Denitrification serves as an important pathway for fixed nitrogen loss, producing inert dinitrogen when complete but the potent greenhouse gas nitrous oxide when incomplete. Comparative genomics and physiology have allowed our understanding of how denitrifiers denitrify to come along in leaps and bounds, but has also opened a number of questions regarding why denitrification is such a diverse and branching process. The objective of this work is to evaluate how denitrification pathways differ between habitats, and to determine which environmental factors drive these differences.