Consumption of residual pyrogenic carbon by wildfire

Pyrogenic carbon (PyC) produced during vegetation fires represents one of the most degradation resistant organic carbon pools and has important implications for the global carbon cycle. Its long-term fate in the environment and the processes leading to its degradation are the subject of much debate. Its consumption in subsequent fires is considered a potential major mechanism of abiotic PyC degradation; however, no quantitative data supporting this removal pathway have been published to date. To address this gap, we quantified consumption of residual PyC at the forest floor during an experimental fire, representative of a typical boreal wildfire, complemented by exploratory laboratory heating experiments. Labelled PyC (pinewood charcoal from a slash pile burn), in granular form contained in stainless steel mesh bags and as individual pieces, were placed at 2-cm depth within the forest floor. The median mass loss of granular charcoal was 6.6%, with 75% of the samples losing <15%, and of individual pieces 15.1% with 75% of the samples losing <25%. The mass losses under laboratory conditions, although somewhat higher than in the field, confirm an overall low consumption of PyC. The limited losses of PyC found here do not support the widely held notion that wildfire is a major cause of loss for residual PyC.