| As the largest reservoir in Guangdong province,Xinfengjiang Reservoir is the main source of water supply for Guangdong,Hong Kong and Macao Bay Area(including Hong Kong,Guangzhou,Shenzhen),its water supply covers more than 40 million people,and is a very strategic "life water","economic water","political water" in China.In recent years,the concentration of total nitrogen and total phosphorus in Xinfengjiang Reservoir and the eutrophication index of water bodies are on the rise,and the fertilizer surface source pollution in the catchment area of Xinfengjiang Reservoir has become a potential threat to the water supply and water ecological environment security of Dongjiang River Basin.At present,there is an extreme lack of scientific research reports on the characteristics of nitrogen and phosphorus loss from agricultural fields in the catchment area of Xinfengjiang Reservoir,the process of nitrogen phosphorus migration and transformation of local agricultural fields with fertilizers is not clear.Therefore,in this study,a combination of field monitoring,small watershed monitoring,model simulation,and isotope traceability experiments was used from January to December 2022 to select typical crops(rice,peanut,and vegetables)in the Xinfengjiang River catchment as experimental objects,and to study the characteristics of nitrogen and phosphorus loss from agricultural runoff,soil physicochemical properties and nutrient content changes,and soil physicochemical property of typical crops under different fertilizer management(recommended fertilizer application,organic fertilizer substitution,and no fertilizer application)patterns.The study also discussed the characteristics of surface pollution of farmland at the small watershed scale in the catchment area of Xinfengjiang Reservoir,and provided scientific basis for preventing and controling nitrogen and phosphorus source reduction in the downstream of Xinfengjiang Reservoir and the low-carbon and sustainable development of local agriculture.The main conclusion of the study are as follows:(1)Rice,peanut and vegetable had nitrogen loss loads in the order of rice>peanut>vegetable;phosphorus loss loads in the order of rice>peanut>vegetable during their respective fertility periods in 2022.Rainfall intensity was positively correlated with runoff losses of nitrogen and phosphorus nutrients.The runoff loss of nitrogen from rice,peanut and vegetables was mainly in the form of ammoniacal nitrogen;the runoff loss of phosphorus was mainly in the form of particulate phosphorus.The total nitrogen loss coefficients of rice,peanut and vegetables were 2.68%,3.72%and 3.65%,and the total phosphorus loss coefficients were 1.28%,0.93%and 0.67%,respectively.The loss coefficients of various crops were generally slightly higher compared with the national average,and the relatively high loss coefficients in Heyuan area might be related to the high intensity of rainfall and the poor water and fertilizer retention capacity of red soil soils.Principal component analysis(PCA analysis)revealed that exogenous fertilizer application was the main factor contributing to runoff N and P loss from farm systems relative to soil background N and P.For the three crops,organic fertilizer substitution technology could reduce N and P runoff loss to some extent,but was not significant in preventing and controlling N and P loss.The basal fertilizer period and the first follow-up period are high-risk windows for N and P surface source loss,and heavy rainfall is the main driver of N and P surface source loss.(2)The attribution of nitrogen in the farmland system of the three crops is dominated by soil sequestration,ammonia volatilization and crop utilization,in which the sum of nitrogen output in the process of soil sequestration and ammonia volatilization is the highest,accounting for about 38.64-79.3 1%of the total nitrogen input,followed by nitrogen utilized by crops,accounting for about 11.67-36.78%,with specific utilization rates varying depending on the crop,and runoff loss of nitrogen Only 1.07-2.87%;the environmental attribution of phosphorus tends to be dominated by soil sequestration and fertilizer residues 79.85-90.16%,which is because the red soil in the Heyuan area is rich in iron and manganese oxides,making it easier for soil particles to adsorb phosphorus to form precipitation.This is followed by crop utilization,which accounts for 8.2422.56%of the environmental attribution of phosphorus.And runoff losses accounted for 0.28-1.15%of phosphorus inputs,respectively.Overall,the liquid loss of nitrogen and phosphorus fertilizers with runoff and other pathways accounted for a very low proportion of the input in nitrogen and phosphorus fertilizers,with the greater gaseous loss of nitrogen fertilizers,while most of the phosphorus fertilizers were sequestered in the soil.(3)The regression equation of the LODEAST model between nitrogen and phosphorus loss from field monitoring plots and rainfall yield was 0.72-0.87,and the data from field monitoring plots were used to extrapolate nitrogen and phosphorus loss from different cropping types at the sub-basin scale with high confidence.April-June is the window period for total nitrogen loss from agricultural fields in the sub-basin,and the total nitrogen loss at the entrance and exit of the sub-basin is influenced by the topographic index and the nitrogen retention efficiency of rice fields.The high risk period of total N loss in the sub-basin is from July to August.The main sources of δ15 N and δ18 O in the Shuntian township subwatershed during the abundant,flat and dry periods were rural domestic sewage,with soil background nitrogen and chemical fertilizer nitrogen as secondary sources. |
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