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Nitric Oxide Reductase
There isn't a lot on norB primers out there. This paper by Braker and Tiedje is the only one I could find that developed norB primers for use in environmental samples. The Dandie group (three papers under denitrifcation genes/ first look at functional genes) developed primers from culturable bacteria and applied them to ag fields, but as they are focused on culturable microbes it's hard to tell how well those primers would work in finding other species.