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 (+3.3°C air and soil warming 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.