| Distribution of purple soil is in hilly areas in southwest and southern of China, especially in Chongqing and Sicuan. It is a lithology soil developed from sedimentary rock and its based material is purple sand shale. Purple rock physical weathering is strong, easy to form a loose debris which cementation ability is very weak. Additionally, the purple soil region is soil erosion area because of abundant precipitation, frequent rainstorm, and low coverage of ground vegetation. Meanwhile, the purple soil accounted for more than 70% of the arable land in Three Gorges Reservoir area, majority of the slope-land. Therefore, soil erosion in the area is very serious. There are many reports of soil erosion in this area. However, soil erosion is a complex process with the interaction of rainfall, topography, vegetation and soil characteristics. Moreover, the process of soil erosion with interaction of runoff and leaching in dynamic conditions rarely reported.This study took purple soil slope farmland in Chongqing as the research object, combining field experiments and laboratory simulation tests to analyze aggregate stability before and after rainfall of different intensities, mechanism of soil erosion process driven by surface runoff and interflow, mechanism of nutrient loss of the slope soil for the purpose of reducing soil and water erosion, controlling non-point source pollution, maintaining sustainable development of ecological environment of the study area. The conclusions are as follows:(1) Size distribution of soil aggregates: under different rainfall intensities, mechanical stable aggregates dominated by macro-aggregates, while water stable aggregates dominated by micro-aggregates. Water stability of purple soil micro-aggregates splashed by rainfall correlated little with the degree of surface coverage. Macro-aggregate content of soil covered by gauze was more than bare land.(2) Soil aggregate stability: With the increase of rainfall intensity, aggregate stability decreased. When the rainfall intensity was 140mm/h, splash erosion of the raindrop became weaken because of the runoff, resulting in the increase of aggregate stability. The results showed that low rainfall intensities had little effect on aggregate stability, aggregation ability, anti-dispersion ability of purple soil.(3) Runoff generation characteristics: Covering gauze on soil could help delay the runoff. When the rainfall intensity was 140mm/h, shortest time was spend to generate surface runoff of bare land, while longest time was spend to generate interflow. Runoff volume and cumulative runoff volume kept increase with the persistent rainfall. The runoff showed that: surface runoff of bare land > surface runoff of gauze covered soil > interflow of gauze covered soil > interflow of bare land.(4) Sediment produced characteristics: the greater the rainfall intensity, the earlier that sand content of the runoff peaked after runoff began and the time was always in 20 min. With the increase of rainfall intensity, soil particles slipped longer. 120mm/h was the critical intensity to distinguish increasing amplitude. Covering gauze on soil could reduce sediment removed by runoff effectively thus decreasing erosion intensity.(5) Characteristics of nutrient in runoff: Over runoff time, various forms of nitrogen and phosphorus in runoff decreased with fluctuation, total nitrogen in interflow increased, total phosphorus, soluble phosphorus, orthophosphate decreased and most loss occurred in the early time of runoff. Various forms of nitrogen and phosphorus in runoff in surface runoff and interflow peaked when the rainfall intensity was 90mm/h and larger than 90mm/h. Covering gauze on soil reduced total nitrogen, nitric nitrogen, total phosphate, soluble phosphorus, orthophosphate content in both surface runoff and interflow effectively. Various forms of nitrogen was: interflow > surface runoff. Various forms of phosphate was: surface runoff > interflow.(6) Characteristics of soil nutrient after rainfall: The loss of total nitrogen, total phosphate, soluble phosphate peaked when the rainfall intensity were 60mm/h, 140mm/h, 22mm/h respectively. The loss of nitric nitrogen and orthophosphate of soil covered by gauze peaked when the rainfall intensity was 22mm/h while the loss of nitric nitrogen and orthophosphate of bear land peaked when the rainfall intensity was 140mm/h.To sum up, when the runoff became larger, the sand content of the runoff became more and more nutrient loss would occur. Covering gauze on soil could weak the hit of raindrop, maintain aggregate stability, reduce soil and water erosion and nitrogen and phosphate loss effectively. |
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