Deadwood in the final decay class: Shifts in functions, processes and active microbial communities (Woodstock II)

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Lying deadwood (logs) represents a habitat and substrate for many organisms that is transformed to humus through several stages of decay. The transformation to humus is critical for organisms that rely on specific decay stages of deadwood and for long-term carbon (C) storage in forests. Understanding processes, microbial community structures and functions of decaying logs is difficult because several decay classes typically exist in parallel. In this proposal, we will study an average of parallel existing (hereafter bulk) decay classes, the final decay class and deadwood humus from logs of 13 tree species in the BELongDead experiment. Our overarching hypothesis is that the function of deadwood as habitat and substrate for microorganisms changes in the transition from bulk to humus. These changes are related to shifts in the substrate quality, microbial community structure and process rates and differ among tree species, regions or plots, and forest management intensity, and result into different humus and C accumulation in forests. The substrate quality and active bacterial and fungal community members of these physically separated deadwood fractions will be assessed by chemical/spectroscopic analyses and by labelling of newly formed DNA and amplicon sequencing, respectively. Tree species identity and plot specific differences in substrate quality of deadwood fractions will be linked to microbial process rates and active microbial community members. Process rates include respiration, biological N2 fixation, microbial carbon use efficiency, microbial growth, microbial biomass and necromass turnover. We expect that the fungal saprotrophic contribution will decrease and the diversity of bacterial functions will increase towards deadwood humus. Further, we expect that deterministic processes as well as specialists of the active microbial communities will also increase towards deadwood humus. Combined expertise will allow us to reappraise the relevance of the link between microbial diversity, microbial processes and substrate quality of deadwood in the final decay class and humus for wood-inhabiting organisms. By working on a coordinated and full replicated experiment across regions, we expect robust results with high scientific impact and benefits for deadwood management in forest ecosystems.