Examining the Impact of Coal Contamination on Soil Structural and Moisture Properties: A Comparative Study of Coal-Free and Coal-Impacted Soils

Soil porosity and moisture are critical indicators of soil quality. In coal–grain intercropping areas, centuries of coal industry activities have resulted in coal particle contamination, which has affected soil properties; however, its impact on soil porosity and moisture remains underexplored. This study compares coal-contaminated soils (CCS) and coal-free soils (CFS) in Jiaozuo, employing computed tomography (CT) scanning and moisture measurements to analyze how coal pollution influences soil porosity and moisture. Our findings indicate that CCS, compared to CFS, exhibit significant reductions in total porosity (TP), CT-measured porosity (CTP), number of pores (CTN), and the proportion and volume of water–air regulating pores (CTNWA/CTN and CTPWA). These results underscore that coal pollution substantially alters soil porosity and pore numbers. Additionally, coal pollution modifies soil pore morphology, leading to reductions in the number and length of pore throats (N throat and L throat ) and causing the pores to become more flattened and rounded, with an increased inclination angle of interconnected pores (IA ic ). As coal pollution levels increase, interconnected porosity (P ic ) and coordination number (CN ic ) decrease, while isolated porosity (P isolated ) increases. In terms of moisture parameters, coal pollution diminishes the maximum water holding capacity, soil permanent wilting point, saturated hydraulic conductivity, and moisture evaporation rate. In contrast, field water-holding capacity and maximum effective water content are enhanced. Furthermore, with increased coal pollution, maximum water holding capacity, soil permanent wilting point, and saturated hydraulic conductivity decreased, whereas field water-holding capacity and maximum effective water content increased. Correlation analysis reveals that changes in CTN, N throat , and L throat significantly influence moisture parameter variations, with most pore parameter changes affecting saturated hydraulic conductivity. The observed effects of coal pollution on soil pore parameters are attributed to the filling and clogging actions of coal particles, while its impact on moisture parameters primarily results from these particles filling and clogging soil pores. This study provides a scientific basis for managing soil moisture in areas affected by coal pollution, particularly in coal–grain intercropping regions.