Early‐season fires in boreal black spruce forests produce pyrogenic carbon with low intrinsic recalcitrance

Abstract

Early-season fires in boreal black spruce forests produce pyrogenic carbon with low intrinsic recalcitrance.Ecology, 96 (6) Abstract. Pyrogenic carbon (PyC), a major by-product of wildfires in boreal forests, plays several critical roles in soil biogeochemical processes. However, PyC properties, including its potential recalcitrance, may vary depending on its formation conditions. Our study aimed to characterize the chemical and physical properties of PyC formed under variable fire severity in Eastern Canada boreal forests; these latter represent an important fraction of fire-affected circumboreal ecosystems. A total of 267 PyC samples, produced by early-season wildfires in 2005-2007, were collected 5 years after fire from the forest floors of 14 black spruce sites distributed across Quebec, to cover the range of fire severity encountered in these forests. Early-season fires occur frequently in Eastern Canada, and are predicted to increase in regional and global scenarios of future fire regimes associated with climate change. Selected PyC samples were analyzed using elemental analysis, solid-state 13 C nuclear magnetic resonance (NMR) spectroscopy, scanning electron microscopy, and surface area analysis. The NMR spectra of the PyC collected on low-fire-severity sites were dominated by peaks indicative of cellulose, while those for PyC from higher-severity sites were dominated by a broad peak assigned to aromatic carbons. Atomic H/C and O/C ratios decreased with increasing fire severity. By comparing field samples to samples produced in the laboratory under controlled formation conditions, we were able to infer that the temperature of formation in the field was low (758-2508C). In addition, for all PyC samples, the aromatic carbon : total carbon ratio was small, suggesting that PyC produced by early-season fires in these boreal forests may be susceptible to relatively rapid degradation. Taken together, our data suggest that boreal PyC may not be as recalcitrant as previously assumed, and that its influence on soil biogeochemical processes may be short lived.

Charcoal in Organic Horizon and Surface Mineral Soil in a Boreal Forest Fire Chronosequence of Western Quebec: Stocks, Depth Distribution, Chemical Properties and a Synthesis of Related Studies

“…These structural variations are consistent with observations of increasing aromaticity with temperature in laboratory charring studies (Baldock and Smernik, 2002;Czimczik et al, 2002;Ascough et al, 2008;Soucémarianadin et al, 2013;Hart and Luckai, 2014). Organic horizon/biomass char produced in field settings (wildfire or prescribed fire) also shows a range of intensity distributions depending on fire severity, from little alteration to high aromaticity (Nocentini et al, 2010;Miesel et al, 2015;Soucémarianadin et al, 2015b). Similar to laboratory charring, samples referenced to actual temperatures documented in the field show increasing aromaticity with temperature (Alexis et al, 2010;Santín et al, 2016b), and illustrate the variability in formation conditions.…”

Modification of the weak nitric acid digestion method for the quantification of black carbon in organic matrices

“…This observation is consistent with the widely accepted paradigm on the chemistry of pyrolyzed materials, suggesting that the aromatic fraction of pyrolyzed material-C increases with formation temperature (Keiluweit et al, 2010). This interpretation is consistent with observations realized using various techniques, including molecular markers (Schneider et al, 2010;Wiedemeier et al, 2015), Fourier-Transform infrared (Keiluweit et al, 2010) and 13 C nuclear magnetic resonance spectroscopy (McBeath et al, 2011;Soucémarianadin et al, 2013Soucémarianadin et al, , 2015Wiedemeier et al, 2015). Also, the pyrolysis of wood has been reported to deliver higher concentrations of aromatic molecules than non-woody materials such as grass (Hammes et al, 2006) or leaves (Singh et al, 2012).…”

Post-wildfire Erosion in Mountainous Terrain Leads to Rapid and Major Redistribution of Soil Organic Carbon

“…Generally, PyC is considered to have a longer mean residence time than non-pyrogenic soil C, typically on the multi-centennial time scale (Hammes et al, 2008;Bird et al, 2015). However, in the past two decades, there has been growing evidence for PyC decomposing on shorter time scales, on the order of days to years (Cheng et al, 2006;Nguyen et al, 2009;Soucémarianadin et al, 2015). Furthermore, Bird et al (2015) suggested a multi-pool model of PyC decomposition, where physical or chemical components of PyC are considerably more susceptible to decomposition than others.…”