Contrasting Stabilization Pathways of Soil Organic Matter Emerge from Clay Inheritance and Calcium Interactions in Black and Red Karst Topsoils of the Yucatán Peninsula

Abstract

The Yucatán Peninsula, a tropical karst region, harbors soils with contrasting capacities to store soil organic carbon. The previous evidence suggests this is primarily determined by environmental conditions, but pedogenetic histories are still not fully understood. We investigate the composition of soil organic matter (SOM) in previously deeply studied Rendzic Leptosols (black topsoil) and Chromic Luvisols (red topsoil) using an integrated approach of micromorphology, physical fractionation, isotopic analysis (δ13C, 14C), differential scanning calorimetry (DSC), and ATR-FTIR spectroscopy to infer the main SOM stabilization mechanisms. We found that black topsoil, despite thinner profiles, exhibited more than twice the soil organic carbon (SOC) content (137 g C kg−1) of red topsoil (62.1 g C kg−1). This disparity is attributed to high biological activity and pedogenetic features, including a granular structure and a mineral assemblage (mica-illite, carbonates) that, under neutral pH and high Ca2+ levels, promote strong organo-mineral associations. Spectroscopic and thermal analyses revealed that black topsoil SOM is rich in nitrogenous microbial residues and stabilized aromatic compounds within fine mineral-associated fractions. In contrast, the red topsoil displayed a compact structure, an acidic environment, and a vermiculite clay mineralogy that was less conducive to organo-mineral complexation. Its SOM showed strong signatures of oxidized residues and pyrogenic carbon, indicating a legacy of fire and limited long-term mineral protection. The results underscore that SOC persistence in these karst systems is governed not only by intrinsic recalcitrance but also by the interplay of pedogenetic history, mineralogical context, and organo-mineral interaction pathways. These findings improve the understanding of SOM vulnerability to anthropogenic perturbation and environmental change.