Pyrogenic Carbon Generation From Fire and Forest Restoration Treatments

Thomas H Deluca,Si Gao,Rachel J Brimmer,Michael J Gundale

doi:10.3389/ffgc.2020.00024

Abstract

Pyrogenic carbon (PyC) is a chemically stable form of carbon (C) generated during fire events and is one of the few legacies of fire recorded in soil; however, the significance of this material as a form of C storage in forest ecosystems has received only limited scientific attention, and currently relatively little is known regarding the quantity of PyC generated during forest restoration efforts that include prescribed (Rx) fire. Quantifying the rate of PyC production during fire events is essential to estimating the potential for ecosystem C storage in forest systems where wildfire is a common natural disturbance and Rx fire is used as a fuel management practice. In this paper, we report on the average mass of PyC formation during wildfire and Rx fire events in forest ecosystems through synthesizing data across 12 published studies; and we report empirical data on the mass of PyC generated in a replicated Rx fire study in a ponderosa pine (Pinus ponderosa) forest ecosystem of the Rocky Mountain West. Our data synthesis showed that PyC (consisting of PyC from downed wood, O horizon, and mineral soil) was, on average, produced at a rate of 5.2% of biomass exposed to fire in forest ecosystems, and a single wildfire or Rx fire event generated about 2.2 Mg C ha-1 in the form of PyC. In our empirical study, we collected O horizon and surface mineral soil (0 – 10 cm) samples from the Fire and Fire Surrogates study plots in western Montana (treatments: control, thin, burn, thin&burn) that was burned in 2002 and analyzed for PyC content using a wet digestion method. The two Rx fire treatments had significantly higher PyC contents in the O horizon compared to the control. In the thin&burn treatment, PyC was formed at a rate of about 12% of total biomass consumed yielding approximately 2.24 Mg PyC C ha-1 in the O horizon. Mineral soil PyC contents were not affected by Rx fire. Wildfire and Rx fires generate a substantial amount of PyC which initially resides in the O horizon and serves as a stable form of ecosystem C.

Highlights

Charcoal and pyrogenic carbon (PyC) are general terms given to organic materials that have been subject to partial combustion during biomass burning events (Forbes et al, 2006; Zimmerman and Mitra, 2017)
Our analysis involving 22 independent fire event sites that were previously published on Pyrogenic carbon (PyC) generation rates (N = 31) showed that PyC was generated at an average rate of 2.17 (± standard deviation (SD) 1.72) Mg C ha−1 in a single fire event that ranging from 0.05 to 6.4 Mg C ha−1, regardless of wildfire or prescribed (Rx) fire, with or without fuel reduction treatments (Figure 1A)
There was no difference in the amount of PyC produced by wildfire or Rx fire; the amount of PyC produced by wildfire seemed to exhibit a higher degree of variation than Rx fire (Figure 1A)

Summary

Introduction

Charcoal and pyrogenic carbon (PyC) are general terms given to organic materials that have been subject to partial combustion during biomass burning events (Forbes et al, 2006; Zimmerman and Mitra, 2017). The role of PyC in the soil ecosystem extends beyond its capacity to serve as stored C, PyC can directly and indirectly influence soil microbial processes (Thies et al, 2015), increase rates of organic matter decomposition (Wardle et al, 2008; Pluchon et al, 2015) alter soil nutrient availability (Lehmann et al, 2011; Gul and Whalen, 2016; Gao and DeLuca, 2019; Gao et al, 2019), modify soil nutrient cycling (DeLuca and Sala, 2006; DeLuca et al, 2006, 2015; Gao et al, 2017; Gao and DeLuca, 2018), and adsorb organic compounds in plant root exudates, litter decomposition products, and microbial byproducts (Keech et al, 2005; Gundale and DeLuca, 2006; Quilliam et al, 2013). These facets of PyC have led to a significant increase in the number of studies devoted to the role of naturally formed PyC in terrestrial ecosystems (Pingree and DeLuca, 2017)

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