Study On Nitrogen Distribution And Loss Mechanism In A Small Watershed Of The Middle Dan River

The quality of water environment in the source area of the Dan River is of greatsignificance to the water supply security of the middle route of the South to North WaterDiversion Project. Based on the water quality monitoring and soil sampling inYingwugou Small Watershed of Dan River, geostatistical method, fractal theory, soilerosion prediction model and hydrology method are adopted to research the spatialvariability of soil nitrogen and losing rules of runoff-sediment-nutrient in the smallwatershed. It is expected to provide the basis for the protection of water source area andthe construction of clean small watershed. The main conclusions of this study are asfollows:（1）The total nitrogen content in the soil profile decreased as the soil depthincreased. The total nitrogen contents per square meter in0-40cm soil layer wereforestland> cropland> grassland. There were significant differences among the threesoil layers （P <0.01）. The mean total nitrogen contents in0^-10（A1）,10^-20（A2） and20-40cm （A3） were0.85,0.47and0.30g/kg, respectively. The best fitted models in thethree soil layers were all linear models indicating moderate spatial dependence. Thespatial variations of total nitrogen content under different land use types in A1and A2were significant （P <0.05）. The impact of slope on soil total nitrogen content in the threesoil depths was significant （P <0.05）. There were significant correlations between thetotal nitrogen content and elevation, slope and aspect in different soil layers of cropland（P<0.01）. The soil total nitrogen storage in the study area in0-40cm was562.37t. Thetotal nitrogen contents per square meter in0-40cm soil layer under forestland, croplandand grassland were0.343,0.299and0.289kg/m², respectively.（2）There was a significant relationship between soil total nitrogen content and soilparticle distribution in the different soil depths. The soil silt and clay content in the soil profile increased as the soil depth increased. Soil total nitrogen content had a significantpositive correlation with coarse sand at a depth of0–20cm （P <0.01）. The fractaldimension and soil total nitrogen content both indicated positive correlations with siltand clay content at a depth of20–60cm （P <0.01）. The mean soil silt and clay contentsin the different soil horizons were all cropland> forestland> grassland. The fractaldimensions of soil particle-size distribution among different plant communities alsoindicated significant differences （P <0.05）. Howerver, the fractal dimension of soilparticle-size distribution in10^-20cm soil layer could better represent the differences ofthe different land use types. There was a significant correlation between fractaldimension of soil particle-size distribution and slope （P <0.05）, but no correlation withelevation and aspect. The soil silt and clay contents per square meter in0^-10cm soillayer under cropland, forestland and grassland were74.71,71.54and70.23kg/m²,respectively.（3）The nitrogen migration process in runoff and the characteristics of pollutionload were clarified. The average total nitrogen runoff loss modulus in Yingwugouwatershed was0.89t×km^-2×a^-1. The total nitrogen content was the major exceededpollutant in runoff and nitrate nitrogen content was always greater than the ammoniacontent. Rural production and living pollutant emissions had a great impact on waterquality. The average annual nitrogen loss moduluses of ammonia nitrogen, nitratenitrogen and total nitrogen were all farmland> grassland> forestland. The average annualtotal nitrogen loss moduluses of farmland, grassland and forestland were0.36,0.22and0.09t×km^-2×a^-1, respectively. The average non-point source pollutant concentration ofammonia nitrogen, nitrate nitrogen and total nitrogen in watershed export were0.17,4.71and7.55mg/L, respectively. The average point source pollutant concentration ofammonia nitrogen, nitrate nitrogen and total nitrogen in watershed export were0.20,2.12and4.08mg/L, respectively.（4）The relationship between soil nitrogen loss and soil erosion and water loss wasclarified. The annual loss moduluses of total nitrogen with sediment were0.27t×km^-2×a^-1.The annual soil erosion modulus in Yingwugou watershed was3140t/km². It was in thecategory of moderate degree erosion. The high soil erosion area was mainly distributed insloping cropland with big slopes which was the key management area. The annual soil erosion moduluses of forestland, grassland and cropland were509.7、1511.8and4606.5t/km², respectively. The annual soil erosion amount of forestland and grassland wasrelatively small and the annual soil erosion amount of cropland accounted for95.3%ofthe total soil erosion amount in the study area. For each additional5°slope, the increasedsoil erosion modulus of different land uses was of1to2times greater than that of eachadditional5m for slope length. The annual loss amount of total nitrogen, ammonianitrogen and nitrate nitrogen in topsoil was3799.9,44.8and16.9kg, respectively. Thenitrogen loss of cropland was serious.（5）Different scenarios of soil and water conservation to prevent and controlnitrogen loss in watershed were simulated. When the sloping cropland more than5°wasin the ultimate governance status of forestland in Dan River watershed, the averageannual concentration of total nitrogen in Dan River would reduce0.203mg/L. Soilerosion intensity was in the category of light degree erosion when the slope was allwoodland or grassland. The average erosion modulus was750.1t×km^-2×a^-1and1875.2t×km^-2×a^-1, respectively. The relationship between the proportions of slope forest（grass） measure and soil erosion modulus is close to a linear function. The relationshipbetween the proportions of terrace measure and soil erosion modulus is closer to anexponential function. The effect of forest and grass measures on reducing water andsediment was better than terracing, most of the precipitation outflows in the form ofsubsurface flow, thereby reduces the nitrogen concentration. When the sloping croplandmore than5°was in the ultimate governance status of forestland and grassland in DanRiver watershed, the sediment loads would reduce205.3and196.6million tonsrespectively. The amount of total nitrogen content in sediment would decrease732.8and701.9tons, respectively. The amount of total nitrogen content in runoff would reduce346.9and169.2tons, respectively.