Trade-offs between water storage and transport mediated by three percolating pores in subtropical red soils: X-ray CT evidence from an 11-year tillage experiment

Red soils in subtropical regions are highly susceptible to structural degradation, resulting in poor water retention and restricted water transport. Although tillage strongly influences soil hydraulic properties, the pore-scale mechanisms governing these responses after long-term management remain poorly understood. Here, we investigated how 11 years of no-tillage (NT), rotary tillage (RT), and deep ploughing (DP) reshaped pore architecture and regulated soil hydraulic behavior across a 0–40 cm red soil profile by integrating soil physical measurements, van Genuchten modeling, X-ray computed tomography (CT), and multivariate analyses. Relative to conventional rotary tillage (RT), DP reduced bulk density by 16.8% and increased saturated hydraulic conductivity (Ks) by 138.9–1248.2% and air permeability (Ka) by 25.9–167.0% within the 0–30 cm cultivated layer through enhanced macroporosity and pore connectivity. In contrast, NT maintained higher Ks and Ka in the 20–40 cm subsoil, reflecting the preservation of continuous biopore networks under long-term minimal disturbance. RT preferentially promoted mesopore (200–500 μm) development, resulting in the highest saturated water content, field moisture capacity, and plant-available water. Random forest analysis identified global pore connectivity (Γ), pore anisotropy (An) and pore size distribution, rather than total porosity, as the dominant predictors of hydraulic behavior. Correlation, principal component, and redundancy analyses consistently showed that macropores (>500 μm) primarily controlled water transmission, whereas meso- and micropores (60–200 and 200–500 μm) governed water retention. Structural equation modeling demonstrated that tillage regulated hydraulic function indirectly through pore architecture, with pore conductivity promoting hydraulic conductivity (standardized path coefficient: 0.38) and pore storage enhancing hydraulic storage (0.45), while revealing a strong antagonistic trade-off (-0.93) between conductivity and water storage. These findings establish pore architecture as the mechanistic link between long-term tillage and soil hydraulic function, providing a scientific basis for optimizing tillage strategies to improve water use efficiency and drought resilience in red soil agroecosystems.