Pyrogenic carbon stocks and storage mechanisms in podzolic soils of fire-affected Quebec black spruce forests

Wildfire, a recurrent disturbance in the boreal, converts part of the forest floor into pyrogenic carbon (PyC). The latter is an important component of the global soil carbon pool, yet knowledge of its stocks and storage mechanisms in these boreal ecosystems is scarce. Podzolization processes, which are frequent under boreal vegetation, result in distinctive patterns of soil organic carbon (SOC) accumulation in the mineral subsoil; how this may affect PyC storage remains largely unknown. The objectives of this study were to estimate SOC and PyC stocks in podzolic soils from fire-affected black spruce forests, and to explore the storage mechanisms taking place in their mineral horizons. We also compared PyC stocks in mineral soils to forest floor stocks. Samples were collected from 23 soil profiles under black spruce forests located throughout the province of Quebec. To further explore the relationship between podzolization and PyC storage mechanisms, we measured SOC and PyC contents in size and density fractions of a subset of 11 podzolic B horizons. Total SOC stocks in the mineral horizons and forest floors were comparable. Pyrogenic carbon stocks in the mineral soils, estimated by a H2O2/dilute HNO3 digestion, averaged 0.2(±0.1)kgCm−2. This was significantly lower than forest floor stocks, which ranged from 0.2 to 1.2kgCm−2. Consequently, PyC constituted a smaller fraction of total SOC (2–15%) in mineral soils than in forest floors, where it was as high as 68% (±5) in some horizons. In the mineral soils, SOC and PyC concentrations were strongly correlated. While some PyC was found in unprotected particulate organic matter (POM), the rest was associated with organo-mineral and organo-metallic complexes in the micro-aggregate protected POM and fine fraction. Patterns of PyC accumulation in mineral soils were similar to SOC, and the greater PyC stocks were found in podzolic B horizons.