Fire Impacts on the Soil Metabolome and Organic Matter Biodegradability

Abstract

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 CO 2 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.

Temporal Variations in Fire Impacts on Characteristics and Composition of Soil-Derived Dissolved Organic Matter at Qipan Mountain, China

“…The DOM result aligns well with the result of a meta-analysis that fire decreased SOC contents and did not consistently increase or decrease DOC contents in 383 sites worldwide . Specifically, increased soil DOM contents immediately following fires have been found in some recent wildfire studies in the United States , and Australia, , while the long-term decreased DOM contents after fires have also been found in some other wildfire events. ,, These findings imply that the larger postfire temporal variations observed in DOM than in SOM in natural environments, where DOM could exhibit a short-term increase and a long-term decrease after fire, at least partially explain the inconsistencies observed in DOM (or DOC) responses to fire worldwide.…”

“…With a longer time after the fire, the unique formulas at burned sites compared with unburned sites shifted from the H/C < 1.5 region to the O/C < 0.5 region (Figures a and S8), highlighting differences between short- and long-term fire effects. The high abundance of low H/C compounds at the burned sites within a year after the fire could be due to the pyrogenic aromatics and preferential microbial consumption of biolabile nonaromatics (e.g., aliphatics). , After two years, the enrichment of the low O/C compounds may be caused by the recovered plant DOM inputs and negative priming effects of black carbon that slowed down DOM oxidation . Although the ecosystem started to recover over time and introduce recovered plant- and microbe-derived DOM to soils, the DOM molecular composition at burned sites gradually turned back to the unburned levels.…”

“…Unlike the response of SOM to fire, the response of the DOM content exhibited strong temporal dependence, with short-term pulse releases but long-term declines. Since soil DOM is crucial for microbial activity and serves as a source of greenhouse gases, − our findings support a recent finding of a short-term pulse of greenhouse gas emission immediately after a fire, despite long-term suppression of microbial activity or greenhouse gas emission . Moreover, the temporal variability in DOM quantity should be considered in postfire environmental management.…”