Long-term continuous cropping affects the soil microecological community and leads to nutrient imbalances, which reduces crop yields, and crop rotation can increase soil productivity. To study the effects of the cultivation of tomato (Solanum lycopersicum) and corn (Zea mays) on the microbial community, physical and chemical factors and the structure of aggregates in cotton (Gossypium hirsutum) long-term continuous cropping soils were examined. Four cropping patterns were established, including one continuous cropping pattern and three crop rotation patterns, and the diversity of the soil microecological community was measured using high-throughput sequencing. The physical and chemical properties of different models of soil were measured, and the soil aggregate structure was determined by dry and wet sieving. Planting of aftercrop tomato and corn altered the bacterial community of the cotton continuous soil to a lesser extent and the fungal community to a greater extent. In addition, continuous cropping reduced the diversity and richness of the soil fungal community. Different aftercrop planting patterns showed that there were very high contents of soil organic carbon and organic matter in the cotton-maize rotation model, while the soil aggregate structure was the most stable in the corn-cotton rotation model. Planting tomato in continuous cropping cotton fields has a greater effect on the soil microbial community than planting maize. Therefore, according to the characteristics of different succeeding crop planting patterns, the damage of continuous cropping of cotton to the soil microenvironment can be alleviated directionally, which will enable the sustainable development of cotton production.

In this paper, the Malan loess is considered as the research object, through the direct shear test and electron microscope scanning test (SEM test), and based on studying the macroscopic shear deformation of loess, the formation and evolution of the shear surface of Malan loess are analyzed from the macroscopic, microscopic and granular scales, and the microscopic mechanism of the shear deformation characteristics of Malan loess is deeply revealed. The test results show that the shear strength of soil is controlled by the moisture state and the stress state, and the shear strength parameters c and phi are mainly related to the moisture state. The dry density highly affects the cohesion, but has a small influence on the internal friction angle. On the macroscopic level, the larger the vertical load, the smaller the shear surface roughness. In addition, the higher the water content, the higher the flatness of the shear surface. On the microscopic level, some skeleton particles near the shear surface are damaged and rearranged along the shearing direction during the shearing process, and some surface scratches appear. On the granular level, the basic shearing structural functional units mainly include the core-coat covering structure and the aggregate structure, and the performance of the basic structural functional units is related to the moisture state and the stress state. The results and conclusions obtained in this study have an important reference significance for the in-depth understanding of the strength characteristics and microscopic mechanisms of loess as well as similar soil materials.