New insights into the solute transport processes of red soil combining CT scanning and stable isotope tracing

Understanding soil pore structure’s influence on material transport is crucial for nutrient cycling and pollutant management, particularly in complex hydrological environments. This study investigated the relationship between soil structure and solute transport across three land uses (forest, citrus, and rainfed cropland) in China’s red soil region using X-ray computed tomography and water tracers. Forest soils exhibited well-connected macropores, while rainfed cropland showed numerous discrete micropores. Citrus land displayed heterogeneous pore distribution with high surface macroporosity but low overall macropore content. Breakthrough curves revealed three distinct transport stages: initial transport, rapid replacement, and complete stages, each demonstrating unique characteristics based on land use. The initial transport varied significantly among land uses (0.2 pore volumes for forest/citrus, 0.1 for cropland), with cropland showing nearly double the porosity (24.4 %) compared to other land uses at this stage. Complete stage convergence occurred at approximately 1.6 pore volumes across all land uses. The Continuous Time Random Walk model outperformed the Advection-Dispersion Equation in predicting solute transport (R² = 0.999), particularly in capturing late-time tailing phenomena. Bulk density, mean diameter, soil organic matter, and pore connectivity explained 51.6 % and 61.5 % of the variability in transport parameters ΦA and ΦB, respectively. These findings enhance our understanding of structure-transport relationships in red soil regions and provide valuable insights for sustainable land management practices, pollutant transport prediction, and the development of targeted soil remediation strategies in similar hydrological conditions worldwide.