Abstract
Wildfires represent a fundamental and profound disturbance in many ecosystems, and their frequency and severity are increasing in many regions of the world. Fire affects soil by removing carbon in the form of CO2 and transforming remaining surface carbon into pyrolyzed organic matter (PyOM). Fires also generate substantial necromass at depths where the heat kills soil organisms but does not catalyze the formation of PyOM. Pyronema species strongly dominate soil fungal communities within weeks to months after fire. However, the carbon pool (i.e., necromass or PyOM) that fuels their rise in abundance is unknown. We used a Pyronema domesticum isolate from the catastrophic 2013 Rim Fire (CA, United States) to ask whether P. domesticum is capable of metabolizing PyOM. Pyronema domesticum grew readily on agar media where the sole carbon source was PyOM (specifically, pine wood PyOM produced at 750°C). Using RNAseq, we investigated the response of P. domesticum to PyOM and observed a comprehensive induction of genes involved in the metabolism and mineralization of aromatic compounds, typical of those found in PyOM. Lastly, we used 13C-labeled 750°C PyOM to demonstrate that P. domesticum is capable of mineralizing PyOM to CO2. Collectively, our results indicate a robust potential for P. domesticum to liberate carbon from PyOM in post-fire ecosystems and return it to the bioavailable carbon pool.
Highlights
Wildfires can have substantial effects on nutrient cycling (Wan et al, 2001; Nave et al, 2011) and community composition both above- and belowground (Bond and Keeley, 2005; Pressler et al, 2018), making them important drivers of ecosystem processes (McLauchlan et al, 2020)
13C-labeled 750°C pyrolyzed organic matter (PyOM) was produced from P. strobus as described above, except the biomass was from 13C-labeled seedlings
We incubated the P. domesticum on 13C-labeled 750°C PyOM agar (10 g L−1 PyOM) in 118.29 ml Mason jars, fitted with gas-tight lines, and connected to an automated sample analyzer (“multiplexer”) that automatically samples the jar headspaces at regular intervals and quantifies the amount and isotopic signature of the headspace CO2 in a Picarro cavity ringdown spectrometer
Summary
Wildfires can have substantial effects on nutrient cycling (Wan et al, 2001; Nave et al, 2011) and community composition both above- and belowground (Bond and Keeley, 2005; Pressler et al, 2018), making them important drivers of ecosystem processes (McLauchlan et al, 2020). While organic matter in surface soils may be completely combusted or pyrolyzed during fire, in deeper soil layers, non-pyrolyzed organic carbon is released where the heat from fire was enough to kill cells, but not hot enough for combustion or to catalyze the formation of PyOM. This soil layer defined by a heatinduced release of nutrients has been termed the “necromass zone” (Knicker et al, 2005; Bruns et al, 2020). Relatively little is known about how the metabolism of these respective carbon sources may drive post-fire microbial succession and community recovery
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