Pedogenic Carbon and Calcium Carbonate Accumulation in Semi-Arid Soils of North East Nigeria: Mechanisms, Stocks, and Climate Change Implications

This dissertation provides an original investigation of pedogenic carbon accumulation in semi-arid soils of North East Nigeria, with particular focus on secondary calcium carbonate (CaCO3) formation and its role in long-term carbon stabilisation, addressing a fundamental gap in the understanding of inorganic carbon dynamics in Nigeria's dryland soils. Pedogenic carbonate represents a significant but largely ignored pool of terrestrial carbon in semi-arid environments, and its sensitivity to climate change and land management is poorly constrained in sub-Saharan African contexts. This study samples 45 soil profiles to 150 cm Soil Scienceh across Borno, Yobe, and Gombe States, characterising calcium carbonate morphology, stable isotope composition (delta 13C and delta 18O), and radiocarbon age by AMS dating. Pedogenic carbonate stocks are estimated by gravimetric and volumetric methods. The relationship between present and past vegetation (C3 versus C4 plant dominance as recorded by delta 13C of carbonate) and carbonate formation rates is reconstructed. A sensitivity analysis using the CALDIS model projects pedogenic carbonate stability under RCP 4.5 and RCP 8.5 warming and drying scenarios. The dissertation proposes an original Dryland Soil Inorganic Carbon Framework (DSICF) that explicitly incorporates pedogenic carbonate and hydromorphic calcrete formation into terrestrial carbon budgets for the Nigerian dryland zone. Original contributions include the 45-profile pedogenic carbonate characterisation dataset, AMS radiocarbon chronology, stable isotope palaeoclimate reconstruction for North East Nigerian soils, and the DSICF modelling framework. Keywords: pedogenic carbonate, inorganic carbon, semi-arid soils, North East Nigeria, climate change.