Black carbon in Quebec boreal black spruce forests

Wildfires affect boreal forest carbon stocks through consumption of the vegetation and forest floor, and production of black carbon (BC). This by-product of organic matter’s incomplete combustion is an important component of the global soil carbon pool. My thesis reveals how fire severity is related to BC chemical and physical properties and how it influences both soil organic carbon (SOC) and BC stocks in forest floors and mineral soils of fire-affected Quebec black spruce forests. This work also attempts to uncover BC storage mechanisms in mineral horizons. I collected BC samples produced by 2005–2007 wildfires, and compared them to laboratory-produced samples. As indicated by 13C NMR spectroscopy, elemental analysis, surface area analysis, and scanning electron microscopy, formation conditions – mostly maximum temperature – greatly affected BC properties, which in turn may determine its potential as a carbon sink. BC condensation increased with increasing fire severity, as shown by decreasing atomic H/C and O/C ratios. The fraction of aromatic carbon:total carbon of all these freshly-produced BC was low, suggesting that they may be susceptible to rapid physical alteration and chemical degradation. However, these BC were characteristic of early-season fires, which resulted in low temperatures (≤ 250 °C) and overall, low fire severity, in forest floors. Mineral horizons contained SOC stocks comparable to those in forest floors, but their BC stocks were significantly lower. In the fire sites I studied, forest floor’s BC stocks were mostly influenced by past severe fires, with the deepest layer containing most of the BC stocks. In mineral soils, SOC and BC concentrations were strongly correlated. To further explore the relationship between podzolization and BC storage mechanisms, I measured SOC and BC content in size and density fractions of podzolic B horizons. While some BC 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. Podzolization processes result in idiosyncratic patterns of SOC accumulation in the mineral subsoil; here I show that patterns of BC and SOC accumulation are similar, with the greater BC stocks being found in podzolic B horizons.

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