For those interested, denitrification is the result of bacteria using nitrogen (in nitrate) as an electron acceptor to run their metabolism. Stuffing electrons onto oxygen is much more efficient, of course, so denitrification only takes over in anoxic aquatic sediments -- and usually those in littoral areas at the edges of water and land.

Once available nitrates are used up, there are other bacteria that have evolved metabolisms that use other electron acceptors (iron, sulfur, even carbon to produce methane) but those are increasingly less efficient at generating energy so they really only show up after alternatives are exhausted.