Earthworm casts are recognized as microsites that influence soil organic carbon (SOC) dynamics, yet their stabilization mechanisms in highly weathered tropical soils remain unclear. We investigated organo–mineral interactions in casts from anecic Rhinodrilidae earthworms in a Hapludox soil under a long–term soybean–forage rotation. Casts contained significantly higher total SOC and particulate organic matter (POM) than surrounding soil, while mineral–associated organic matter (MAOM) contents were similar. DRIFT–FTIR analysis revealed MAOM in casts was enriched in nitrogen–rich, hydrophilic functional groups, particularly carboxylates and amide/carbonyl. However, sodium pyrophosphate (NaPPi) and ammonium–oxalate extractions showed reduced pools of Fe and Al short–range–order phases in casts, indicating lower mineral reactivity to MAOM. In contrast, casts exhibited higher Mg–NaPPi levels, suggesting a shift toward Mg–mediated cation bridging. These results indicate a short–term trade–off between organic matter quality and mineral reactivity: although casts act as hotspots for POM accumulation and produce organic matter enriched in mineral–associative functional groups, the depletion of Fe and Al short–range–order phases reduces mineral reactivity, potentially limiting organo–mineral stabilization.