Warming and reduced rainfall alter fungal necromass decomposition rates and associated microbial community composition and functioning at a temperate-boreal forest ecotone

Changes in temperature and rainfall associated with altered climatic
conditions are likely to significantly alter rates of soil organic matter
decomposition. To determine how the combined effects of warming and
drought impact the decomposition of fungal necromass, a large and
fast-cycling portion of the global soil organic carbon (C) pool, we
incubated Hyaloscypha bicolor necromass under both ambient and altered
conditions (air and soil warming +3.3°C and ~40% reduced rainfall) at the
B4Warmed experiment in Minnesota, USA. We conducted two multi-week
incubations, one assessing mass loss and microbial community composition
on decaying necromass after 1, 2, 7, and 14 weeks and the second
characterizing the substrate utilization capacities of
necromass-associated microbial communities after weeks 1 and 7. Warming
and reduced rainfall accelerated the initial rate of necromass decay by
~20%, yet slowed overall mass loss by ~6% at the end of the 14 week
incubation. These different rates of decay over time paralleled shifting
abiotic conditions, with altered plots experiencing warmer and relatively
moist conditions early, but hotter and drier conditions later. The
microbial community composition also varied by treatment and time, with
warming and reduced rainfall stimulating fast-growing fungi as well as
fungal relative to bacterial growth overall. Additionally, the functional
capacity of the microbial community also changed over time, having a
higher metabolic capability to utilize C and N substrates in the altered
plots early in decomposition but a lower capability later in decay.
Collectively, our findings highlight a dynamic, stage-dependent response
of fungal necromass decomposition to altered climate regimes. By linking
these decay dynamics to shifts in environmental conditions as well as
microbial community composition and function, our study highlights the
critical roles of both abiotic and biotic changes in mediating
decomposition responses to climate change.