| Hilly red soil regions of southeast China, which locate in the tropical and subtropical area, have abundant hydrothermal resources. However, due to the uneven seasonal distribution of the hydrothermal resources and the frequent disturbance by human, soil erosion has been one of the main constraints in sustainable developments of agriculture, economy and society. Therefore,it is of great important to carry on the research of soil erosion processes and mechanism. According to the report from “The comprehensive scientific survey of soil and water loss and ecological security”, sheet erosion, which includes splash erosion, interrill erosion and rill erosion, is the most widespread and the largest area of erosion in hilly red soil regions of southeast China. So, it is necessary to research soil erosion in this region. In this paper, studied soils were derived from Quatenrary red clay and shale, which were the main parent materials in this region. Base on the research of aggregate breakdown and pore characteristics, the relationships between characteristics of aggregate breakdown and erosion processes were obtained. The main results as followed:1. The aggregate stability and breakdown mechanisms were measured by traditional wet-sieving and LB method. In addition, aggregate stability and breakdown characteristics were determined under different antecedent moistures.The results showed that aggregate water stability significantly increased with increasing antecedent moisture content and slaking was the most efficient breakdown mechanism.With the increase of antecedent moisture content, the MWD values of soils derived from Quaternary red clay increased initially then decreased with the most stable value at the moisture of 15%. However, MWD values of the soils derived from shale significantly increased.2. Through rainfall simulation and measurements of aggregate size distribution, the characteristics of aggregate breakdown were analyzed in this study. Based on aggregate breakdown and breakdown mechanisms, the splash erosion and interrill erosion processed were analyzed.(1) Total masses of splash erosion decreased as a power function with an increasing rainfall duration. The behaviors of sediment size fractions(<0.25 mm, 0.5–0.25 mm, 1–0.5 mm, 2–1 mm, 5–2 mm) showed that the transport selectivity was decided by the interaction of aggregates size distributions and the characteristics of soil surface structure. Sediment size selectivity led to the enrichment of 0.25–1 mm fraction compared with the breakdown products.(2) Aggregates were prone to breakdown at the beginning of rainfall and a multiple regression equation(D=1.02RMI+1.77T0.06) was obtained to describe aggregate break down dynamics considering a relative aggregate stability index(RMI) and rainfall time(T).(3) Total masses of erosion sediment decreased as a power function with an increasing size fraction.(4) With antecedent moisture content increasing, the total mass of splash for soils derived from shale decreased, but decreased first then increased for soils derived from Quaternary red clay, with a minimum value at the moisture of 15%.The size selectivity was determined by the size distribution and surface structure at different antecedent moisture contents. For more stable soils, size distribution of the splashed fragments is bimodal with a major peak at 1?0.5 mm and <0.05 mm. For less stable soils, size distribution of splashed is unimodal with a 1arge peak at 1?0.25 mm except the ones at the moisture of 20%.(5) Aggregate breakdown during interrill erosion is a complex process. Slaking is the main breakdown mechanism at the beginning of rainfall, and the mechanical breakdown played a major role in aggregate breakdown subsequently. The sediment concentration decreased with the limitation of aggregate breakdown and the development of crust. The size of sediments were mainly distributed in 100?250 μm(45.87%?53.13%) within 0?24 min, and in 100?250 μm、250?500 μm ' 500?1000 μm within 36?60 min for QX2 and SX3 samples. While the size distribution of QX4 and SX1 samples were <200 μm during rainfall. Notably, the contents of sediment were decrease with the increase of sediment size.3. Aggregate pore characteristics were quantitatively analyzed using synchrotron-based X-ray micro-computed tomography(SR-μCT)and image analysis after wetting and drying simulation. On this basis, the relationships between characteristics of aggregate pores and aggregate breakdown and soil detachment in concentrated flow using partial least squares regression(PLSR).(1) An increase in the number of pores after wetting and drying treatments. Moreover, more inter-connected pores embedded in the aggregates were observed after wetting and drying treatments, suggesting amore porous microstructure compared with the control treatments. The results indicated that total porosity, >100 μm porosity, and fraction of elongated pores all significantly increased with an increasing number of wetting and drying cycles, while the opposite trends were observed for the total number of pores, the percentage of pores with a diameter of 75–100, 30–75,and <30 μm.(2) Decrease in wet aggregate stability and tensile strength of both soils was mainly due to higher total porosity, a more extensive and complex pore network, which caused increased air pressure, increased rate of water entry, and high probability of crack propagation and interaction. The total number of pores, 75–100 μm porosity, and >100 μm porosity were identified as the primary factors controlling the wet aggregate stability and tensile strength according to PLSR.(3) The pore characteristics, such as a >100 μm porosity, total porosity, fraction of regular pores, 30–75 lm porosity, fraction of elongated pores, and fraction of irregular pores, were identified as the primary parameters that control the soil detachment. Soil detachment was significantly and positively related to a >100 μm porosity, total porosity, and fraction of elongated pores. Thus, soil fragmentation depended on microcracks that formed during the wetting and drying cycles and on the geometrical characteristics of the pores or cracks. |
PDF Full Text Request