Biochar application to soil has been proposed as a means to sequester carbon and mitigate anthropogenic greenhouse gas (GHG) emissions. However, biochar directly influences soil GHG emissions in a complex-interactive manner that remains poorly understood, and hence further understanding of biochar-soil interactions is needed to evaluate biochar’s efficacy as a tool for greenhouse gas mitigation. The goals of this dissertation are to (1) quantify the organic and inorganic alkalis of several biochars, (2) quantify the impact of biochar properties on GHG emissions from diverse soils, and (3) identify mechanisms by which biochar properties influence GHG emissions from soils. To achieve these goals, biochar alkalis and thermochemical properties were analyzed, and three laboratory incubations as well as two field studies and one greenhouse study were conducted, employing eight biochars, two field sites, and four cropping systems. It was found that biochar properties are highly diverse, and effects on GHG emissions varied with respect to biochar properties and soil properties. Both carbonates and bicarbonate-extractable organic carbon in biochar contributed directly to very short term CO2 emissions but did not influence N2O emissions. Effects of biochar on N2O emissions were found to be more complex, with biochar increasing, decreasing or not affecting emissions depending on the context. Results highlight perturbation of N transformations, direct sorption of N, and enhanced water retention as potential mechanisms of biochar’s influence on N2O emissions and suggest that multiple mechanisms likely operate simultaneously.