Decoupling of MAOC accumulation and relative allocation under slope-dependent mineral activation in forest soils of a north–south transition zone

Mineral-associated organic carbon (MAOC) is central to long-term soil organic carbon (SOC) preservation, yet greater mineral activation does not necessarily imply a proportional allocation of SOC to MAOC. Whether dissolved organic matter (DOM) spectral characteristics represented by the humification index (HIX) are linked to Fe/Al mineral activation and whether this coupling is associated with MAOC accumulation and MAOC/SOC allocation under field-scale hydrothermal divergence remain unresolved. We sampled soils along an elevational gradient on contrasting north- and south-facing slopes of the Funiu Mountains, central China, integrating DOM fluorescence indices, multi-form Fe/Al (hydr)oxides, and SOC physical fractionation. In a relative-importance model excluding total nitrogen (TN), Al-related mineral variables explained the largest share of MAOC variance (45.6%), exceeding Fe-related variables (32.5%) and DOM optical indices (17.1%); the relative order among these three groups was retained in a TN-included sensitivity analysis. The humification index (HIX) was positively associated with Alo/Ald on both slope aspects, indicating a consistent but modest association between HIX-defined DOM spectral characteristics and Al activation. However, reactive Fe/Al indicators were associated with higher MAOC concentrations, particularly on north-facing slopes, without producing a proportional increase in MAOC/SOC. This decoupling was most evident on the south-facing slope, where Alo/Ald and Feo + Alo were negatively related to MAOC/SOC but positively associated with particulate organic carbon (POC) accumulation. Bootstrap mediation showed offsetting indirect effects on the north-facing slope, whereas only the negative Al activation-mediated pathway was statistically supported on the south-facing slope, consistent with a potential mismatch between organic precursor supply and effective mineral binding availability. These findings suggest that mineral activation can support MAOC accumulation without necessarily increasing relative SOC allocation to MAOC, highlighting the need to incorporate spatial heterogeneity in mineral protection capacity into SOC stabilization models.