Low‐severity fire as a mechanism of organic matter protection in global peatlands: Thermal alteration slows decomposition

Abstract

Worldwide, regularly recurring wildfires shape many peatland ecosystems to the extent that fire‐adapted species often dominate plant communities, suggesting that wildfire is an integral part of peatland ecology rather than an anomaly. The most destructive blazes are smoldering fires that are usually initiated in periods of drought and can combust entire peatland carbon stores. However, peatland wildfires more typically occur as low‐severity surface burns that arise in the dormant season when vegetation is desiccated, and soil moisture is high. In such low‐severity fires, surface layers experience flash heating, but there is little loss of underlying peat to combustion. This study examines the potential importance of such processes in several peatlands that span a gradient from hemiboreal to tropical ecozones and experience a wide range of fire return intervals. We show that low‐severity fires can increase the pool of stable soil carbon by thermally altering the chemistry of soil organic matter (SOM), thereby reducing rates of microbial respiration. Using X‐ray photoelectron spectroscopy and Fourier transform infrared, we demonstrate that low‐severity fires significantly increase the degree of carbon condensation and aromatization of SOM functional groups, particularly on the surface of peat aggregates. Laboratory incubations show lower CO2 emissions from peat subjected to low‐severity fire and predict lower cumulative CO2 emissions from burned peat after 1–3 years. Also, low‐severity fires reduce the temperature sensitivity (Q10) of peat, indicating that these fires can inhibit microbial access to SOM. The increased stability of thermally altered SOM may allow a greater proportion of organic matter to survive vertical migration into saturated and anaerobic zones of peatlands where environmental conditions physiochemically protect carbon stores from decomposition for thousands of years. Thus, across latitudes, low‐severity fire is an overlooked factor influencing carbon cycling in peatlands, which is relevant to global carbon budgets as climate change alters fire regimes worldwide.

Effects of water level alteration on carbon cycling in peatlands

“…Turetsky, Donahue, and Benscoter (2011) reported that after long-term experimental drainage, combustion carbon losses during wildfire became ninefold higher in a boreal peatland, even though the carbon accumulation rates have doubled. Low-severity fire modifies the chemistry of soil organic matter and further, stabilizes C storage in peatlands from all regions (Flanagan et al 2020), but these positive effects may be weakened with increased fire severity after drainage or drought. Indirect effects of water table alteration through wildfire are overlooked and therefore, need to be incorporated in future research.…”

An outline summary document of the current knowledge about prescribed vegetation burning impacts on ecosystem services compared to alternative mowing or no management

“…Finally, the recent criticism of two studies, Heinemeyer et al (2018) and Marrs et al (2019a), showing considerable C accumulation under rotational burning, was based on a model by Young et al (2019), which remains non-validated, unspecified, irrelevant (only deep drainage and not burning impacts), and omits key C storage aspects via charcoal (see pre-print responses by and Heinemeyer et al (2021)). In fact, recent studies by Leifeld et al (2018) and Flanagan et al (2020) support the findings of Heinemeyer et al (2018) and Marrs et al (2019a) that low-severity (i.e. prescribed) fires can result in high soil C accumulation rates in peatlands.…”

Constructive criticism of “Misinterpreting carbon accumulation rates in records from near-surface peat” by Young et al: Further evidence of charcoal impacts in relation to long-term carbon storage on blanket bog under rotational burn management

“…Indeed, Young et al's criticism of two recent burning studies [2,3] is based on nonvalidated and unexplained model scenarios that omit crucial fire-mediated C cycle processes; we urge peatland researchers to include such burn-related C-cycle processes within future model scenarios [2,7] . For example, (i) net C fluxes at the three Heinemeyer et al sites [2] (see the Peatland-ES-UK website [12] ) indicate a potentially high net C uptake on regrowing burnt plots (possibly higher than that of unburnt plots with aging/degenerating heather and soon to offset combustion losses, but this requires a complete burn rotation to allow comparing cumulative C flux budgets versus C stocks estimates); (ii) recent work [14] adds to the prescribed heather burning work [2,3] in highlighting the positive role that charcoal (produced during cool burns or low-severity fires) can have on peatland C storage (e.g. it has the potential to increase peat C stocks by adding recalcitrant Corg in the form of charcoal and further reducing C losses via decomposition of soil organic matter); finally, (iii) a recent study suggests that low-severity fires may reduce methane emissions relative to no burning or high-severity fires in cooler climates [15] .…”