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

Stefan H Doerr,Claire M Belcher,Greg Baxter,Cristina Santín,Agustín Merino

doi:10.3389/feart.2018.00127

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 the consumption of PyC in subsequent fires, however, only three studies to date have tested this hypothesis with reported losses of 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 Tmax 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 fires. 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 results support the usefulness of these analyses in reflecting thermal degradation resistance and temperature exposure under actual

Highlights

Pyrogenic carbon (PyC, known as charcoal, pyrogenic organic matter or black carbon) is a ubiquitous organic residue resulting from the incomplete combustion of organic fuels during vegetation fires
Two types of PyC and two types of uncharred wood materials were used in the fire exposure experiments (Figures 1A,B): (A) Wildfire charcoal generated during an experimental wildfire (June, 2012) in a mature jack pine stand in this region and collected in June 2013 (61◦34’55” N; 117◦11’55” W, see Santín et al (2015), for site and fire characteristics)
Due to its production in a longburning slash pile fire, this type of charcoal was speculated in previous work (Santín et al, 2013) to exhibit a higher degree of charring and greater resistance to combustion when exposed to a subsequent fire than wildfire charcoal

Summary

Introduction

Pyrogenic carbon (PyC, known as charcoal, pyrogenic organic matter or black carbon) is a ubiquitous organic residue resulting from the incomplete combustion of organic fuels during vegetation fires (both natural and anthropogenic fires) It comprises a continuum of pyrogenic organic compounds ranging from partly charred material to soot (Santín et al, 2016a). PyC produced in a fire and not transported off site by wind or water, or protected through its transfer to sufficient depths below litter or soil surface layers, can act as fuel and be subject to combustion in subsequent fires This abiotic loss mechanism for PyC could be important in environments with short fire-return intervals, low bioturbation and topography, which limit, respectively, its vertical movement and off-site transport

Methods

Results

Conclusion