Fire Impacts on the Soil Metabolome and Organic Matter Biodegradability

Global wildfire activity has increased
since the 1970s and is projected
to intensify throughout the 21st century. Wildfires change the composition
and biodegradability of soil organic matter (SOM) which contains nutrients
that fuel microbial metabolism. Though persistent forms of SOM often
increase postfire, the response of more biodegradable SOM remains
unclear. Here we simulated severe wildfires through a controlled “pyrocosm”
approach to identify biodegradable sources of SOM and characterize
the soil metabolome immediately postfire. Using microbial amplicon
(16S/ITS) sequencing and gas chromatography–mass spectrometry,
heterotrophic microbes (Actinobacteria, Firmicutes, and Protobacteria) and specific metabolites (glycine,
protocatechuate, citric cycle intermediates) were enriched in burned
soils, indicating that burned soils contain a variety of substrates
that support microbial metabolism. Molecular formulas assigned by
21 T Fourier transform ion cyclotron resonance mass spectrometry showed
that SOM in burned soil was lower in molecular weight and featured
20 to 43% more nitrogen-containing molecular formulas than unburned
soil. We also measured higher water extractable organic carbon concentrations
and higher CO2 efflux in burned soils. The observed enrichment
of biodegradable SOM and microbial heterotrophs demonstrates the resilience
of these soils to severe burning, providing important implications
for postfire soil microbial and plant recolonization and ecosystem
recovery.