Effects of root-soil environment changes on the spatial heterogeneity of soil moisture under different land preparation measures in sloping farmland

Root-soil environment is often required for the functional restoration of sloping farmland after land preparation. However, the current separate effects of roots or land preparation cannot fully consider the coupled mechanisms between the root-soil environment and soil moisture spatial heterogeneity after land preparation. This study aims to clarify the regulatory mechanisms of the root-soil environment for the spatial heterogeneity of soil moisture. The study area was selected from the pepper (Capsicum annuum L.) planted on sloping farmland in the hilly regions of Guangxi, southern China. A series of approaches were combined, including ground penetrating radar (GRP) detection, root morphological and functional trait analysis, and soil physicochemical property measurement. Two typical measures of land preparation were set as the horizontal and reverse-slope farmland. Image interpretation, index evaluation, and grey relational analysis were applied to investigate the effects of root-soil environment on soil moisture spatial heterogeneity. The results showed that the land preparation significantly altered the spatial distribution pattern of soil moisture, indicating the a more concentrated distribution. Soil moisture was concentrated in the 0~30 cm layer under horizontal farmland, and in the >20~40 cm layer under reverse-slope farmland, whereas it was continuously distributed throughout the 0~50 cm layer in sloping farmland. Among the three treatments, the spatial variation of soil moisture in the root zone was greater than that in the non-root zone. Land preparation significantly modified the spatial heterogeneity of soil moisture in both the root and non-root zones. The largest spatial heterogeneity of soil moisture was observed in the horizontal platform (comprehensive index values of 2.16 and 2.26 for root and non-root zones, respectively), followed by the reverse-slope platform (2.12 and 2.08), and the sloping farmland shared the lowest values (1.74 and 1.70). Land preparation significantly improved the root-soil environment. Both horizontal and reverse-slope farmland exhibited higher bulk density, silt, clay, soil organic matter, total nitrogen, and total phosphorus contents, as well as higher soil enzyme activities (sucrase, urease, and catalase activities) in the root zone, while sand content and saturated hydraulic conductivity were reduced significantly, compared with sloping farmland. The contents of soil organic matter, total nitrogen, and total phosphorus were higher in the root zone of horizontal farmland than those in the non-root zone, as well as in reverse-slope and sloping farmland. Root morphological and functional traits were significantly varied after land preparation. Horizontal farmland showed significantly greater specific root length and mean root diameter than reverse-slope and sloping farmland. The primary root advantage was the most significant, with the concentrated lateral branch roots. The synergistic variations in the root-soil environment also reconfigured the main factors to regulate spatial heterogeneity of soil moisture. Soil physical (bulk density and silt content) and chemical factors (total phosphorus, soil organic matter, and total nitrogen) dominated in the non-root zone, with silt content (grey relational degree 0.978-0.993) and bulk density (0.971-0.992) remained remaining the primary drivers. In the root zone, however, a co-dominance emerged between soil physical properties and root traits: specific root length (correlation degree 0.967) for horizontal, and topological index (0.973) for reverse-slope, as well as silt content and bulk density as leading factors, whereas root traits were attributed to sloping farmland, and root diameter (0.987). Land preparation effectively improved the root-soil environment in hilly sloping farmland. Horizontal farmland outperformed reverse-slope farmland, indicating the highest spatial heterogeneity of soil moisture. Synergistic effects between root development and soil quality enhanced physicochemical properties and enzyme activities in the root zone. The horizontal farmland achieved in the better root-soil performance than the reverse-slope farmland under identical climatic and cropping conditions.