Concentrations of soil C pools at the Calhoun Critical Zone Observatory. Soil organic C (SOC, a) and extractable organic C (EOC, b), and the ratio of EOC/SOC (c), at multiple depths in three different land uses. Each background point represents one soil sample from that depth and land use, and bold-overlaid points represent values averaged for 0–25, 25–50, 50–100, 100–300, and 300–500 cm depth intervals in each land use. Error bars represent one standard error of the mean (note that the 300–500 cm depth interval is comprised of data obtained between 400 and 500 cm). Depth was a significant driver of both [SOC] and [EOC]. There was a near-significant trend of forest type influencing [EOC] (P = 0.11). An interaction between depth and forest type influenced EOC/SOC (P = 0.001); at the deepest depth interval, old-growth forest values were greater than regenerating pine values (P = 0.006). Agricultural soil EOC/SOC values were significantly lower than soils in either forest type. See text for results details. DOI: https://doi.org/10.1525/elementa.287.f5