INTRODUCTION
Microbial denitrification is a key pathway in global nitrogen cycling and has been studied extensively for its role in fixed nitrogen loss and as a source of potent greenhouse gases (Zumft, 1997; Decleyre et al. , 2016). Diverse bacteria are capable of denitrification, often facultatively using nitrate or nitrite as an alternative electron acceptor in oxygen-limited zones. Several diverse microorganisms have the genomic capacity to perform complete denitrification (Fig. 1), reducing nitrate to dinitrogen gas (Philippot, 2002; Canfield, Glazer and Falkowski, 2010).