Divergent Proton-Buffering Processes and Acidification Risks in Permanent and Variable-Charge Soils

Soil acidification threatens agroecosystems, yet the coupled, soil-specific proton-buffering mechanisms in permanent-charge soils (PCSs) and variable-charge soils (VCSs) remain insufficiently quantified. This study systematically investigated surface cation exchange, vacant site H+ sorption, and mineral dissolution, using batch and kinetic incubation experiments. Results showed that H+ buffering in PCSs was dominated by rapid, stoichiometric surface ion exchange, whereas approximately 42% of the total exchangeable acidity increment in VCSs originated from specific H+ sorption on vacant, high-affinity surface sites. VCSs exhibited ~10-fold-higher Langmuir proton sorption affinity and Temkin acid-buffering capacity than PCSs, driven by their more homogeneous, pH-dependent surface properties favoring inner-sphere coordination. Base cation release followed Ca2+ ≫ Mg2+ ≫ Na+ ≈ K+ across all soils; VCSs showed a twofold-higher Mg2+ pseudo-second-order rate constant and a strong Mg2+-Mn2+ positive correlation (R2 > 0.804, p < 0.0001), exposing them to dual risks of Mn phytotoxicity and Mg deficiency during acidification. The well-fitted parabolic diffusion model for Al3+ and Mn2+ release further indicated prolonged, diffusion-limited metal toxicity risk in VCSs. A critical soil organic carbon (SOC) threshold of 8.1 g kg−1 was identified, exceeding this value effectively retarded acidification via enhanced cation exchange capacity (CEC) and base retention. These findings provided a mechanistic framework for developing soil-specific strategies to manage and mitigate agricultural soil acidification.