Deadwood is an important structural component in forest ecosystems and its degradation is mainly controlled through microorganisms and their secreted digestion enzymes. While the process of wood decay is rather well understood, in situ data about the factors that explain the heterogeneous distribution patterns of deadwood microorganisms, their corresponding enzyme activities and the resulting changes of wood physico-chemical parameters and element bioavailability are scarce. We therefore investigated the deadwood decomposition in a long-term experiment, which has been designed and launched within the German Biodiversity Exploratories in 2009. Here, we especially address the following hypotheses: i) the structure of microbial communities is explained by the tree species under decay and their spatial localization, i.e. sap- and heartwood, ii) specific groups of microorganisms explain the presence of particular enzymes and their distinct spatial patterns; and iii) fungal depolymerization processes increase the bioavailability of essential elements in decomposing wood.
Samples were taken from logs of 13 different tree species (n=3) exposed to decomposition for 6 years in beech forests in Central Germany. Microbial communities were amplicon-sequenced using Illumina MiSeq, 15 extracellular enzyme activities, wood parameters (e.g. water-soluble lignin fragments by HPSEC) and element contents were measured.
i) Microbial community structure and species richness significantly differed among the 13 decaying tree species, which can partially be explained by the different physico-chemical traits of the logs. For example, the lignin content, pH, extractives and element contents were found to considerably vary between gymnosperm and angiosperm wood. The structure of the mainly unicellular bacterial community and its richness significantly differed between sapwood and heartwood, while the filamentous growth of fungi enabled them to occupy both sapwood and heartwood.
ii) The presence of particular fungal families (e.g. Polyporaceae or Xylariaceae) was found to correlate with the secretion of characteristic lignin-modifying and/or (hemi)cellulolytic enzymes.
iii) Fungal depolymerisation activity determined by analysis and quantification of small (0.5-5 kDa) water-soluble lignin fragments significantly correlated with element bioavailability, which may be an important result regarding flux processes in forest ecosystems. Causal agent of this relation are ligninolytic white-rot basidiomycetes.
Altogether we gathered an in-depth dataset of the microbial activities and resulting changes in decaying wood, which enables us to mechanistically understand the complex processes in the course of the biodegradation of woody biomass.