Influence of pepper roots on the spatial movement differentiation of soil moisture in farmland

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.