Aggregate Stability Characteristic Of Red Soils And Its Effects On Erosion Processes At Hillslope

Hilly red soil region are abundant in hydrothermal resources, and play an important role in agriculture and economy development. Improper land use and soil management, however, have caused severe soil erosion by water of red soils, and is one of the most important environmental problems in China. Analysis of soil structure characteristic and soil erosion is very important for comprehensive watershed management and sustainable development. Laboratory and field experiments of rainstorm simulations with different initial condition and rainfall characteristic were conducted on typical red soils derived from Quaternary red clay, Shale and Granite, and the soil aggregate stability was determined by the wet-sieving and LB-method. The improved formulae for assessing interrill erosion rate were established by incorporating the aggregate characteristic index in the prediction evaluations for soil erodibilites of red soils. The results of this study have promoted the development of soil erosion process research and provided scientific bases for the establishment of soil and water conservation in hilly red soil region. The main results were listed as following:1. The aggregates of selected red soils were used to understand the stability and the breakdown mechanisms by applying wet-sieving and LB-method. The relative slaking index (RSI) and the relative mechanical breakdown index (RMI) were used to determine the resistance to slaking and the mechanical breakdown of the soils of this study. Also, the relationship between aggregate stability and some soil properties, such as different forms of Fe oxides and Al oxides, organic matters, CEC and clay content were studied in condition of cultivated land.(1) The results of wet-sieving and LB-method indicated that the aggregate stability was highest in soils derived from Quaternary red clay, moderate in Granite, and lowest in Shale. There was some discrepancy in aggregate stability with different initial aggregate size, however, the stability of aggregate was found to be decreased with the increase of initial size in the selected soils.(2) The range of MWD (mean weight diameter) values differed widely, and values did not always show the same trend across the three treatments in LB-method. Across all three treatments, the MWD ranked in the order of fast wetting (FS), Wet Stirring (WS), and slow wetting (SW). The MWD_FW and MWD_WS was more smaller than MWD_SW, indicating that the slaking and the mechanical breakdown were the main breakdown mechanisms in red soils. However, the vulnerabilities of slaking and mechanical breakdown were different in study soils. The soils derived from Quaternary red clay showed relatively high values in RSI and low in values RMI, while the soil derived from Granite showed a low RSI value and a high RMI value. The soils derived from Shale showed both high values in RSI and RMI.(3) Due to the subtropical conditions of red soil region, red soils were heavily weathered and characterized by enrichment of sesquioxide and poor in soil organic matter. Hence, the effect of soil organic matter and humus acid on aggregate stability was relatively weak, and clay content and sesquioxide were the most important bonding agents for aggregation of red soils. However, an increase in clay content in the soil might have two opposing effects on aggregate stability: (i) clay acted as a cementing material that holds particles together in aggregate and (ii) an increase in clay content could also increase slaking forces during soil wetting. Moreover, the parameter of PAD > 2 mm (percentage of aggregate disruption) was highly related to humus acid, indicating that humus acid was relatively important in formed processes of aggregates in large size.2. The study was designed to reveal the effects of aggregate breakdown mechanisms on interrill erosion dynamics. Based on laboratory experiments, the effects of slaking force, mechanical breakdown, and initial size of aggregate on the interrill erosion processes were discussed. A new aggregate instability index (K_a), which could reflect the main mechanisms of aggregate breakdown in interrill erosion process, was proposed to measure the erodibility of red soils.(1) For all soils, the runoff rates followed the order slowing wetting (2 mm/h) < medium wetting (10 mm/h) < fast wetting (60 mm/h). However, magnitude of changes in runoff rate depended on soil aggregate stability and texture. Based on runoff data, soils were divided into different groups lay on their response to wetting rate. The effect of wetting rate on runoff generation was significant in soils with high clay content while negligible in soils with low clay content. A significant effect of wetting rate on soil loss was found except soil GT2. The soil loss was affected by slaking and aggregate stability in two ways (runoff production and detachment), and was more sensitive than runoff amount to wetting rate. The content of sediment fragment in diameter of < 0.25 mm was highest in the fast wetting treatment, also, the MWD was found to be lower than that in the slow wetting treatment, which was highly related to the form of surface sealing.(2) In present study, the runoff rate curves of all soils departed from each other evidently, indicting that the raindrop impact affected runoff generation essentially. The soils (SH2, GT1, and GT2) showed low aggregate stabilities and high susceptibilities to mechanical breakdown, and the aggregates of these soils were easily destroyed by rainfall impact to produce considerable fine particles. The aggregates in soil QT1, QT2, and SH1 were stable and could resistant the raindrop impact, however, the soil surface also became compacted. In this respect the main effect of rain impact was to produce sealing of the surface by redistributing already-detached material, rather than causing aggregate breakdown. The soil loss without raindrop impact was higher than that with raindrop impact, however, the difference between them was merely significant in soil with low aggregate stability. The MWD of sediment was significantly lower under raindrop impact, indicating that the dispersing effect of raindrop impact was essential on selecting sediment even though in soils with high aggregate stability.(3) Aggregate size <2, 2 to 3, 3 to 5 mm of each soil were exposed to simulated rainfall with an intensity of 60 mm/h. There was a significant interaction between soil loss, runoff rate and aggregate size. The results showed that as clod size increased, the runoff rate decreased. The small aggregate size was likely to form a seal, consequently, the soil loss increased with decreasing aggregate size. The largest MWD of sediment materials were found in aggregate size < 2 mm of each soil.(4) The new aggregate stability index (K_a), reflected both the susceptibility of soil aggregates to slaking in sheet erosion as well as the detachment of soil material by raindrop impact. The simple correlation coefficients between the interrill soil loss and parameters of aggregate stability were calculated and the soil loss was found to be most strongly related to the new index, K_a. It could be thus supported that K_a was a suitable parameter to evaluate the interrill erosion in red soils with different rainfall conditions. However, all the erosion tests were accomplished in laboratory and the soil samples suffered the effect of sieving and rehandling. Therefore, the credibility of K_a to assess the interrill erosion needed to be validated in the field.3. Due to serious soil loss by water and seasonal drought in red soil region, this study was designed to investigate the relationship between aggregate stability and slope erosion. The result showed that the state of aggregate affected erosion process essentially, and aggregate stability was found to be highly related to runoff rate, erosion rate, and infiltration rate in the field scale. By introducing K_a into the WEPP model frame as a substitute for soil erodibility factor, statistical formulae for estimating soil loss were established with good correlation coefficient. It was concluded that these formulae based on the stability index, K_a, had the potential to improve methodology for assessing interrill erosion rates for the subtropical Chinese red soils. Considering the time-consuming and costly experimentation of runoff rate measurements, the equation without runoff rate,D_i =0.23K_aI²(1.05 -0.85 exp^-4sinθ) , was the more convenient and effective one topredict interrill erosion rates on red soils of subtropical China. These results extend the validation of soil aggregation characterization as an appropriate indicator of soil susceptibility to runoff and erosion in red soils, especially in subtropical China where intense rainfall is frequent. They also confirm that simple laboratory determination can provide data closely correlated with those resulting from more expensive or time-consuming field investigations.