Developing a Soil Carbon Stock Inventory Methodology for Forest-Agriculture Transitions in Cross River State, South South Nigeria

This study develops a standardised soil carbon stock inventory methodology for quantifying soil organic carbon changes at forest-to-agriculture transition boundaries in Cross River State, located in Nigeria's South South geopolitical zone. Cross River State hosts Nigeria's largest remaining lowland tropical forest, but deforestation at forest margins for smallholder agriculture and oil palm expansion is ongoing. Accurate soil carbon stock measurement at transition zones is required for REDD+ reporting, carbon credit certification, and land use policy formulation. Using a professional methodology development approach, four forest-agriculture transition sites were selected in Obudu, Boki, and Afi River Forest Reserve margins. At each site, soil profiles to 100 cm Soil Scienceh were sampled at five positions from closed-canopy forest to five-year-old farmland. Bulk density was measured by core method at each Soil Scienceh increment, and organic carbon was determined by the modified Walkley-Black method. Stock calculations followed IPCC Tier 2 methodology. Three repeated sampling events over 18 months assessed within-site temporal variability. The methodology was compared against reference estimates from published allometric equations and soil carbon modelling (RothC model). Findings revealed that deforestation reduced soil carbon stocks by an average of 48 tonnes C per hectare in the top 100 cm over five years, with the greatest losses in the top 30 cm. The methodology showed high reproducibility (coefficient of variation below 12%). The study delivers a practical soil carbon inventory protocol for Cross River State REDD+ implementation and forest conservation finance applications. Keywords: soil carbon inventory, REDD+, forest-agriculture transition, Cross River State, carbon stock.