| The South-to-North Water Transfer Project is a water transfer project from Danjiangkou Reservoir in the upper and middle reaches of the Han River to Beijing and Tianjin,aiming to solve the water shortage problem in the North China Plain,and to take into account the ecological environment and agricultural water in the surrounding areas.The water source area of the South-North Water Transfer Project includes the southern slope of the Qinling Mountains,the Han River Basin,the northern slope of the Daba Mountains and the area around the Danjiangkou Reservoir,which is a national ecological civilization demonstration area.The distribution of nitrogen and phosphorus content in the water sources of the South-North Water Transfer Project and its ecological environment directly affect the water environment of Han River,Dan River and Danjiangkou Reservoir,which in turn determines whether the Central Line Project can bring its great socio-economic and ecological benefits.In this study,we collect 408 soil samples according to different land use types,and measure the p H,organic matter,total phosphorus,total nitrogen and total potassium factors in the study area,and the differences in soil nitrogen and phosphorus contents under different land use types and different forest land types in the study area were analyzed by statistics.We determine the differences in soil nitrogen and phosphorus contents by ordinary kriging(OK),geographically weighted kriging(GWRK),and synergistic kriging(COK).By three methods,we study the spatial distribution of nitrogen and phosphorus content in the study area: ordinary kriging(OK),geographically weighted kriging(GWRK),and cooperative kriging(COK),and evaluate the accuracy of the three methods at the same time.Finally,use the nitrogen and phosphorus index method evaluate the risk of nitrogen and phosphorus loss in the study area.Last,we clarify the spatial distribution pattern of nitrogen and phosphorus loss risk by dividing the risk areas,and calculate the main driving factors of nitrogen and phosphorus loss through model inversion.The results of the study are as follows:(1)The study area is rich in nitrogen and phosphorus reserves,with significant differences in soil nitrogen and phosphorus contents between different land use types and different forest land types.The total nitrogen content of the soil in the water source area of the South-North Water Transfer Project is 1.45 g/kg,with a total storage capacity of 6,509,700 t.The total phosphorus content of the soil is 1.53 g/kg,with a total storage capacity of 7,014,000 t.The soil nitrogen and phosphorus contents of different land use types differ significantly,with the nitrogen content of forest land(1.62 g/kg)> farmland(1.11 g/kg)> grassland(0.5 g/kg)and the phosphorus content of farmland(0.5 g/kg).The phosphorus content was 1.91g/kg in farmland > 1.72g/kg in grassland > 1.4g/kg in woodland.The soil N content differed significantly(p<0.01)and phosphorus content did not differ significantly(p>0.05)among the different woodland types.Among the different stand types,soil N content was broadleaf forest(1.88g/kg)> scrub(1.48g/kg)>mixed coniferous forest(1.45g/kg)> coniferous forest(1.36g/kg)and phosphorus content was coniferous forest(1.48g/kg)> scrub(1.29g/kg)= mixed coniferous forest(1.29g/kg)> broadleaf forest(1.26g/ kg).(2)Soil nitrogen and phosphorus content in the study area were significantly correlated with p H and organic matter content.Soil total nitrogen content was positively correlated with organic matter content,while the effect of p H on soil total nitrogen was positively correlated in the western,central and southern parts of the study area,and negatively correlated in the eastern part of the study area.The effect of soil organic matter content on total soil phosphorus was positively correlated in the central and eastern part of the study area and negatively correlated in the western part of the study area.Microorganisms in the soil decompose organic matter to provide the nutrients they need and further synthesize them into organic substances such as proteins and nucleic acids in the microorganisms,which increase the nitrogen and phosphorus content of the soil as the microorganisms die or metabolize and excrete out of the soil surface,while the effect of soil p H on the nitrogen and phosphorus content depends on a variety of factors,including soil type,plant species and management practices.Soil total nitrogen content was lower in the central part of the study area and higher in the western,eastern and southern parts of the soil.Soil total phosphorus content,on the other hand,showed higher levels in the east and west,while the central,northern,and southern parts of the study area were all areas with low values of soil phosphorus content.The Co-operative kriging(COK)method has lower mean absolute error,root mean square error and higher correlation coefficient,and the difference with the geographically weighted kriging(GWRK)method is small,but both of them are significantly better than the ordinary kriging(OK)method,and the prediction accuracy of soil nitrogen and phosphorus content in the water sources of the South-North Water Transfer Central Project can be significantly improved by using co-operative kriging.<COK.(3)The overall risk of nitrogen and phosphorus loss in the water source area of the Middle Route Project of the South to North Water Diversion is relatively low.The highrisk areas of soil nitrogen and phosphorus loss in the study area overlap,all located in the southeast of the study area,near the Danjiangkou Reservoir,but the risk of phosphorus loss is higher than that of nitrogen.The main reasons for the high risk of nitrogen and phosphorus loss in the study area are the large surface runoff coefficient,close distance to the receiving water body,and large terrain drop.In addition,by ranking the importance of the output factors of the best Random forest model among the four models,vegetation coverage,economy and population are also important factors of soil nitrogen and phosphorus loss in the study area. |
PDF Full Text Request