Soil Carbon Sequestration Mechanisms and Stabilisation Pathways in Tropical Savanna Soils of the Nigerian Middle Belt: An Original Molecular Framework

This dissertation develops an original molecular framework for understanding the mechanisms and pathways of soil organic carbon stabilisation in tropical savanna soils of the Nigerian Middle Belt, encompassing the North Central geopolitical zone states of Benue, Kogi, Kwara, Nasarawa, Niger, and Plateau. Current understanding of soil organic carbon stabilisation is predominantly derived from temperate soils and is inadequate for predicting carbon dynamics in tropical savanna soils with their distinctive mineralogy, high biological activity, and seasonal dryness-rewetting cycles. This study employs a combination of density fractionation, sequential chemical extraction, solid-state 13C NMR spectroscopy, mid-infrared spectroscopy, X-ray diffraction, and scanning electron microscopy with energy dispersive X-ray spectroscopy to characterise the composition, mineral associations, and stabilisation mechanisms of organic carbon fractions across a gradient of management intensities on the Jos Plateau and Benue Valley. Long-term (greater than 20-year) incubation kinetics of radiocarbon-dated organic fractions provide stability data. Three soil types representing dominant parent materials are compared. The dissertation proposes the Savanna Carbon Stabilisation Pathway Model as an original theoretical contribution, identifying ferrihydrite-organic matter coprecipitation, microaggregate occlusion, and fungal necromass-mineral bonding as the three primary stabilisation mechanisms in the study soils, and demonstrating their differential sensitivity to management disturbance. Original contributions include the molecular framework, a mineral-organic association characterisation methodology applicable to highly weathered tropical soils, and management guidelines derived from mechanistic understanding. Keywords: soil carbon stabilisation, molecular fractionation, Middle Belt savanna, organic matter mechanisms, 13C NMR.