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Soil organic carbon (SOC) storage and persistence are strongly controlled by reactive metal phases, particularly organically complexed aluminum (Al) and iron (Fe) and short-range-order (SRO) minerals. However, their global relevance and the specific metal phases involved remain uncertain due to substantial variability in parent material, soil age, and rock-climate-SOC interactions. Andisols, derived from volcaniclastic materials and enriched in SOC and reactive metals, provide an ideal system to assess metal-SOC associations across broad pedogenic gradients. We compiled a global Andisol database of over 2850 soil samples across 34 countries, covering wide ranges of mean annual temperature (−2 °C to 30 °C), precipitation (60–6000 mm y⁻¹), and soil pH in water (3.1–9.3). Most samples clustered within pH 4.5–6.5, corresponding to an Al-buffered domain where soil pH is predominantly regulated by Al hydrolysis reactions and equilibria among reactive Al pools. Generalized additive mixed model analyses identified organically complexed Al (pyrophosphate-extractable Al, Alₚ) as the strongest global predictor of SOC (relative importance = 40%) after accounting for soil depth. SRO Al minerals (acid oxalate-extractable Al minus Alₚ) showed moderate importance (relative importance = 10%), whereas reactive Fe and clay content had minor effects. Exchangeable calcium contributed significantly only at pH > ∼6.3, consistent with a transition toward base-cation buffering. The persistence of strong SOC-Alₚ relationships within the Al-buffered domain, together with consistent pH-dependent shifts in reactive Al and Fe pools, suggests that complexation with pedogenic Al released through weathering may exert a first-order control on mineral-protected SOC beyond Andisols and provides a mechanistic basis for incorporation into global-scale models. Identifying dominant stabilization mechanisms remains critical for determining whether SOC persistence is primarily regulated by carbon inputs, metal supply, or their combined effects. Given its integration of organically complexed and SRO Al phases and its broad data availability, acid oxalate-extractable Al emerges as the most practical proxy for mineral-protected SOC at the global scale.
Kida et al. (Fri,) studied this question.