The carbon source is an organic food. The example below is for methanol (CH3OH), it would need to be balanced differently if another carbon source is used.
NO3- + 5/6 CH3OH => 1/2 N2 + 5/6CO2 +7/6 H2O +OH-
Ref. The Reef Aquarium Vol III, J Charles Delbeek, Julian Sprung, Chapter 6, pg. 260.