Governing role of pore structure in solute transport processes in plain river network areas: Insights from CT imaging

Abstract

The plain river network (PRN) areas are key high-yield agricultural zones in China, but the excessive use of fertilizers and pesticides has caused severe non-point source (NPS) pollution. Soil pore structure plays a crucial role in regulating NPS generation and transport by controlling water flow and solute transport. However, conventional static fractal parameters are insufficient to capture the actual pathways of solute transport. To overcome this limitation, we introduced the spectral fractal dimension ($d$), a dynamic measure of pore structure derived from random walk theory. In this study, we investigated paddy fields, orchards and vegetable fields in the Jiangsu PRN region, analyzing both pore morphology, the static fractal dimension (mass fractal dimension ($Dₘ$) and surface fractal dimension ($Dₛ$)) and the dynamic pore characteristic ($d$) across soil depths. Solute transport parameters were determined using breakthrough curves and the continuous time random walk (CTRW) approach to assess the governing role of pore structure. Our results revealed that surface soils exhibited higher pore abundance and more complex pore structures, resulting in increased solute transport velocity ($v$) and dispersion coefficient ($E$) compared to deeper layers. Pore morphology (e.g. Volume, Surface, Mean Breadth) and heterogeneity ($Dₘ$ and $Dₛ$) controlled $v$ and $E$ by providing fast flow paths and extensive surface interactions, whereas connectivity ($d$) governed anomalous transport ($\beta$) and timing ($t_1$, $t_2$) via tortuous, retention-enhancing pathways. Pore shape further affected solute transport indirectly through connectivity. Among land use types, paddy soils showed a less complex and poorly connected pore structure compared to orchard soils and vegetable soils. To improve the pore structure of paddy soils and mitigate the risk of agricultural non-point source (NPS) pollution in PRN areas, the adoption of water-saving irrigation combined with straw return is recommended.