Contrasting Fe and Al controls on mineral-associated organic carbon across a tropical wetland toposequence in palm swamps (Vereda)

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Wetlands are major global natural carbon reservoirs, yet the mechanisms controlling organic carbon persistence in tropical systems remain poorly constrained. In the Brazilian Cerrado, Veredas (groundwater-fed palm swamp wetlands) are characterized by strong hydrological gradients, dynamic redox conditions, and highly weathered soils. Despite their ecological importance, the mechanisms controlling organic carbon stabilization in these systems remain poorly understood, particularly regarding the relative roles of Fe and Al under persistent saturation. Here, we examined relationships between organic carbon and reactive Fe and Al phases along a border–intermediate–bottom toposequence using sequential selective dissolution and regression analyses. Soil moisture increased and pH decreased downslope, accompanied by a sharp decline in total Fe and substantial organic carbon accumulation in permanently saturated bottom soils. Across all positions, organic-complex (pyrophosphate-extractable) fractions dominated the extracted pools of Fe, Al, and carbon. However, contrasting behaviors emerged between metals. Iron-associated carbon decreased along the gradient and showed no consistent relationship with organic carbon, reflecting redox-driven dissolution and mobilization. In contrast, aluminum remained consistently associated with organic carbon across all positions, with significant relationships observed in the organic-complex fraction (β = 0.75, p = 0.003) and the low-crystallinity fraction (β = 0.59, p = 0.011). These results indicate a shift in dominant stabilization mechanisms along the hydrological gradient. While Fe may contribute to carbon retention under oxic conditions, its role diminishes under prolonged saturation due to redox sensitivity. In contrast, Al-mediated associations persist under reducing conditions and represent a more stable pathway for organic carbon stabilization in deeper hydromorphic environments. Overall, our findings highlight aluminum as a key, yet often overlooked, regulator of carbon persistence in highly weathered tropical wetlands.

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