| Biochar soil amendment has the potential to simultaneously promote soil health and crop productivity and mitigate climate change by sequestering carbon and reducing greenhouse gas (GHG) emissions. Yet the underlying mechanisms are obscure, as are the influential factors that determine the net outcomes. This thesis investigates biochar effects on the soil microbiome, whose structure and functionality can translate to soil ecosystem services. A three-level meta-analysis of up-to-date literature on soil microbial responses to biochar amendment identified feedstock as the most important predictor for the largest number of microbial responses, with the largest positive effect seen in dehydrogenase activity. Other less important influential factors of biochar performance include pyrolysis temperature and biochar application rate, in addition to initial soil properties and field conditions. Controlled microcosm experiments revealed sequential changes of the microbial community of agricultural soil over 10 weeks, with stronger responses from bacteria and archaea than fungi. By week 10, the prokaryotic communities clustered significantly based on biochar feedstock (cattle manure, corn stover, pine sawdust and wheat straw) and application rate (0.25% and 2.5%, w/w). Biochar addition, regardless of feedstock, enriched many plant-beneficial bacteria whose potential ecosystem functions included symbiotic nitrogen fixation, denitrification, decomposition of organic matter, and suppression of soil-borne pathogens. In line with these shifts in microbial community structure, biochar induced alterations in substrate utilization, stimulating community-level usage of amino acids, carbohydrates, carboxylic and ketonic acids, phenolic compounds, and polymers at varying microcosm stages. Feedstock repeatedly emerged as the main driver of differential patterns of community-level substrate usage and GHG production. Collectively, this thesis work provides novel insights into the mechanisms of biochar’s beneficial effects and highlights the importance of feedstock selection in magnifying desired microbial responses towards sustainable agriculture. Key Words: sustainable agriculture, soil microbiome, bacteria, archaea, fungi, biochar, feedstock selection, meta-analysis, metagenomics, nutrient cycling, enzyme activity, greenhouse gas emission |