Pyrogenic organic matter production from wildfires: a missing sink in the global carbon cycle

Abstract

Wildfires release substantial quantities of carbon (C) into the atmosphere but they also convert part of the burnt biomass into pyrogenic organic matter (PyOM). This is richer in C and, overall, more resistant to environmental degradation than the original biomass, and, therefore, PyOM production is an efficient mechanism for C sequestration. The magnitude of this C sink, however, remains poorly quantified, and current production estimates, which suggest that ∽1-5% of the C affected by fire is converted to PyOM, are based on incomplete inventories. Here, we quantify, for the first time, the complete range of PyOM components found in-situ immediately after a typical boreal forest fire. We utilized an experimental high-intensity crown fire in a jack pine forest (Pinus banksiana) and carried out a detailed pre- and postfire inventory and quantification of all fuel components, and the PyOM (i.e., all visually charred, blackened materials) produced in each of them. Our results show that, overall, 27.6% of the C affected by fire was retained in PyOM (4.8 ± 0.8 t C ha−1), rather than emitted to the atmosphere (12.6 ± 4.5 t C ha−1). The conversion rates varied substantially between fuel components. For down wood and bark, over half of the C affected was converted to PyOM, whereas for forest floor it was only one quarter, and less than a tenth for needles. If the overall conversion rate found here were applicable to boreal wildfire in general, it would translate into a PyOM production of ∽100 Tg C yr−1 by wildfire in the global boreal regions, more than five times the amount estimated previously. Our findings suggest that PyOM production from boreal wildfires, and potentially also from other fire-prone ecosystems, may have been underestimated and that its quantitative importance as a C sink warrants its inclusion in the global C budget estimates.

Source Material and Concentration of Wildfire-Produced Pyrogenic Carbon Influence Post-Fire Soil Nutrient Dynamics

“…Together, the temperature ranges reported from these experimental fires suggest that the mean temperature at the floor forest during fire in conifer forests may range between 300 and 750 °C and that the temperature in the canopy may reach 650 °C. Although the study conducted by Santín, et al [12] also focused on jack pine forest, their study was conducted in the boreal region and their study site supported a deep moss layer and saturated mineral soil. Jack pine forests in our study site are more ecologically similar to the pitch pine forests in the New Jersey, USA pine barrens because both forest types have thin forest floors and are found on dry sandy soils [37].…”

“…Wildfires can have a greater range in temperature than prescribed fires. For example, Hartford and Frandsen [35] found that the litter surface, duff, and soil surface reached 300, 515, and 400 °C, respectively, in a prescribed ground fire in mixed-conifer forests (Larix occidentalis and P. contorta) in western Montana, whereas Santín, et al [12] found that the mean temperature during an experimental stand-replacing crown fire in a jack pine forest in Canada was <70 °C at the mineral soil surface and 750 °C at the surface of the forest floor. Furthermore, Schneider, et al [36] in a study of experimental fire in pitch pine (P. rigida Mill) forest in New Jersey, found that temperatures in the canopy can reach 510-650 °C during a crown fire whereas temperatures in the canopy during surface fire range between 0 and 260 °C.…”

“…High concentrations of BK added to soils caused a significant decrease in soil respiration. In a study of PyC production in a stand-replacing fire boreal jack pine forest in Canada, Santín, et al [12] found that one third (1.4 t ha ) of PyC produced is in the form of BK. Santín, et al [12] reported similar % C for charred bark (62.9%) and similar percent increase in C due to charring (15.6%) compared to the 15.0% change we observed.…”

“…In a study of PyC production in a stand-replacing fire boreal jack pine forest in Canada, Santín, et al [12] found that one third (1.4 t ha ) of PyC produced is in the form of BK. Santín, et al [12] reported similar % C for charred bark (62.9%) and similar percent increase in C due to charring (15.6%) compared to the 15.0% change we observed. BK represents a large pool of PyC in jack pine forests, and may be added to soils at high concentration after a fire.…”

“…However, PyC can increase nitrogen (N) mineralization rates [10] and interact physically with organic compounds that may affect N cycling [11]. PyC is produced from various source materials and at differing temperatures within individual wildfires [12,13], yet the current scientific understanding of how variation in PyC influences post-fire C and N ecosystem dynamics is limited.…”