Physical Structure Overrides Short‐Term Nitrogen Effects on Preferential Flow in Heterogeneous Rock‐Soil Media: Implications for Mountain Ecohydrology

ABSTRACT

Mountainous critical zones, with heterogeneous rock‐soil structures, regulate water storage and flux through complex subsurface flow paths. Preferential flow, often triggered by textural contrasts, controls rapid water redistribution and eco‐hydrological connectivity. This study investigates the divergent infiltration dynamics between two dominant media—coarse, weathered clastic rock and finer brown soil—in the Taihang Mountains through controlled column experiments under varied nitrogen (N) concentrations (0–125 mg N L ⁻¹ ). A quantitative multi‐criteria framework was applied to discriminate matrix flow from preferential flow, including volume‐balance closure via the piston displacement index ( ν ), velocity ratio, flux‐capacity ratio, wetting‐front morphology with Monte Carlo noise calibration, and Green–Ampt consistency. Brown soil was matrix‐dominated ( ν * = 0.93–1.12; smooth wetting fronts) and retained significantly more water, with cumulative infiltration of 18.1–21.8 cm and effective storage capacity of 19.4 cm over the 45‐cm profile. Clastic rock exhibited strong bypass behaviour ( ν * = 1.35–1.53; p  = 0.005), with 26%–35% of infiltrated water moving beyond the visible wetting front, despite its late‐time flux remaining below the matrix saturated conductivity. Cumulative infiltration in clastic rock was significantly lower (13.3–15.2 cm; p  = 0.005), and its leaching risk index was medium‐to‐high (0.44–0.88) compared with low values in brown soil (0.08–0.24; p  = 0.002). Wetting front velocity was significantly higher in clastic rock in all phases (), but morphology remained near‐piston‐like (), indicating that rapid infiltration was a matrix‐capacity and storage‐capacity effect rather than finger‐like preferential flow. Nitrogen addition, applied as a short‐term chemical perturbation, did not significantly alter these physical controls across the tested range. These findings provide a mechanistic basis for understanding water partitioning, retention, and leaching risk in heterogeneous rock‐soil media, while field‐scale validation remains necessary.