Parent Material, Water Regime Shifts, and Fertilization Regulate Aggregate Stability and Cadmium Adsorption in Typical Subtropical Paddy Soils

Cadmium (Cd) contamination in paddy soils poses health risks, yet the role of aggregate-scale Cd binding remains poorly understood under interactive agricultural management. Unlike primary textural separates that are fixed by parent material, aggregates are dynamic and manageable. This study investigates the interactive effects of parent material, water regime shifts in annual tobacco–rice rotation, and chlorine-containing fertilizers on aggregate stability, carbon/nitrogen sequestration, and cadmium (Cd) adsorption in paddy soils from subtropical paddy fields of Hunan Province, South China. Two paddy soils, Masaniutan (granite-derived sandy loam) and Shanniutan (slate/shale-derived clayey soil), were divided into four size classes. Selective removal of soil organic carbon (SOC) and free iron oxides (Fed), followed by Cd adsorption experiments. Shanniutan had greater aggregate stability than Masaniutan. Water regime shifts did not significantly alter aggregate structure. Chlorine fertilizers disrupted aggregates, increasing finer fractions. Cd adsorption rose as aggregate size decreased. SOC removal reduced the maximum adsorption capacity (qm) by 36.8%, while Fed played a supporting role. Partial least squares structural equation modeling (PLS-SEM) revealed that parent material had a strong positive direct effect on aggregate stability (path coefficient = 2.346), while chlorine fertilization (−0.750) and water regime shifts (−0.344) had negative effects. Parent material, water regime shifts, and fertilization jointly influence aggregate stability and the distribution of cementing agents, thereby controlling Cd adsorption capacity—with SOC as the dominant factor. These findings provide a basis for sustainable paddy soil management by promoting SOC retention and avoiding excessive chloride inputs.