Soil organic carbon stocks in a forest and an adjacent cultivated land in southern Ontario

Accurate quantification of soil organic carbon (SOC) stocks is essential for evaluating land-use impacts, informing soil health assessments, and supporting carbon accounting frameworks. Comparisons between forest soils and adjacent cultivated lands provide insight into the magnitude and depth distribution of SOC losses following agricultural conversion. In this study, soil samples were collected to a depth of 1 m from a cultivated field and an adjacent forest at the Elora Research Station (Guelph, Ontario) to quantify SOC stocks and assess topsoil and subsoil contributions. Soil organic carbon was determined using a muffle furnace method, and SOC stocks were calculated using an equivalent soil mass approach to account for land-use-related differences in bulk density. The effects of land use and soil depth on SOC were evaluated statistically. Soil organic carbon stocks were 180.6 Mg C ha⁻¹ in the cultivated site and 219.6 Mg C ha⁻¹ in the forest at an equivalent soil mass of 13,000 Mg ha⁻¹, representing an approximately 18% reduction following conversion from forest to agriculture. The majority of SOC loss (93%) occurred within the upper 30 cm, while subsoil layers accounted for a substantial proportion of total SOC stocks (52% in cultivated soils and 44% in forest soils). These results indicate that surface SOC is highly sensitive to land-use change, whereas subsoil SOC constitutes a significant and comparatively stable component of total SOC stocks. Overall, this study highlights the importance of depth-resolved SOC assessment and standardized stock calculations for improving land-use comparisons and strengthening soil carbon accounting.