Soil microbial responses to wood ash addition and forest fire in managed Ontario forests

Wood ash is typically applied to soils to counteract acidification and base cation depletion, but the effects of this ash on the organic layer microbial community are rarely studied. We analyzed microbial responses to ash addition in two field-scale experiments in Ontario, Canada. One experiment was in a young boreal forest soil and the other was in an uneven-aged hardwood north-temperate forest soil. We also investigated the effects of a natural forest fire near the boreal experiment. In both cases, ash addition had no effect on overall microbial biomass and respiration, but increased the phylogenetic diversity of bacterial communities and the relative abundance of Bacteroidetes taxa, though effects on other bacterial taxa were site-specific. Eukaryotic effects also varied by experiment; at the boreal site, ash increased eukaryotic phylogenetic diversity and decreased the fungi:bacteria ribosomal marker ratio, but at the temperate site ash decreased eukaryotic diversity and did not affect the fungi:bacteria ratio. There was limited additional effect on the boreal soil microbial community of increasing ash addition from 0.7 to 5.7 t ha−1, as determined by T-RFLP analysis, though soil pH in both experiments increased with higher addition rates. At the temperate site, ash effects were consistently stronger for fly ash than for bottom ash. In both experiments, only 14 unique bacterial taxa were found after ash addition, and the strongest driver of overall community composition was the forest type, not ash treatment. In contrast, the soil microbial community observed at the forest fire site was clearly different. Overall, these results indicate that wood ash addition has only minimal effects on the composition of the soil microbial community in sites across two distinct global forest biomes.