| Hilly red soil region of southeast China locate in the tropical and subtropical area, and they are abundant in hydrothermal resources. However, many reasons, such as undulating topography, poor soil properties, improper land use and soil management, have caused severe soil erosion in this region. Besides, due to the effects of high intensity rainfalls and topographic, the rills and gullies caused by washing are obvious on the surface of the soil in this area. It is important and essential to understand the red soil detachment mechanism and aggregate (Red soil structure factor) abrasion characteristic in concentrated flow. In this paper, selected soils were derived from Quaternary red clay and Shale, which were main parent materials in this region. The relationship between hydraulic characteristics, red soil structure factor and red soil detachment processes were obtained. In addition, we analyzed the quantitative effects of hydraulic characteristics, transport distance, particle size and stability on aggregate abrasion degree, and the distribution of aggregate size, shape and cementing material after it abrasion in concentrated flow was also studied. The main results were listed as following:1. The red soil aggregate stability and breakdown mechanisms were estimated by traditional wet-sieving and LB method. Moreover, due to the subtropical conditions of the red soil region, the relationship between and aggregate stability and some basic physical and chemical properties were studied. Especially, we focused on analyzing the effects of clay content, organic matter, and different forms of Fe oxides and Al oxides on selected red soil aggregate stability.(1) The trend of the aggregate stability was not always the same because of the difference of pretreatment and selected indexes. In the wet-sieving method, for similar land use of the two parent materials, the water stability of the aggregates from Quaternary red clay was stronger than those from Shale except QJ4sample. Because of the relatively small farming activities, the water stability of the aggregates from tea garden, woodland and weed land was also stronger than cropland. In the LB method, values of these parameters did not always show the same trend in different treatments. In all the soil samples of the three treatments, aggregate stability was the greatest for the slow-wetting treatment (MWDSW), followed by the wet-stirring treatment(MWDws) and the fast-wetting treatment (MWDFW).This result showed that slaking and mechanical breakdown were probably the main mechanisms of aggregate breakdown in these study soils.(2) Because of the difference of the parameters from aggregate stability, the correlation between the red soil physical and chemical properties and the aggregate stability was significantly different. In this study, the aggregate stability was significantly correlated with the contents of soil organic matter, free iron and aluminum oxides, while not significantly related to cation exchange capacity and acid ammonium oxalate extracted aluminum oxides. The clay content played an important role while the aggregate was suffered the fast wetting conditions. Organic matter and free iron and aluminum oxides played a leading role in maintaining the aggregate stability.2. Through selected disturbed and undisturbed soil samples, concentrated flow simulation and measurements of detachment was carried out in this study. The quantitative effects of hydrodynamics on red soil detachment rate in concentrated flow were analyzed. Further, this study was conducted to establish the quantitative relationship between soil detachment rate in concentrated flow with the aggregate stability index (As), root density (Rd) and saturated soil strength (σs).(1) The flow regime was mainly turbulent and torrent in this concentrated flow. The effect of slope was smaller than discharge on flow regime. It could be used the power function to simulate the relationship between velocity or flow deep with slope and discharge. The red soil detachment rate would be predicted by the power function of discharge and slope, or flow deep and slope, or velocity. Among shear stress, stream power and unit stream power, all of them were a linear relation with red soil detachment rate, and stream power was the best related to soil detachment rate.(2) The results showed a linear relationship between the detachment rates and shear stress for different soils. It was clear that erodibilities and critical shear stresses varied among different soil samples. The correlation between the aggregate stability index (As) and the soil detachment rates showed that As was well related with soil detachment rate. The As values had better linear relationship with concentrated flow erodibility factors (R2=0.70, p<0.01), and a significant negative exponential relationship between Kc and root density (Rd) was obtained at the10%level (R2=0.40, p=0.09). A positive linear relationship between saturated soil shear strength and critical flow shear stress could be also observed for study soils (R2=0.64, p=0.02). By introducing As, Rd and σs into the WEPP model frame as a substitute for soil erodibility factor and critical flow shear stress, statistical formulae for estimating red soil detachment rate were established with a good correlation coefficient.3. Fore related test of the aggregate abrasion were conducted in concentrated flow. We analyzed the quantitative effects of transport distance, hydraulic characteristics, particle size and aggregate stability on aggregate abrasion degree. And the abrasion regularities, the distribution of aggregate size, shape and cementing material (organic matter, iron and aluminum oxides) after it abrasion in concentrated flow was also studied.(1) The abrasion of aggregates in concentrated flow was affected by its stability and particle size. Aggregate with higher stability or smaller size suffered less extent of abrasion in different transport distances and discharges. The study revealed that the mechanism of abrasion and destruction of soil aggregates in concentrated flow was different from the mechanism of the abrasion of rock fragments in river flow, and two stages of aggregate breakdown could be identified. These results demonstrated that flow depth and friction factor should be appropriation indicators to reflect aggregate abrasion in concentrated flow. The aggregate abrasion degree under different slopes and discharges could be predicted from the slope and flow deep. Among three hydrodynamic parameters, unit stream power was the best related to aggregate abrasion.(2) In the present study, the Wr/Wi(%) values were highly correlated to RMI from LB method in different transport distances (p<0.05), and therefore it would be logical to include the RMI and transport distance x in the aggregate abrasion prediction equation. A multiple regression equation was then developed, relating Wr/Wi(%) to RMI and x for all the tested samples(3) The aggregate with weaker susceptible to mechanical breakdown, the content of large particles and the value of MWD were higher under them suffered to abrade in a transport distance. There are small changes in the particle size and shape after the aggregate abraded, and the D50of shape factor (Cir) was logarithmic function increasing with the transport distance increasing. The aggregate with weaker susceptible to mechanical breakdown, the micro-aggregate was more nearly spherical under them suffered to abrade in a transport distance.(4) From the preceding study, the dominantly cementing agents of the aggregate were organic matter, free iron and aluminum oxides (SOM, Fed and Ald). The organic matter, free iron and aluminum oxides contents distribute mainly near the inside of the aggregate and in the aggregates inside were higher than the aggregates outside. The SOM, Fed and Ald contents of the aggregates retained on the0.25mm sieve after subjecting to abrade upon72m distances in overland flow (Ag72m>o.25) were larger contributed to the bulk SOM, Fed and Ald contents in the study soils from the high correlation coefficients. |
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