Microbial community dynamics during the storage of industrial-scale wood chip piles of Picea abies and Populus canadensis and the impact of an alkaline stabilization agent

Storage of woody biomass in large wood chip piles is unavoidable for biotechnological applications, but comes along with considerable biomass-, energy- and thus, economic losses due to exothermic reactions and microbial degradation. The homogeneous amendment of the storage piles with an alkaline stabilization agent, calcium hydroxide (Ca(OH)₂), was found to decrease dry matter loss in Picea abies; for Populus canadensis piles the effects cannot clearly be deduced. Here we investigated the bacterial and fungal communities of industrial-scale wood chip piles (250 m³) of these two different tree species and related them to physicochemical conditions and enzymatic activities after 35 and 120 d, representing short- and long-term storage of the wood chips, respectively. Coming from different wood types (hard vs. softwood), we expected the communities to converge over time, due to similar storage conditions. Despite pH posing selective pressure, we expected a minor Ca(OH)₂ effect as already known from previous studies. We found that the effectiveness of Ca(OH)₂ addition depended on the wood type that determined both the native microbial seeding community and temperature pattern of all piles, thereby exerting selective forces of differing strength. Generally, a thermophilic community consisting of single fungal and variable bacterial taxa were identified. As expected, the microbial communities from P. abies and P. canadensis converged over time. Biomass loss was connected to C-cycle related enzymatic activities and to the abundance and composition of fungal communities. Chaetomium sp. was identified as potential key taxon determining biomass degradation under the given storage conditions.