| Most of the organic carbon is enriched in the soil surface.During the rainfall process,the raindrops damage the surface soil structure,the soil particles are broken by the raindrops,and the organic carbon is released.The sediment taken away by the erosion usually has a high organic carbon content.Separation and transportation of runoff,low-density fine particles and particle-bound organic carbon are preferentially transferred with runoff,and a part of organic carbon is released when exposed to mineralization in an ecological environment that is prone to mineralization,and a part migrates with runoff,separated soils,along with soil organic carbon,are transported and redistributed in landscape environments.At present,the research on the relationship between runoff carbon and sediment carbon loss under different land uses during the secondary rainfall process mainly focuses on the research on the carbon loss pathway of a single land use type by artificial simulated rainfall experiments,runoff plot observation experiments and digital simulation methods.During the natural secondary rainfall process,there are few studies on the carbon loss pathway under the comprehensive consideration of land use allocation system.Therefore,this study takes the four sub-watersheds A,B,C,and D with different land use configurations in the Wangjiagou watershed in Fuling,Chongqing and the Shipanqiu small watershed in Zhongxian County as the research objects.The characteristics and laws of runoff dissolved organic carbon(DOC)and total sediment carbon(TC)loss under the configuration.Runoff and sediment samples were collected at the exits of four sub-watersheds A,B,C,and D,respectively,for a total of 16 rainfall events,430 sediment and runoff samples during rainfall events,and 213 samples from different land-use sediment sources in the watershed.In this way,the dynamic changes of runoff DOC and sediment TC in the four sub-watersheds during the sub-rainfall process were monitored,and the characteristics of runoff carbon and sediment carbon loss and the sources of sediment carbon during the sub-rainfall process were analyzed to provide the basis for the increase of carbon sequestration in agricultural small watersheds in the Three Gorges Reservoir area.It provides a theoretical basis.The main findings are as follows:(1)In the Wangjiagou watershed,the sub-basin A adopts the technology system of“agricultural mulberry configuration with dry slope terraces” and “agricultural mulberry configuration paddy field” in typical reservoir areas to absorb pollutants.These different land use configurations will significantly reduce agricultural watersheds.The surface runoff velocity can be increased and the runoff residence time can be increased,which can intercept and filter the surface runoff and inhibit the sand-carrying capacity of the runoff.For the same reason,in the sub-basin C of the Shipanqiu watershed,a large-scale configuration including mountain top woodland,hillside grassland and ecological ponds,terraced paddy fields,ecological hedge ridges,and ecological wetlands at the bottom of the basin has Sand played an active role in different stages.(2)The monitoring of the two small watersheds showed that during the secondary rainfall,runoff carbon was the main source of carbon loss in the watershed,while sediment carbon,although its contribution was less than that of runoff carbon,was also an important part of carbon loss in the watershed.Among the 4 sub-watersheds,only 3rainfall events had more sediment carbon than runoff carbon.The proportion of DOC and sediment carbon will have certain dynamic characteristics due to changes in runoff and rainfall intensity.The runoff carbon loss flux exhibits linear,exponential and logarithmic distribution characteristics,which are related to rainfall intensity changes during rainfall events(3)In Wangjiagou sub-watershed A,sloping farmland contributed 54.01% of carbon,hedged terraces contributed 34.67% of carbon,and mulberry orchards contributed only 11.32% of carbon;while in sub-watershed B,sloping farmland contributed 33.17% of the sediment.% of the carbon,the mulberry orchards,the hedge-free terraces and the hedged terraces contributed 29.50%,25.23% and 12.09% of the carbon in the sediment,respectively.In the Shipanqiu watershed of Zhongxian County,in sub-watershed C,citrus orchards contributed 40.98% of the sediment carbon,sloping farmland contributed 38.37% of the sediment carbon,and temporary fallow land contributed 20.64% of the sediment carbon;in sub-D In the basin,most of the sediment carbon is contributed by sloping farmland,accounting for 67.77%,followed by terraced fields,accounting for 20.27%,and the smallest is sloping farmland,accounting for11.96%.(4)High-intensity human activities(i.e.unreasonable trench excavation and high-intensity mechanical farming)have made some soil and water conservation measures ineffective in the sloping farmland in sub-watershed A and in sub-watershed B.According to the experimental results,some corresponding treatment measures are proposed.For the same reason in the Shipanqiu watershed,its ecosystems with different configurations intercepted sediment significantly.For example,the surface roughness significantly reduces the sand-carrying capacity of the surface runoff,thereby reducing soil erosion.Due to different vegetation coverage and measure systems,the surface roughness of the C and D sub-basins is also different.The greater the roughness,the slower the sediment migration.will also increase,while traditional farming in catchment D reduces surface roughness,resulting in rapid sediment migration.Temporary fallow measures can significantly reduce surface runoff and reduce sediment production capacity;building hedges or filter belts can intercept runoff and sediment;therefore,to prevent and control the loss of sediment and carbon,a combination of different measures and a variety of soils are required It will be more advantageous to use the combination of agriculture,forestry and water composite measures system. |
PDF Full Text Request