Microstructure Characteristics Of Soil Aggregate And Their Relevance To Slop Erosion Processes In Typical Climatic Zones Of China

Soil structure is the basis for maintaining soil functions and reflects the morphological characteristics and pattern of variability in soil.Soil aggregate,as an important soil structural unit,play a vital role in soil water,air and nutrient transport and soil erosion.The soil types developed in different climate zones are diverse and soil properties are significant differences due to the latitudinal zonality of China.At present,some limitations in the systematic research on the microstructure and slope erosion process for different types of zonal soil aggregates still exist.In this study,six types of typical soils（Black soil,Brown soil,Cinnamon soil,Yellow-cinnamon soil,Red soil,and Latosol）were selected as the research objects based on geogenesis.The microstructure characteristics of aggregates were quantitatively characterized by SR-μCT visualization.The aggregate stability mechanism and slope erosion process of zonal soil aggregates with different types were systematically studied by indoor analysis and artificial simulated rainfall experiment.The key factors causing the difference of aggregate microstructure were analyzed,and the relationship between aggregate stability,slope erosion parameters and aggregate pore characteristics were obtained by quantitative analysis.Main results were shown as follows:（1）The pores of the six soil aggregates were mainly distributed in the middle,and the pores were scattered around the aggregates.The internal pores of the Yellow-cinnamon soil aggregates were mainly flat and elongated,with uneven and scattered pore distribution and simple pore network.More complex pore structures and more uniform pore distribution were found in Black soil,Red soil and Latosol aggregates was more complex and the pore distribution was more uniform,especially in the eluvial horizon,with many continuous elongated pores,which were related to the high content of soil organic carbon（SOC）or iron and aluminum oxides.The porosity of different equivalent diameters decreased first and increased from north to south.The pore sizes in most aggregates were observed to be30-75μm,however,the pores larger than 100μm were dominant in the Yellow-cinnamon soil,which was connected with the original particle arrangement and the low content of cementing material in the soil.The aggregate pore morphology was dominated by elongated porosity with few regular porosity and irregular porosity.Stepwise regression analysis and path analysis showed that SOC and Al_d had positive effects on pore structure development of aggregates,whereas iron oxides manifested as opposite effects（P<0.05）.Besides,different types of cementing substances will act on the aggregate pore aggregate in their own ways and other cementing substances will affect the cementing effect of certain materials in various degrees.（2）The mean weight diameter（MWD）was used to evaluate the water stability of aggregates.From north soil to south,the water stability of aggregates decreased first and increased,the eluvial horizon in Red soil was the highest（1.49 mm）,and the parent material horizon in Yellow-cinnamon soil was the lowest（0.19 mm）.The aggregate stability under three treatments showed slow wetting（SW）>shaking（WS）>fast wetting（FW）,indicating that dissipation and external mechanical failure were the main fragmentation mechanisms of aggregates.The aggregates stability showed significant positive correlations with total porosity,total pore numbers,elongated porosity,<30μm porosity,30-75μm porosity,75-100μm porosity and>100μm porosity,and inversely correlated with regular porosity and irregular porosity（p<0.01）.The results of partial least squares regression revealed that the number of pores,porosity,and elongated porosity had a positive effect on the aggregate stability.75-100μm porosity,>100μm porosity,elongated porosity,and regular porosity were the main pore characteristic factors controlling the aggregate stability under the LB method.（3）From north to south,the initial runoff time of the five zonal soils decreased and increased.The initial runoff time of Red soil aggregates was significantly greater than other soil aggregates,which could be explained by the aggregate stability and clay mineral composition（kaolinite）.Among all soil aggregate types,the sediment yield of Red soil was the lowest,and yellow-brown soil was the highest.Red soil aggregates were more stable and less prone to crust formation.The broken parts of aggregates tend to fill pores with runoff infiltration rather than moving with runoff.The total porosity,total pore number,pore shape factor and porosity with different equivalent diameters were significantly negatively correlated with runoff coefficient,soil sediment yield and runoff sediment concentration（P<0.05）.Larger aggregate porosity represents better pore connectivity and a richer pore network,resulting in the extension and diffusion of micropores along their edges,which may require more erosion sediment particles to fill the pores and reduce the content of sediment particles moving on the slope.According to the results of partial least squares regression,75-100μm porosity and pore morphology parameters（regular,irregular and elongated porosity）were the main pore characteristic factors for slope erosion.In addition,the variation of surface roughness before and after rainfall increased with sediment yield rate and runoff sediment concentration as a power function,which can be expressed as:ΔSSR=10.02（D_i）^0.3 andΔSSR=4.45（C_s）^0.417,whereΔSSR is the surface roughness,D_i is the soil sediment yield rate,and C_s is the runoff sediment concentration.