Soil Carbon Stocks and Soil Carbon Quality in the Upland Portion of a Boreal Landscape, James Bay, Quebec

As part of a multidisciplinary project on carbon (C) dynamics of the ecosystems characterizing the Eastmain Region Watershed (James Bay, Quebec), the objective of this study is to compare the soil C stocks and soil organic matter quality among the main upland vegetation types in a boreal region subjected to a high fire frequency. On average, the organic layer contained twice the amount of C than the mineral soil. Closed canopy vegetation types had greater C stocks both in the mineral and in the organic layers than the other more open canopy vegetation types. Landscape features such as drainage and surficial deposit could not discriminate between vegetation types although closed vegetation types were on average found on wetter site conditions. Average soil C contents varied more than 2-fold across vegetation types. On the opposite, except for the organic layer C:N ratio, which was smaller in closed vegetation types, other measured soil organic matter properties (namely specific rate of evolved C after a long-term incubation, hydrolysis acid-resistant C as well as the rate of changes in soil heterotrophic respiration with increasing temperature (Q10)) remained within a narrow range between vegetation types. Therefore, total soil C stocks were a major determinant of both labile C and estimated summer soil heterotrophic respiration rate. The homogeneity of soil organic matter quality across the vegetation types could be attributable to the positive relationship between soil C storage and soil C fluxes observed in this landscape experiencing a high fire frequency. The low variability in soil C quality could help simplify the modelling of soil C fluxes in this environment.