Geological and Pedoclimatic Controls on Soil Evolution and Degradation in Humid Tropical Gabon: Insights from Ferralitic and Coastal Sedimentary Terrains

Tropical soils are critical components of the Earth system, sustaining ecosystem functions, food production, and biogeochemical regulation. In humid equatorial regions, their fertility and resilience are strongly shaped by lithological inheritance and climate-driven weathering. This study investigates how parent material and pedoclimatic factors influence soil evolution, fertility, and degradation dynamics in two contrasting systems in Gabon: ferralitic soils over ancient granito-gneissic basement at Koula-Moutou and coastal sedimentary sands at Kango. Physicochemical analyses revealed that both systems are inherently nutrient-poor but exhibit divergent characteristics. Ferralitic soils, shaped by advanced weathering, were highly acidic (pH < 4.5), with low cation exchange capacity, strong phosphorus fixation, and severe base cation depletion. Coastal sands, derived from younger fluviomarine deposits, had higher porosity, moderate organic matter, and improved nutrient retention, despite structural fragility. Indices of degradation (SDR), vulnerability (Vp), and the Cumulative Rating Index (CRI) confirmed these contrasts. Ferralitic soils showed higher degradation but lower vulnerability due to persistent weathering. Coastal sands, while less degraded, were more sensitive to nutrient loss. These findings support the hypothesis that lithology governs fertility dynamics and degradation susceptibility in tropical soils. This work provides new insights into geologically conditioned soil functioning in tropical landscapes. It underscores the need for site-specific soil management strategies such as pH correction, organic matter inputs, and conservation practices that align with substrate properties. Integrating geoscientific and pedological knowledge is essential to restore degraded soils, sustain agroecosystem services, and inform land governance in the Congo Basin and similar tropical regions.