Spatial Characteristics Of Ccementing Material And Pores Of Red Earth And Their Relationship With Aggregate Stability

Red earth is an important agricultural production resource of Southern China.However,the vulnerability and instability of soil ecosystem and long-term cultivation lead to the declined water-stable aggregates content,poor tillage quality,and deteriorated pore structure.Based on above backgrounds,the red soils formed quaternary red earth was taken to examine the quantitative relationship between aggregate stability and spatial characteristics of cementing material and pore structures of red earth by using field emission scanning electron microscope(FESEM)combined with energy dispersive spectrometer(EDS)and synchrotron radiation based X-ray micro-computed tomography(SR-mCT).The obtained main results were summarized as follows:1?The pore characteristics of soil aggregate section was determined using SEM and digital image processing technique.Results indicated that the pores were mainly distributed in the central area of the aggregate.These pores were small in size,high diversity in shape,and well connected.Element contour map of aggregate section showed that elements Fe and Si had irregular spatial distribution in the aggregate.Fe element had better connectivity than Si and Al.Correlation analysis showed that Fe content was significantly negatively correlated with Si content in the aggregate.By combining the principal component analysis(PCA)and matrix clustering analysis,it was found that iron oxide played a leading role in the formation of cementing material and pore structure,while Al oxides mainly influenced the pore size and shape.Morphological observation of the aggregate pores by FESEM presented a complex component of aggregate.EDS analysis further proved that iron and aluminum oxide interacted with clay mineral cementation in section distribution.The morphology of quartz presented different weathering degrees,which had an important influence on cementing materials and structural stability of aggregates.2?Water stable aggregates in the soils were separated into six size classes:>5 mm?5-2 mm?2-1 mm?1-0.5 mm?0.5-0.25 mm and<0.25 mm.The aggregate stability were evaluated by>0.25 mm water-stable aggregates(WSA),mean weight diameter(MWD),and geometric mean diameter(GMD).Results showed that land use type of soils had obvious influence on aggregate stability.The orchard land and forest land had more stable aggregates than bare land and crop land.Correlation analysis showed that organic matter had significant positive correlation with WSA?MWD and GMD.There was no significant correlation between aggregate stability and iron content.Mercury intrusion porosimetry(MIP)was used to determine pore volume and size distribution.It was observed that MWD had significant positive correlation with average pore diameter(APD)and 0.1-5 ?m pore volume of soils,but was negatively correlated with<0.01 ?m pore volume.3?Incubation experiment showed that removal of organic matter could significantly increase the compressive strength of red soil,while addition of coal ash and lime decreased the compressive strength.Correlation analysis indicated that compressive strength had significant correlation with total iron and free iron content,but no significant correlation was observed with organic matter?soil texture and amorphous iron.Porosity determined by MIP method showed that APD and 0.1-5 ?m pores had significant negative correlation with compressive strength,while positive correlation was observed for 0.01-0.?m pose size.Multi-regression equation indicated positive relationship between compressive strength and APD?Fed?0.1-5 pm?0.01-0.1 ?m.The equation with four variables could be used to predict the compressive strength of soils(R=0.732).4?SR-mCT was applied to examine the intra-aggregate pore characteristics and pore spatial distribution within the aggregate.The red earth sample Q4 had more developed pore structures and complicate spatial arrangements than soil sample Q12.Firstly,the morphology of two-dimension(2D)?three-dimension(3D)and anisotropy degree showed that soil Q4 contained abundance of macropores,multifarious pore growth directions and pore size distribution,while most of pores in soil Q12 were micropores with density distribution,and the pore growth tend to be consistent.Secondly,two soils showed similar single pore shape but along with different interconnected pores shape.However,soil Q4 had smooth transition between pores whereas soil Q12 appear a“bottleneck”effect in the pore shape.Thirdly,soil Q4 had larger total porosity than that of soil Q12.The porosity of different size pores was increased from 3.7-5 ?m to>80 ?m pores.Fourthly,pore space distribution showed that soil Q4 had a uniform pore distribution pattern of>30 ?m,while<30 ?m pores were mainly distributed in the center of the aggregate.In contrast,>30 ?m pore size of soil Q12 was mainly distributed in edge of the aggregate and<30 ?m pores were similar to soil Q4.SR-mCT explained the formation mechanism of different pore structure and aggregate stability of soils.