Reactive minerals and physicochemical protection drive soil organic carbon accumulation along a rainforest toposequence

• Clear transition from particulate to mineral associated pools with increasing elevation. • Clear increase in saturation capacity with increasing elevation. • Minimal alteration to functional composition among fractions or across elevation. • Physicochemical protection mechanisms facilitate SOC accumulation and partitioning. Understanding the mechanisms determining soil organic carbon (SOC) persistence along environmental gradients is central for climate change mitigation and carbon stewardship, but much remains unknown in this regard. This study investigated SOC accumulation, partitioning, and molecular composition along a toposequence (300 to 1100 m above sea level) within the Gondwanan subtropical rainforest of Australia. We assessed how climatic, mineralogical, and pedogenic factors regulate the distribution of particulate (POM) and mineral-associated organic matter (MAOM) and influence SOC functional composition. It was found that SOC concentrations increased by 77% with elevation, predominantly accumulating as MAOM which increased from 26.5 to 73.2 g C kg −1 soil, reaching 81% of the theoretical mineral saturation capacity. This increase in OC in the stable MAOM fraction was associated with an increase in 1:1 clays and highly reactive, weathered soil minerals at higher elevation (Fe- and Al-oxyhydroxides), which increased from 18% of the total mineral content at 300 m to 87% at 1100 m. Despite there being a 77% increase in SOC with elevation, SOC functional group composition remained consistent. These findings indicate that SOC persistence along the toposequence is driven primarily by soil mineralogy and physicochemical protection rather than selective preservation or molecular recalcitrance.