Imposing primary colonisation success of wood-decomposing fungi in birch wood alters microbiome composition and carbon release rates

Fungi dominate the decomposition of deadwood, with white rot-type species removing more lignin than brown rot-type species to gain access to wood carbohydrates. These fungi often compete to colonise the same tree species, and globally, a small shift in the success of either rot type could have massive greenhouse gas implications. For this reason, we need to know what controls white vs. brown rot outcomes, starting with field studies that track fungi and the fate of wood lignin under real-world conditions. Such experiments, however, have had skewed, white-rot-only outcomes and have lacked representation of bacteria that compete for wood sugars. To address this, we pre-inoculated small-diameter birch Betula papyrifera stem sections with a brown rot fungus Fomitopsis betulina and compared these to non-inoculated birch in a treatment design that had been dominated by white rot fungi in past field studies. This approach encouraged more brown rot, widening the range of wood physicochemical outcomes (density, pH, lignin and carbohydrate profiles). Achieving the brown rot outcome allowed us to better connect the fungal rot type as a trait to its functional outcomes. Specifically, it enabled us to test for definitive links between dominant fungi (identified via ITS2 amplicon sequencing), their C release patterns, and their bacterial associates (identified via 16S rRNAV4 amplicon sequencing). We observed a clear link between rot type and fungal dominance but did not find parallel bacterial codominance. While the Shannon index for fungi was lower in brown-rotted wood than in white-rotted wood, the pattern was reversed for bacteria. Bacterial beta diversity, too, was different between brown- and white-rotted wood. This fungal influence on bacterial diversity may be due to increased niche space (i.e., higher availability of soluble sugars) for bacteria when brown rot fungi dominate. Collectively, achieving these more directed wood rot-type outcomes enabled us to clearly link fungal dominance to wood physicochemical changes, including C release, while also revealing a lack of relationship between dominant fungi and the success of associated bacteria. Read the free Plain Language Summary for this article on the Journal blog.