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Depth Distribution and Predictors of Soil Organic Carbon in Podzols of a Forested Watershed in Southwestern Canada

Forest soils of coastal British Columbia, Canada, may store significant amounts of organic matter because of the cool climate and high forest productivity of the area. The objectives of this study were to determine the distribution of soil organic carbon (SOC) in the profile and to identify the most important predictors of SOC in Podzols of a forested watershed in southwestern British Columbia. We sampled 9 soil profiles in undisturbed forest plots by morphological horizon and measured SOC using a dry combustion method. We also determined soil pH, texture, moisture content, total nitrogen, loss on ignition, and pyrophosphate- and oxalate-extractable Fe and Al. The average soil profile stored 15.9 kg C/m2 over a depth of 100 cm, which is higher than SOC stocks estimates for inland Canadian forests. The organic layer (LFH) only accounted for one fourth of the C stock. Sixty percent of the profile SOC (including the forest floor) was found in the subsoil of depth greater than 20 cm. Studies of SOC dynamics that only sample the topsoil are therefore inappropriate. Although the clay concentration was low (∼5%), the clay fraction accounted for one third of SOC. This suggests that organo-mineral interactions were an important factor for SOC storage. The major predictors of SOC in the mineral horizons were organically complexed Al and Fe and short-range order inorganic material. Crystalline clays also seemed to play a role in organic matter accumulation, but were not as important as poorly crystalline compounds. In the organic layer, organically complexed Fe forms correlated negatively with SOC, indicating that the amount of Fe available for adsorption to organic matter is limited. Organically complexed Al did not show the same negative association, suggesting the existence of a mechanism for upward translocation of Al into the FH horizon.