Pyrogenic C as a Component of the C Cycle in Boreal Forests.

Despite high interest in black carbon (BC) as a stable component of soil C, it has hardly been integrated into C cycle models, especially for forests. We refer to the whole range of fire-transformed biomass and soil organic matter as pyrogenic C (PyC), while the fraction that is resistant to laboratory oxidation is defined as BC. Fire is the major disturbance in boreal forests, with global production estimated at 7-17 Tg BC/y as solid residues (around 1-3% of original mass consumed by fire) and 2-2.5 Tg BC/y as aerosols. However, there is insufficient information on boreal production, stocks, or loss rates, exacerbated by differences in methods used to measure various forms of PyC. Incorporation of efficiently-produced “biochar” into agricultural soil is being considered to enhance both fertility and C sequestration. However, several factors may limit the sequestration potential of PyC in boreal forests. Compared to production charcoal, wildfire-produced PyC is generally less highly transformed, but higher in N, other nutrients, oxygen-bearing functional groups, and porosity, all of which make it less resistant to physical, chemical and microbial degradation. It is produced, and largely remains on or above the surface. Charred snags may remain standing or suspended for many years. While this prevents soil contact, the PyC is still vulnerable to physical and chemical processes, including possible effects of photochemical oxidation followed by leaching of water-soluble fragments. It is also vulnerable to oxidation by subsequent fires. We are exploring requirements, approaches and available data for integration of PyC into the Carbon Budget Model of the Canadian Forest Sector 3 (CBM-CFS3). Using this complete ecosystem carbon dynamics model and available data, we investigate the relative contribution of loss rates and oxidation in subsequent fires to the dynamics of BC in boreal forest ecosystem.