BACKGROUND: Fine woody debris (FWD; deadwood < 10 cm diameter) is a crucial but often overlooked component of forest ecosystems. It provides habitat for microbial communities and enhances soil fertility through nutrient cycling. This role is especially important in managed forests, which typically have limited deadwood stocks. Climate change is increasing forest disturbances and expanding early successional forests with low canopy cover, yet the effects on microbial communities and related processes remain poorly understood. RESULTS: In a ten-year canopy manipulation experiment, we examined the decomposition of FWD of Fagus sylvatica and Abies alba. Increased canopy openness significantly decreased bacterial diversity in decomposing FWD and altered the community composition in surrounding soil. Decomposition time was the main factor shaping bacterial community structure in FWD, with tree species and canopy cover also contributing. We identified bacterial groups involved in carbohydrate degradation, fungal biomass breakdown, and nitrogen fixation. Importantly, bacterial communities in fully decomposed FWD remained distinct from soil communities. CONCLUSIONS: Deadwood decomposition and nutrient cycling are driven by complex ecological interactions. Microbial community dynamics are influenced by the interplay of FWD decomposition stage, tree species, and microclimatic conditions. Bacterial communities, although less frequently studied in this context, appear more stable over time than previously studied fungi. This stability may help sustain decomposition processes and nutrient turnover under the environmental variability associated with global change.

• Keywords
• Bacterial community, Canopy cover, Deadwood, Decomposition, Ecology, Fine woody debris, Microclimate, Succession, Temperate forest,
• Publication type
• Journal Article MeSH

Decomposition is a crucial process in terrestrial ecosystems, driving nutrient cycling and carbon storage dynamics. Considering the amount of fungal necromass produced in soils annually, its decomposition represents an important nutrient recycling process. Understanding the decomposition dynamics and associated microbial communities of fungal necromass is essential for elucidating ecosystem functioning, especially in environmentally sensitive regions such as the Arctic tundra, which remain under-explored. In a three-year field experiment conducted in the Svalbard archipelago, we investigated the decomposition of two types of fungal necromass with differing biochemical properties. We studied the decomposition rate, changes in chemical composition, and the succession of fungal and bacterial communities associated with the decaying fungal necromass. We discovered that up to 20% of fungal necromass remained even after three years of decomposition, indicating that the decomposition process was incomplete. Our results indicate the crucial role of Pseudogymnoascus in decomposing low-quality, highly melanized necromass with a high C:N ratio in Arctic soils, underscoring its importance in carbon cycling in the Arctic tundra. Notably, we observed dynamic changes in bacterial communities, with increasing richness over time and a shift from copiotrophic to oligotrophic species specializing in decomposing recalcitrant material. Our study indicates the strong potential that fungal necromass can play in carbon sequestration of arctic soils and reveals the distinct dynamics between rather stable fungal and rapidly changing bacterial communities associated with the decomposing fungal necromass in the Arctic tundra. These findings enhance our understanding of microbial succession during decomposition in extreme environments and highlight the potentially differing roles of fungi and bacteria in these processes.

• Keywords
• Arctic tundra, Bacterial communities, Decomposition, Fungal communities, Fungal necromass,
• Publication type
• Journal Article MeSH

Fungi are an integral part of the nitrogen and phosphorus cycling in trophic networks, as they participate in biomass decomposition and facilitate plant nutrition through root symbioses. Nutrient content varies considerably between the main fungal habitats, such as soil, plant litter or decomposing dead wood, but there are also large differences within habitats. While some soils are heavily loaded with N, others are limited by N or P. One way in which nutrient availability can be reflected in fungi is their content in biomass. In this study, we determined the C, N, and P content (in dry mass) of fruiting bodies of 214 fungal species to inspect how phylogeny and membership in ecological guilds (soil saprotrophs, wood saprotrophs, and ectomycorrhizal fungi) affect the nutrient content of fungal biomass. The C content of fruiting bodies (415 ± 25 mg g-1) showed little variation (324-494 mg g-1), while the range of N (46 ± 20 mg g-1) and P (5.5 ± 3.0 mg g-1) contents was within one order of magnitude (8-103 mg g-1 and 1.0-18.9 mg g-1, respectively). Importantly, the N and P contents were significantly higher in the biomass of soil saprotrophic fungi compared to wood saprotrophic and ectomycorrhizal fungi. While the average C/N ratio in fungal biomass was 11.2, values exceeding 40 were recorded for some fungi living on dead wood, typically characterized by low N content. The N and P content of fungal mycelium also showed a significant phylogenetic signal, with differences in nutrient content being relatively low within species and genera of fungi. A strong correlation was found between N and P content in fungal biomass, while the correlation of N content and the N-containing fungal cell wall biopolymer-chitin showed only weak significance. The content of macronutrients in fungal biomass is influenced by the fungal life style and nutrient availability and is also limited by phylogeny.

• Keywords
• ecological traits, fungal biomass composition, nutrient content variation, nutrient stoichiometry, phylogenetic signal,
• Publication type
• Journal Article MeSH

Deadwood decomposition is responsible for a significant amount of carbon (C) turnover in natural forests. While fresh deadwood contains mainly plant compounds and is extremely low in nitrogen (N), fungal biomass and N content increase during decomposition. Here, we examined 18 genome-sequenced bacterial strains representing the dominant deadwood taxa to assess their adaptations to C and N utilization in deadwood. Diverse gene sets for the efficient decomposition of plant and fungal cell wall biopolymers were found in Acidobacteria, Bacteroidetes, and Actinobacteria. In contrast to these groups, Alphaproteobacteria and Gammaproteobacteria contained fewer carbohydrate-active enzymes and depended either on low-molecular-mass C sources or on mycophagy. This group, however, showed rich gene complements for N2 fixation and nitrate/nitrite reduction-key assimilatory and dissimilatory steps in the deadwood N cycle. We show that N2 fixers can obtain C independently from either plant biopolymers or fungal biomass. The succession of bacteria on decomposing deadwood reflects their ability to cope with the changing quality of C-containing compounds and increasing N content.

• Keywords
• bacterial genomes, cellulose, deadwood, mycophagy, nitrogen fixation,
• Publication type
• Journal Article MeSH