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.