Biochar and Wood Ash Impacts on Soil Microbial Community Structure and Biogeochemical Functioning in Managed Ontario Forests
Biochars and wood ash could be used as valuable soil amendments in managed Canadian forests to counteract effects of full-tree harvesting and N deposition, such as soil acidification and nutrient depletion. However, large-scale application cannot be recommended without a thorough assessment of the impacts of these amendments on key aspects of the ecosystem. Characterizing microbial responses is crucial for predicting the overall forest response to biochar and wood ash additions, because soil microbial communities affect ecosystem health and functioning and aboveground plant communities. However, for biochars in particular, the long-term effects on the soil microbial community in temperate forested ecosystems are still unknown. Similarly, while there have been ongoing ash-addition experiments in northern European and South American forests, the variability in microbial responses makes it challenging to extrapolate to other forest types and few field-scale ash addition studies have been conducted in Canadian forests. This thesis examines the effects of adding biochars and wood ash to Great Lakes' St. Lawrence and Boreal forest soils, particularly focusing on changes to the structure of the soil microbial community and its biogeochemical functional ability. In a north-temperate, selection-harvested forest, biochar application had only minor effects on bacterial and fungal community composition, fungi:bacteria ratios, microbial biomass, and microbial C mineralization two years after addition. Ash addition to a clear-cut boreal forest and the same selection-harvested forest had similarly minor effects on microbial community composition and did not alter overall microbial biomass or microbial C mineralization. In a microcosm experiment, biochar and ash addition effects on P bioavailability depended on soil texture and tree species. A comparison of four-year-old biochar particles and adjacent soil showed that even when there were phylogenetic differences between biochar and bulk soil microbial communities, both groups had very similar functional abilities. Across all experiments, site-specific factors played a strong role in chemical and biological responses to biochar and ash addition. However, most observed effects were minor and transient, indicating that biochar and ash can likely be used as soil amendments in these systems without negatively disrupting the soil microbial community.