Slope land preparation has a significant effect on soil and water conservation in sloping farmland in hilly and mountainous areas. Soil structure and water holding capacity can be improved to reduce soil erosion for land use efficiency. Existing research focused on the influence of slope land preparation on soil macroscopic features. However, it is still unclear on the specific mechanism of how slope land preparation regulates soil microstructure to determine the soil water holding capacity, especially soil aggregate structure and its related functional groups. From the microscopic point of view of soil, this study aims to explore the effects of different slope land preparation measures on soil aggregate structure, functional groups, and their quantitative relationship with soil water retention capacity. The results showed that different measures of slope land preparation played a significant role in the soil aggregate structure. Compared with the slope farmland (control group), slope land preparation effectively altered the composition and structure of soil aggregates, and then weakened the cementation among soil aggregates. A small and complex aggregate pore structure was formed, especially the mosaic pores. Therefore, slope land preparation significantly improved the spatial uniformity and structural stability of soil aggregate pores. In addition, the gradation composition of soil aggregates was optimized to improve the water permeability and retention of soil. There was a variation in the content of hydrophilic functional groups in the soil after the slope land preparation. Specifically, the soil aggregate porosity was closely related to the relative content of C-OH groups in the hydrophilic functional groups, providing for a more favorable micro-environment to maintain soil moisture. Furthermore, a systematic analysis was conducted to explore the effects of different slope land preparation on soil water retention. The results showed that the soil preparation of the horizontal platform dominated the soil water retention capacity. Specifically, the field soil maximum water holding capacity (27.06 mm), capillary water holding capacity (25.42 mm), and field water holding capacity (22.57 mm) of the horizontal platform were significantly higher than those of the reverse slope platform and slope farmland. The comprehensive index of soil water holding characteristics from large to small was horizontal platform (75.05), reverse slope platform (57.20) and slope farmland (53.49). The slope land preparation also improved soil moisture retention. The fractal dimension of hydrophilic functional groups (especially C-OH groups) and soil aggregate pores were the main factors affecting soil water holding capacity. Compared with the slope farmland, slope land preparation was used to regulate the pore structure of soil aggregates for the water storage capacity of soil, thus effectively improving the soil moisture r retention. Water and soil resources were optimized for slope farmland in hilly and mountainous areas.

Root system in soil is closely related to the distribution of soil moisture in farmland. The growth of roots can change the pore distribution of soil, leading to the heterogeneity of soil moisture. The soil in the root zone can be utilized to improve the soil moisture storage for the better water conductivity and storage capacity in the field. The flow of material and energy can also be promoted in the soil environment for the better crop growth and development. This study aims to explore the spatial variation characteristics of soil water movement in farmland under the influence of pepper root system. The research area was taken as the agricultural flat land in Lingui District, Guilin City, Guangxi, China. The field pepper was also selected as the research object. The field dyeing tracer test was combined with the indoor morphological image and grey correlation analysis, in order to quantitatively analyze the difference of soil dyeing narrow degree, dyeing area ratio, and water movement in the root and non-root zone of the field. A systematic investigation was implemented on the effects of root morphology and structure on the spatial movement of soil moisture. The results showed that the root system of pepper was similar to the fishtail structure, indicating the dense lateral roots, and the relatively large volume of root expansion in the soil. The rooting depth and width (root morphology) were highly correlated with the root structure (P＜0.05). Under the same external conditions of water supply, the water in the non-root zone was distributed in the surface layer of the field soil (average narrow degree 0.89), while the soil moisture in the root zone was changed from the overall to the agglomerated state, and then to the 'finger-like' (average narrow degree 0.61) in the whole soil depth (0-50 cm). There was a decrease in the average dyeing area ratio of soil profile in the non-root and root zone with the increase of soil depth. By contrast, the average dyeing area ratio in the root zone was significantly higher than that in the non-root one (23.62%), indicating the more concentrated soil moisture distribution. The change degree of water movement in the non-root zone (average turbulence intensity of 116.09) was significantly higher than that in the root zone (P＜0.05) with the increase of soil depth. Furthermore, a better performance was achieved in the high integrity and connectivity of water spatial movement in the root zone, and the strong capacity of vertical infiltration under the influence of roots. Grey correlation analysis showed that the root morphology posed a much more significant effect on the soil water movement than the root structure. The greatest influence on the soil water turbulence intensity was the specific root length (grey correlation degree was 0.984), followed by the root width (grey correlation degree was 0.684). The average total gray value of soil dyeing showed that the closer the soil profile was to the main root of pepper, the deeper the soil particle dyeing and the higher the soil moisture content were. The average total gray value frequency of the root zone (31.34%) was 1.6 times that of the non-root zone (19.36%) in the range of 51-153 gray levels, indicating the higher content of soil water and the more active water movement. The root system can be expected as one of the most important component structures to form the root soil environment in the soil layer for the better growth and development of field crops. Therefore, the presence of the root system can be expected to increase the infiltration capacity of the soil water, due to the more active water environment around the root system. The fertilization and irrigation can then be implemented to improve the utilization rate of soil resources for the high crop yield, according to the soil environment of the root zone.

Soil aggregates can often form the basis of soil structure. This study aims to analyze the relationship between soil aggregates and soil water from the microscopic perspective of soil in the hilly and mountainous slopes. Theoretical support was then provided to further clarify how the soil structure regulated the field water and fertilizer use after land preparation. Two typical measures of land preparation were selected as the horizontal and reverse slope, with the slope farmland as the control. A series of experiments were carried out to explore the interaction between soil aggregates and soil water status. The staining tracer test was also conducted on the plots under different measures of land preparation. Imaging and statistical analysis were finally employed using a scanning electron microscope. The results indicate that the staining was characterized by the movement and distribution of water in the soil, particularly in the same conditions of external water supply. The spatial distribution of soil water was concentrated in the shallow space of the 0-10 cm depth of the soil in the field. A significantly smaller range was observed on the distribution of soil water in the slope farmland field. After horizontal and reverse slope preparation, the degrees of change in the spatial movement of soil water in the field were 11.09% and 7.14% higher than those in the slope land, respectively. Microscopic images of soil aggregates were magnified by 1000 times using scanning electron microscopy. Then the quantitative analysis was conducted using pore analysis software. It was found that the spatial distribution of soil aggregates after horizontal and reverse slope cultivation was more uniform (with average fractal dimensions of 1.16 and 1.18, respectively) than that of the slope-cultivated land (with an average fractal dimension of 1.19). Land preparation reduced the difference in the pore size and composition in the soil aggregates by 5.81% (horizontal) and 10.10% (reverse slope), respectively. The complex behavior was found along the spatial distribution direction of pores in the soil aggregates in the horizontal field (with a probability entropy of 0.957 8). The spatial distribution and movement of soil water were analyzed to couple the coordination degree model. A reference value was obtained to determine the correlation degree of soil water. The coupling coordination degrees of the horizontal platform, reverse slope platform, and slope farmland were 0.62, 0.58, and 0.56, respectively. Gray correlation was employed to analyze the reference value of soil water status and various indexes of aggregates. There was the greatest influence of the spatial distribution in the soil aggregates of the horizontal platform on the spatial distribution and movement of soil water in the field (with a correlation degree of 0.812). However, the size of soil aggregates in the reverse and undisturbed slope cultivated land shared the greatest influence on the spatial distribution and movement of soil water (with correlation degrees of 0.708 and 0.698, respectively). Therefore, the land preparation measures can significantly alter the soil aggregate status of slope farmland, and subsequently affect the soil moisture in the field. Therefore, it is highly required to the optimal measures of land preparation for the sloping farmland in the hilly and mountainous areas. The finding can provide a strong reference to effectively regulate the soil structure and soil water during agricultural production.