Fire as a Removal Mechanism of Pyrogenic Carbon From the Environment: Effects of Fire and Pyrogenic Carbon Characteristics

Abstract

Pyrogenic carbon (PyC, charcoal) is produced during vegetation fires at a rate of ∼116-385 Tg C yr −1 globally. It represents one of the most degradation-resistant organic carbon pools, but its long-term fate and the processes leading to its degradation remain subject of debate. A frequently highlighted potential loss mechanism of PyC is its consumption in subsequent fires. However, only three studies to date have tested this hypothesis with reported losses of <8-37%, with the effects of PyC chemical characteristics and fire conditions on PyC loss in wildfires remaining unexplored. To address this, we placed materials with different degrees of thermal and chemical recalcitrance (A: wildfire charcoal, B: slash-pile charcoal, C: pine wood and D: cedar wood) on the ground surface just prior to a high-intensity and a low-intensity boreal forest wildfire. Mass losses were highly variable and dependent on fire-and sample characteristics. Mass losses across both fires (as % of dry weight) were for A: 66.5 ± 25.2, B: 41.7 ± 27.2, C: 78.2 ± 14.9, and D: 83.8 ± 18.9. Mass loss correlated significantly with maximum temperature (T max ) recorded on sample surfaces (r = 0.65, p = 0.01), but only weakly (r = 0.33) with time >300 • C. Mass losses also showed a significant negative correlation (r = −0.38, p = 0.05) with thermal recalcitrance (T50) determined using Differential Scanning Calorimetry (DSC) and T max with charcoal reflectance (Ro) determined after the fires (r = 0.46, p = 0.05). Losses in the high-intensity fire were significantly higher (p = 0.05) than in the low-intensity fire, but the latter had a higher rate of conversion of fuel to PyC. Our results demonstrate that exposure to fire can indeed be a significant removal mechanism for PyC that remains exposed on the ground after a previous fire. The losses found, however, are likely to represent an extreme upper range as most PyC produced in a fire would not remain exposed on the ground surface by the time the next fire occurs. Our data also demonstrate, for real wildfire conditions, the (i) contrasting resistance of different PyC types to combustion and (ii) contrasting net PyC losses between different fire intensities. The DSC and reflectance (Ro) results support the usefulness of these analyses in reflecting thermal degradation resistance and temperature exposure under actual wildfire conditions.

What Can Charcoal Reflectance Tell Us About Energy Release in Wildfires and the Properties of Pyrogenic Carbon?

“…In terms of charcoal reactivity/recalcitrance, our laboratory experiments suggest that the Pine Point fire would have produced charcoal that remained more reactive (due to the lower Ro%), while those from the Triangle fire are likely to be more recalcitrant owing to their higher Ro%. This is in agreement with the results of Doerr et al (2018), where Ro% has been proven to be positively related to several indicators of chemical recalcitrance and the properties of these same fires. For example, the temperature at which 50% of the total energy was released showed positive correlations with Ro% (r 2 = 0.77) where the percentage of carbon in the charred remains was also found to increase with Ro% (Doerr et al, 2018).…”

“…This is in agreement with the results of Doerr et al (2018), where Ro% has been proven to be positively related to several indicators of chemical recalcitrance and the properties of these same fires. For example, the temperature at which 50% of the total energy was released showed positive correlations with Ro% (r 2 = 0.77) where the percentage of carbon in the charred remains was also found to increase with Ro% (Doerr et al, 2018). Doerr et al (2018) also report that previously generated charcoals exposed to a second fire (the two fires in this study) increased their thermal recalcitrance particularly in the high-intensity fire at the Triangle site.…”

“…For example, the temperature at which 50% of the total energy was released showed positive correlations with Ro% (r 2 = 0.77) where the percentage of carbon in the charred remains was also found to increase with Ro% (Doerr et al, 2018). Doerr et al (2018) also report that previously generated charcoals exposed to a second fire (the two fires in this study) increased their thermal recalcitrance particularly in the high-intensity fire at the Triangle site. Therefore, charcoals with higher reflectances will not only be more chemically resistance to post-fire degradative processes but can become increasingly resistant if subject to subsequent high intensity fires.…”

“…Due to the patchy nature of the low-intensity fire in Pine Point, not all samples were exposed to burning and, in some cases, the combustion of the organic soil led to the destruction and associated data loss of some of the loggers. For more details on the characteristics of the fires, sites, and experimental settings, see Doerr et al (2018). Following the fires, the integrated area under each thermocouple curve was calculated for the duration each sample spent at >300 • C. This served as a guide to the total heating experienced by the sample throughout the duration of the fire.…”

“…These properties should also determine how different charcoals are then subsequently modified chemically and physically in the post fire environment (Harvey et al, 2012;Santín et al, 2017). However, because fire is a complex chemical reaction that also relates to the physical state of the fuel it has been difficult to disentangle how the nature of fire might impact the production of different charcoals and therefore to what extent individual fire and fuel characteristics might lead to charcoals with varying levels of resistance to biological and physical degradation or to combustion in subsequent fires (e.g., Doerr et al, 2018).…”