Spatial and vertical distribution of redox‐sensitive elements and their controlling factors in deep soils under contrasting land cover in the Gulf Coastal Plains

Abstract

Redox processes in soil governs key biogeochemical cycles and influence carbon, nitrogen, and phosphorus dynamics. However, their role in deep vadose zone soils remains poorly understood, particularly in variably saturated environments with heterogeneous hydroclimatic, landscape position, and land use conditions. This study investigates the spatial and vertical distribution of redox‐sensitive elements (Fe(II), Mn ³⁺ , SO ₄ ²⁻ , NO ₃ ⁻ , and PO ₄ ³⁻ ) and their controlling factors in deep vadose zone soils (up to 15 m) under contrasting land covers in the Gulf Coastal Plains of Texas. We combined field sampling, laboratory analyses, statistical modeling, and reactive transport simulations by PFLOTRAN to evaluate how soil physicochemical properties and hydrological conditions influence redox transformations, the coupled dynamics of iron and phosphorus under variable moisture regimes, and the dominant geochemical mechanisms controlling deep subsurface redox biogeochemistry. Our results reveal significant heterogeneity in soil texture and moisture retention across sites, which dictate redox zonation and the vertical mobility of redox‐sensitive elements. In waterlogged environments, reducing conditions promote Fe(III) reduction and PO ₄ ³⁻ release, whereas well‐drained upland soils exhibit oxidizing conditions with lower dissolved organic carbon and nutrient mobility. Random Forest and Shapley Additive Explanations analyses identified soil moisture, clay content, and base cations as key predictors of redox variability, reinforcing the strong interplay between soil physical structure and biogeochemical reactivity. Reactive transport modeling further highlights the kinetics of Fe(III) accumulation and phosphorus release under dynamic redox transitions, emphasizing the need to integrate hydrological fluctuations into subsurface redox process studies.