Dryland-to-paddy conversion enhances hydrological functioning by reshaping soil structure in the hilly red soil region of southern China

Soil organic carbon (SOC) and soil structure are key determinants of soil hydraulic properties. However, the roles of SOC fractions and soil structure in regulating hydraulic behavior during dryland (DL) conversion to paddy fields (PF) in the hilly red soil region of southern China remain insufficiently understood. This study evaluated dryland and paddy fields cultivated for 1 year (PF1), 3 years (PF3), 6 years (PF6), and 30 years (PF30) to quantify their effects on SOC composition, soil structure, and soil hydraulic properties. The results indicated that the SOC content increased with rice cultivation duration in the surface soil (0–20 cm), with PF30 exhibiting 181.8–187.6% higher SOC than PF1. In contrast, SOC downward translocation was evident in young dryland-to-paddy (PF1–6) in the subsurface layer (20–50 cm). The SOC, particulate organic carbon (POC), humic extractable carbon (HE-C), and humin carbon (HU-C) of PF1–6 decreased by only 17.5–35.6% compared with the surface soil, while PF30 decreased by 71.5–87.6%. Short-term dryland-to-paddy conversion (PF1–6) induced macropore collapse, with soil porosity (Φ > 116 µm) in the 0–10 cm layer reduced by 61.7–78.9% compared with PF30. In contrast, organic carbon pool accumulation and progressive soil agglomeration reconstructed water-retentive micropores, suggests that soil structural function is undergoing a transition from aeration to water retention. PF30 produced a porous and stable soil structure, significantly enhancing soil water retention, as indicated by the increases in saturated water content ( θs , 16.0–33.1%), field capacity (FC, 20.1–39.6%), and plant-available water (PAW, 29.2–136.5%) compared with PF1–6 in the surface soil (0–20 cm). These findings provide new insights into soil structural reconfiguration and hydraulic functioning during dryland-to-paddy conversion.