The expansion of agricultural activities into previously undisturbed ecosystems has significant implications for carbon storage and climate regulation. This study investigates the effects of agro-ecological expansion on carbon stock in the southeastern part of Osun State, Nigeria, a region characterized by a mosaic of forest patches, farmlands, and secondary vegetation. The primary objective was to quantify changes in aboveground and belowground carbon stocks associated with land-use conversion and to assess the potential implications for regional carbon dynamics.

A combination of field sampling and remote sensing techniques was employed. Forest and farmland plots were systematically selected across three local government areas, and aboveground biomass was estimated using standard allometric equations. Soil samples were collected at 0–30 cm depth to determine soil organic carbon content. Historical land-use changes over a 20-year period were analyzed using high-resolution satellite imagery to quantify the spatial extent of agro-ecological expansion.

Results indicate that areas converted from forest to agricultural land exhibited a significant decline in carbon stock, with aboveground biomass decreasing by an average of 42% and soil organic carbon declining by approximately 18%. Secondary vegetation and fallow lands retained moderate carbon levels, highlighting their potential role in carbon sequestration. Spatial analysis revealed that intensive cultivation and expansion into marginal forest areas were the primary drivers of carbon loss. The study also found that agroforestry practices within farmlands contributed to partial carbon retention, emphasizing the importance of sustainable land management strategies.

In conclusion, agro-ecological expansion in southeastern Osun State has led to substantial reductions in both biomass and soil carbon stocks, posing risks to local carbon balance and climate mitigation efforts. Promoting sustainable agricultural practices, including agroforestry and conservation agriculture, is crucial for mitigating carbon loss while supporting rural livelihoods. These findings provide valuable insights for policymakers and land managers aiming to balance agricultural development with environmental sustainability.