| The purple soil sloping field is the main type of cultivated land in the Three Gorges Reservoir area.The terrain in this area has large slopes,abundant rainfall,and concentrated rainstorms,resulting in serious soil erosion in the purple soil area.The purple soil is a purple mud shale with a shallow soil layer and extremely weak water permeability.The soil structure is poor.Due to its own soil characteristics,the interflow is extremely developed.During rainfall,nutrients in the soil tend to enter the water body with the loss of surface runoff and interflow,causing agricultural non-point source pollution in the Three Gorges reservoir area.Excessive or unreasonable application of chemical fertilizers in agricultural production activities is an important source of agricultural non-point source pollution.Therefore,exploring the fertilization methods in this area is of great significance for improving the soil quality of the purple soil dry slope land and controlling agricultural non-point source pollution in the Three Gorges Reservoir area.In this paper,the purple soil dry slope rapeseed/corn rotation farmland ecosystem in the Three Gorges Reservoir area was used as the research object.Standard runoff plot was adopted,and 5 fertilization treatments were set up,including no fertilization（CK）,conventional fertilization,optimized fertilization,biochar（fertilizer reduction and biochar）and straw mulching（fertilizer reduction and straw mulching）.By comparing the effects of different fertilization methods on surface runoff,interflow,sediment,carbon and nitrogen nutrients along with runoff loss and distribution on slopes.To find a suitable fertilization method for the purple soil in the Three Gorges Reservoir area,which aims to provide a theoretical basis for improving the soil quality of the purple soil dry slope land in the Three Gorges Reservoir area,controlling agricultural non-point source pollution,and reducing global greenhouse gas emissions.The main research results are as follows:（1）The runoff and soluble organic carbon（DOC）in each fertilization treatment were mainly lost through interflow.The loss of interflow in each fertilization treatment accounted for 52.84%-92.23%of the total runoff loss,while the surface runoff accounted for 7.77%-47.16%.And the DOC loss flux of all fertilization treatment was 7.47-18.82 kg·hm-2·a-1,among which interflow accounted for43.64%-87.35%,surface runoff accounted for 12.65%-56.36%.The total surface runoff,DOC loss flux and sediment loss of surface runoff in straw treatment are the smallest.Compared with CK,conventional and optimized treatments,the total surface runoff of straw treatment decreased by320.13%,191.36%and 43.66%,and the surface runoff DOC loss flux of straw treatment was significantly reduced by 181.74%,146.88%,and 40.26%,respectively.Compared with CK and conventional fertilization,the optimized fertilization treatment can also reduce the total surface runoff and surface DOC loss flux.Although the total surface runoff,surface runoff DOC loss flux and sediment loss of biochar treatment are the largest,except for two heavy rains on August 8 and August30,The total surface runoff,surface runoff DOC loss flux and sediment loss are less than CK treatment and conventional fertilization treatment.Compared with conventional treatments,straw and biochar treatments increased the interflow loss and interflow DOC loss flux.It can be seen that straw treatment has a good effect on reducing the loss of surface runoff and surface runoff DOC,while biochar only has a certain effect on reducing the loss of surface runoff and surface runoff DOC when the rainfall is small.（2）The nitrate nitrogen,soluble total nitrogen and total nitrogen loss flux of each fertilization treatment were 2.39-36.92,3.63-42.88 and 5.73-55.31 kg·hm-2·a-1,respectively.among which the surface runoff flux accounted for 0.40%-6.48%,0.38%-6.48%2.26%-39.04%,respectively.The fluxes of interflow accounted for 93.52%-99.60%,95.52%-99.62%and 60.96%-97.74%,respectively.Ammonium nitrogen in CK,conventional and biochar treatment is mainly lost through surface runoff,and the ammonium nitrogen loss fluxes from surface runoff account for 70.38%,67.85%and 67.92%of the total ammonium nitrogen loss fluxes,respectively.The optimization and straw treatment of ammonium nitrogen is mainly through the interflow.Compared with conventional fertilization,biochar and straw treatments can reduce the surface runoff nitrate nitrogen loss flux,which is reduced by 2.77%and 33.48%,respectively;the surface runoff fluxes of ammonium nitrogen,soluble total nitrogen and total nitrogen in straw treatment decreased by 219.34%,89.69%and 73.54%,respectively;Biochar treatment（except August 8th and30th）can reduce the surface runoff losses of ammonium nitrogen,soluble total nitrogen and total nitrogen than conventional and optimized fertilization treatment.Compared with conventional fertilization treatments,straw and biochar treatments increased the loss flux of nitrate nitrogen,ammonium nitrogen,soluble total nitrogen and total nitrogen in interflow.The surface runoff ammonium nitrogen loss of each treatment accounted for 1.17%-4.16%of the total nitrogen loss,while the nitrate nitrogen loss accounted for 5.86%-12.87%;The interflow ammonium nitrogen loss of each treatment accounted for 0.11%-0.86%of the total nitrogen loss,while the nitrate nitrogen loss accounted for 59.85%-76.17%.Straw can reduce the nitrogen loss in surface runoff,and biochar can only reduce the nitrogen loss in surface runoff under certain rainfall conditions,but both straw and biochar increase the nitrogen loss in interflow.（3）The average concentrations of soil organic carbon,soluble organic carbon and microbial biomass carbon were the highest in biochar treatment(6.61 g·kg-1,78.46 mg·kg-1 and 94.99 mg·kg-1respectively),followed by straw treatment(4.46 g·kg-1,72.55 mg·kg-1 and 73.93 mg·kg-1 respectively).Compared with CK,conventional,optimized and straw treatment,the average concentration of soil organic carbon in biochar treatment was significantly increased by 142.45%,68.04%,95.79%and48.14%,respectively.The average concentrations of soil organic carbon in straw treatment was increased by 63.66%,13.43%and 32.16%,respectively,compared with CK,conventional and optimized treatments.Compared with conventional treatment,the average concentration of soil soluble organic carbon in optimized treatment,biochar treatment and straw treatment increased significantly by 42.04%,77.92%and 64.52%respectively,while the average concentration of soil microbial biomass carbon increased significantly by 34.47%,89.61%and 47.57%.The distribution of soil organic carbon content in CK treatment,conventional treatment and optimized treatment in different slope positions is down slope position>middle slope position>up slope position,while the biochar and straw treatment soil organic carbon content was the largest at the middle slope.Except for straw treatment,the contents of soil soluble organic carbon in the other four treatments increased from top to bottom in sequence,that is,down slope position>middle slope position>up slope position,while straw treatment soil soluble organic carbon content is down slope position>up slope position>middle slope position;In addition to biochar treatment,the distribution of soil microbial biomass carbon content in different slope positions in the other four treatments was down slope position>middle slope position>up slope position,while biochar treatment was middle slope position>down slope position>up slope position.Both straw and biochar can increase the content of soil organic carbon,soluble organic carbon and microbial biomass carbon,and can delay the migration of soil organic carbon and microbial biomass carbon to the downslope.（4）Compared with the conventional treatment,the average total nitrogen concentration of the soil in biochar treatment and straw treatment increased by 17.96%and 10.42%,respectively.Compared with the optimized treatment,the average concentration of total nitrogen of the soil in biochar and straw treatment was significantly increased by 39.50%and 30.58%,respectively;The average concentration of alkali nitrogen in biochar treatment and straw treatment increased by 14.18%and 23.13%respectively compared with the conventional treatment,and increased by 42.33%and53.49%compared with the optimized treatment.The average concentration of ammonium nitrogen in the optimized treatment,biochar treatment and straw treatment was significantly higher than the conventional treatment,which increased by 170.22%,219.78%and 140.72%,respectively.However,straw and biochar treatments reduced the average nitrate nitrogen concentration compared with conventional fertilization treatments.The contents of total nitrogen,alkali nitrogen and nitrate nitrogen in different slope positions under conventional treatment were:up slope position>middle slope position>down slope position.The optimization process is the opposite,that is,down slope position>middle slope position>up slope position.The contents of total nitrogen,alkali nitrogen and nitrate nitrogen in different slope positions under straw and biochar treatment were middle slope position>down slope position>up slope position.The content of ammonium nitrogen in the straw-treated soil was as follows:down slope position>middle slope position>up slope position,and the down slope position was 39.13%and 20.30%higher than the up and middle slope positions,respectively.The conventional treatment and biochar treatment showed that the ammonium nitrogen content in the soil at different slope positions was middle slope position>down slope position>up slope position.The optimal treatment of soil ammonium nitrogen content in different slope positions showed that the middle slope position(3.39 mg·kg-1)>up slope position(3.27 mg·kg-1)>down slope position(3.05 mg·kg-1).In summary,straw treatment is the best method of fertilization to reduce surface runoff carbon and nitrogen losses,increase soil carbon and nitrogen nutrients,and improve the distribution of carbon and nitrogen on slopes. |
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