitrogen where the potential of loss with runoff erosion will be higher. The critical loss content of soil Olsen-P is 60.79mg/kg and the according critical loss content of water soluble phosphate is 3.04 mg/kg. The ranking of the phosphorus loss risk content of different land use from high to low is forest land, dry land and paddy field. |
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