Aggregate Mechanical Stability And Its Relation To Slope Erosion Of Typical Red Soils

The soil erosion of the hilly red soil region in Southeast China is a most serious environmental problem of China. Aggregate is the basic unit of soil structure, so aggregate stability is a good indicator of soil resistance to erosion. Numerous methods have been developed to assess aggregate water stability, such as wet sieving, end-over-end shaking and water-drop impact, but a major limitation of these procedures is that they apply an arbitrary application of mechanical energy that is not quantified, resulting in a stability assessment that can only be related to the context in which it is being used. Ultrasonic technique provided a new way to assess aggregate stability. In theory, aggregate bond energy can be quantified by the ultrasonic aggregate stability assessment. However, there still some problems need to be solved in theory. Tensile strength is a quantitative aggregate index and very sentive to soil structure. It has been used widely in the studies of soil tillage and wind erosion. Some researchers have found the relationships between tensile strength and aggregate water stability and some erosion characteristics.However, there is still little information about the relationship between tensile and soil erosion.Therefore, objectives of this research were to (i) understand the stable mechanism of mechanical stability and water stability in Ultisols (Subtropical China),especially the effect of organic matter on aggregate stability;(ii) to compare the mechanism of aggregate breakdown in tension and the different mechanism of aggregate breakdown under rainfall (iii) to analyze the relationship between aggregate tensile strength and aggregate breakdown process;(iv) to explore the possibility of predicting soil detachment by interrill erosion in Ultisols (subtropical China).First, some typical Ultisols developed from Quaternary red clay and Shale were selected from Hubei Province and Jiangxi Province. First, aggregate tensile strength and water stability were compared, especially the relationship between tensile strength and different mechanism of aggregate breakdown during soil erosion process. Considering the confusion about the effect of organic matter on aggregate stability of Ultisols, we selected the same soils with different fertilizer management, which had the similar oxides content and soil texture, to analyse the relationship between organic matter and aggregate stability.Second, the process of different size aggregate breskdown under rainfall was observed.We analysed the compared the process of aggregate breakdown in tension and the process of breakdown in erosion. The relationship between tensile strength and these indexes of aggregate breakdown, such as particle size distribution and splash detachment, were analysed as well. In addition, we discussed the relation between friability and soil erosion.Third, we analysed the breskdown process of3-5mm aggregate from different soil under rainfall, and compared these relationships between soil erosion indexes (size distribution and splash detachment) and aggregate stability indexes, including aggregate water stability, tensile strength and friability.At last, laboratory experiments of simulated rainfall involving three different slope wereconducted on six Ultisols to evaluate the possibility for assessing interrill erosion rates for the subtropical Chinese Ultisols by tensile strength.The main results were listed as following:1According to the research of aggregate from different parent material and different land use, soil texture and oxides of which varied greatly, cation exchange capacity, clay content and oxides were the most important stable factors contributed to aggregate water stability, whereas cation exchange capacity and organic matter were the most important stable factors contributed to tensile strength.According to the research of different fertilizer management, organic matter is a strong determinant of tensile strength and aggregate water stability in soils with similar soil texture and oxides content. Organic carbon and hot water extractable carbohy were related to the three mechanisms of aggregate breakdown, aggregate repellency and other aggregate propertirs significantly. They are good indexs of the effect of fertilizer management on aggregate stability.2On the basis of WEPP erosion model, we developed two prediction equations for assessing interrill erosion rate by incorporating the reciprocal of aggregate tensile strength in the prediction evaluations for soil erodibilites, both of which had high efficiency in assessing soil detachment rate in interrill erosion in our study. R2of the equations with runoff and without runoff were0.68and0.80, respectively. To further improve the efficiency in prediction, water-repellency index was introduced in the two prediction equations, which enhances more precision of two prediction equations. R2of these new equations were0.74and0.86, respectively.3Tensile strength is closely related with aggregate water stability. The results of wet-sieving method, including the content of water stable aggregate and mean weight diameter, showed good correlation with tensile strength. For the three aggregate breskdown mechanisms in erosion, both breakdown caused by slaking and by differential swelling showed significant correlation with aggregate breakdown in tension, especially for aggregate breakdown caused by slaking, which showed very significant correlation with.tensile strength.4Both aggregate break in tension and break in slaking were depended on aggregate microstructure. Under the simulated rainfall with60mm/h intensity, slaking is the dominant mechanism of aggregate breakdown. As a result, aggregate breakdown during rainfall was closely related to tensile strength. For different size aggregates, good correlations were observed between tensile strength and normalized mean weight diameters under different amounts of cumulative rainfall. For the same size aggregates of different soils, good correlations were observed between tensile strength and mean weight diameters under different amounts of cumulative rainfall.5As aggregate size increased, the resistance to aggregate breakdown was decreased whereas the resistance to detachmen t was increased. The rates of splash exhibited delayed detachment peaks with the larger-size aggregates and that the delay became greater as the initial aggregate size increased. For two soils with different frisbility, the tendency that aggregate splash decreased with increasing aggregate size was more obvious for the soil with low friability. Because splsh detachment is mainly affected by the tensile strength of small aggregate, for all size classes, the aggregates of soil with low friability produced more splash materials. However, no significant linear relationship between friability and the amount of splash or mean weight diameters after rainfall were observed.At alst, tensile strength is a quantitative aggregate stability index, which can be determined by simple tests, used for aggregates of very different size, and is very sensitive to soil structure. Considering the main problems of other methods for testing aggregate stability, it was concluded that these formulae based on the aggregate tensile strength have the potential to assesse intertill erosion rates for the subtropical Chinese Ultisols.