Water loss in paddy fields occurs through various pathways, and previous studies have primarily focused on water seepage in the field, often overlooking the potential for the field-bund area. In this study, 3 typical paddy fields in the plain river network area of southeastern China were selected to clarify the differences in the soil structure and hydraulic characteristics at different positions within the field-bund area: the field, inner bund, middle bund and outer bund. The interactions between basic soil properties and hydraulic characteristics were also evaluated. The results revealed that the outer bund presented the lowest soil porosity (6.92 %), followed by the field (7.52 %), middle bund (7.77 %), and inner bund (8.09 %). The soil pores in the field presented the smallest mean diameter and fractal dimension and the highest degree of anisotropy. The deep layer of the bund contained more macropores, and the soil pores exhibited greater spatial distribution heterogeneity. The bottom layer in the field and bund presented the lowest average Ks value of only 0.05 mm min(-1), indicating the presence of a plow pan and a notable tendency for lateral seepage. Differences in the soil structure and hydraulic parameters between the field and bund created a driving force for lateral seepage and rendered the field-bund area a hotspot for water loss. For the analysis of the underlying water loss mechanism, the structural equation model represented 65 % of the total variance in the hydraulic parameters. The micropore characteristics had the greatest positive direct effect on the hydraulic parameters, with a standardized path coefficient of 0.39 (p < 0.001). The soil physical properties were not directly related to the hydraulic parameters but exerted an indirect effect through aggregate stability and micropore and macropore characteristics, with a total indirect standardized path coefficient of -0.41.

The development of ground fissures in the subsidence area induced by shallow-buried extra-thick coal seam mining leads to a decrease in soil water-holding capacity and vegetation withering. The investigation of the spatial distribution characteristics of soil pore structure in mining-induced subsidence areas and the elucidation of the response mechanism between soil physical parameters and soil structure are essential prerequisites for achieving effective ecological environment protection and restoration in mining areas. In this study, three-dimensional visualization reconstruction of soil porosity, pore diameter, roundness rate and pore connectivity at different profile depths in subsidence area caused by ultra-thick coal seam mining was conducted by using the soil CT scanning technology. Additionally, a correlation analysis model was established between physical parameters such as soil moisture content, bulk density, and pore structure parameters. The results indicate that: (1) Compared to the non-subsidence area, the number, size, and proportion of soil pores significantly increase in the tension zone, compression zone, and neutral zone, and the pore network and connectivity are extensively interconnected, and the fissure development in the tension zone is most significant, and the soil pores in the subsidence compression zone are concentrated on one side due to the influence of soil deformation. (2) As the depth of the soil profile increases, the level of soil pore development significantly decreases. The large profile depth (40-100 cm) causes a significant decrease in pore development. (3) There is a significant correlation between soil pore structure parameters and soil physical parameters (P = 0.05), a highly significant correlation between soil pore structure parameters and soil moisture content (P = 0.01), and a highly significant correlation between soil texture and soil moisture content (P = 0.01). This research is of great significance for guiding underground production layout and repairing damaged soil.

Straw returning and tillage measures are one of the key measures to improve soil structure and fertilize soil. Different rotation systems and tillage methods (no-tillage or conventional tillage) affect the physical structure and the C sequestration efficiency of soil. Here, we examined the response of soil organic C stocks, soil C fractions and soil structure to straw returning and tillage management based on a 15-year rice-rice-oilseed rape rotation. Our results indicated that the soil carbon stock reached C equivalent within 10 years with straw returning and 5-6 years without straw returning. No-tillage improve the C sequestration efficiency only in the early stage. Soil organic C (SOC) fractions significantly increased after straw returning, that is, dissolved organic carbon (DOC) increased by 10.3%-21.4%, particulate organic carbon (POC) increased by 32.0%-44.2%, light fraction organic carbon (LFOC) increased by 37.9%-61.2%, and microbial biomass carbon (MBC) increased by 7.1%-12.1%. Except for LFOC, no significant difference was observed between no-tillage and tillage for the other SOC fractions. Straw returning significantly improved the proportion of >2 mm aggregates (+40.0%) and the SOC content of >0.25 mm aggregates. Meanwhile, straw returning with conventional tillage (CTS) enhanced the SOC content of <0.25 mm aggregates. The soil structure became irregular (anisotropy increased by 115.0%), more complex (fractal dimension increased by 10.2%) and the number of soil pores increased by 108.5% after straw returning. LFOC and MBC played important roles in promoting the changes in the soil structure and the formation of macro-aggregates. Overall, straw returning was more effective in increasing the SOC, the accumulation of macro-aggregates, and the number of soil macropores, as well as improving the soil structure compared with tillage under the triple upland-paddy rotation system.