Abstract:
Coal mining subsidence can substantially modify soil hydrological conditions and aggregate structures, thereby influencing the distribution and stability of soil organic carbon. However, the responses and underlying mechanisms of aggregate-associated organic carbon under different subsidence depths in cropland soils remain unclear. This study aimed to elucidate the distribution patterns and driving mechanisms of aggregate-associated organic carbon under different coal mining subsidence depths in high groundwater-level subsided croplands. A typical subsided cropland area in Huaibei City, Anhui Province, China, was selected as the study site. Non-subsided cropland was used as the control (CK), and subsided croplands were classified into four depth gradients: 0-50 cm (D50), 50-100 cm (D100), 100-150 cm (D150), and 150-200 cm (D200). Water-stable aggregates were separated into different size fractions using wet sieving. Combined with measurements of organic carbon fractions, constant-temperature incubation experiments, random forest analysis, and partial least squares path modeling (PLS-PM), the responses of aggregate-associated organic carbon distribution and stability to subsidence depth and their underlying pathways were investigated. The results showed that: 1) Increasing subsidence depth significantly changed soil aggregate composition and structural stability. The proportion of large macroaggregates decreased from 64.42% in CK to 33.18% in D200, accompanied by decreases in mean weight diameter, geometric mean diameter, and soil structural stability index. These results indicated a shift toward finer aggregate size distributions and reduced soil aggregate stability under deeper subsidence conditions. 2) The organic carbon content associated with different aggregate fractions generally decreased with increasing subsidence depth. Specifically, organic carbon content in large macroaggregates declined by 20.65-37.49% compared with CK, and their contribution to total soil organic carbon decreased from 70.31% to 37.76%. These results suggested that subsidence disturbance weakened the physical protection of organic carbon by large macroaggregates and reshaped the distribution pattern of organic carbon among aggregate fractions. 3) With increasing subsidence depth, the proportions of dissolved organic carbon (DOC), easily oxidizable organic carbon (EOC), and microbial biomass carbon (MBC) relative to total soil organic carbon generally increased, indicating a greater contribution of active carbon pools to aggregate-associated organic carbon. The mineralization rates of organic carbon among different aggregate fractions followed the order of small macroaggregates > microaggregates > silt and clay fractions > large macroaggregates, suggesting that organic carbon associated with finer fractions had relatively higher mineralization potential, whereas large macroaggregates retained stronger physical protection. 4) Correlation analysis, random forest analysis, and partial least squares path modeling revealed that the proportion of large macroaggregates, soil structural stability, soil water content, organic carbon fractions, and carbon mineralization characteristics were the major factors regulating variations in aggregate-associated organic carbon. Path analysis further indicated that subsidence depth primarily affected aggregate-associated organic carbon indirectly through changes in aggregate size distribution and structural stability rather than through direct effects. Overall, high groundwater-level coal mining subsidence altered soil moisture conditions and aggregate structures, resulting in aggregate size redistribution, weakened physical protection of organic carbon by large macroaggregates, and changes in organic carbon allocation and mineralization characteristics. These alterations ultimately reduced the carbon retention capacity of aggregate-associated organic carbon in topsoil. This study provides a scientific basis for assessing soil carbon pool stability and developing ecological restoration strategies in croplands affected by high groundwater-level coal mining subsidence.