Global patterns of soil organic matter stabilization driven by soil mineralogy

Soil organic matter (SOM) stabilization is strongly mediated by interactions between organic compounds and the mineral fraction of soils, yet mineralogical controls remain underrepresented in global SOM assessments. This study investigates how soil mineralogy regulates SOM stabilization across contrasting pedoclimatic environments at the global scale. We analyzed 88,555 Vis–NIR–SWIR soil spectra from 25,919 soil profiles worldwide, encompassing broad climatic, geomorphological, and pedogenetic conditions. Spectral data (350–2500 nm) were transformed using the Kubelka–Munk function and processed with second-derivative Savitzky–Golay filtering to enhance diagnostic mineral absorption features. Mineral amplitudes were calculated as proxies for the relative abundance of hematite, goethite, kaolinite, and gibbsite. To address heterogeneous depth sampling, mineral amplitudes were vertically harmonized using mass-preserving splines, standardizing soil profiles into 5 cm increments from 0 to 100 cm depth. Results reveal a clear global dominance of goethite, accounting for approximately 65% of mineral-dominant layers, followed by hematite (21%) and kaolinite (14%), whereas gibbsite shows limited occurrence. Mineralogical patterns were strongly associated with soil texture, particularly clay content, confirming the fine fraction as the primary mineralogically active compartment. Importantly, soils dominated by goethite consistently exhibited higher soil organic carbon and SOM contents than soils dominated by hematite or kaolinite. This pattern indicates enhanced stabilization linked to the high specific surface area and surface reactivity of goethite, favoring sorption and organo-mineral associations. Overall, the results demonstrate that mineralogical composition exerts a fundamental control on SOM stabilization beyond total clay content alone, emphasizing the need to explicitly incorporate mineralogical information into global analyses globally.