Organic matter chemistry and bacterial community structure regulate decomposition processes in post-fire forest soils

Wildfires decrease forest aboveground biomass and have long-term legacy effects on carbon (C) stocks in soil via alterations of microbial communities and functions. However, the interactions between soil organic C (SOC) chemodiversity and bacterial communities that drive C decomposition remain unclear. Soils from two boreal forest sites, 3 months (S1) and 15 years (S2) after fire events, were incubated for 53 days to quantify the mineralization of sucrose (mimicking rhizodeposits, δ13C = −11.97‰) and SOC priming. To reveal SOC-bacterial interactions that regulate SOC decomposition, the isotopic abundance, SOC chemical composition (13C NMR), and associated bacterial community structure (16S rRNA gene sequencing) were analyzed. The best multivariate model (DISTLM) analysis indicated that aromatic C (phenolic-C and aryl-C) in S1 and di-O-alkyl C in S2 were the largest contributors to bacterial community structure. The co-occurrence network confirmed SOC-bacteria interactions, and revealed the highly co-occurrent groups, i.e. Paenibacillus in S1 and Bacillus in S2, both of which belong to the Firmicutes, correlated with recalcitrant C and labile C, respectively, and are potentially linked to decomposition. For example, Firmicutes (as well as Proteobacteria and Actinobacteria) were correlated with aryl-C and phenolic-C in S1 and highly correlated with SOC priming intensity. The limited C resources (enriched refractory components, i.e. phenolic substances) in S1 favored oligotrophs to outcompete other bacterial groups, which likely aided decomposition of more recalcitrant SOC via co-metabolisms. The slow decomposition of sucrose and large soil priming effects observed in S1 suggested a faster SOC turnover via bidirectional processes of additional sucrose-C gain and native soil-C loss. Collectively, changes in SOC chemistry were coupled with an altered bacterial community, and their interactions might further correlate to decomposition, with implications for C sequestration in the post-fire boreal forest soils.