Physics major/Engineer...so chem baffles me. Is DNRA bacterial related or is it just a natural chemical reaction based on water conditions?
As you mentioned below, it is bacterial mediated. It occurs in facultative aerobic bacteria, so it really only occurs in the absence of oxygen (anaerobic bacteria).
...my follow on question would be, if this is naturally occuring in nature has it been established in a tank? would seem to me outside of trace elements needing to be refreshed would almost eliminate the need for water changes?
For your second question, it is a very difficult one
Some things that happen in test conditions, do not necessarily happen in nature, and vice versa. On top of that, since nature is not controllable, it makes things even more complicated.
As I mentioned, DNRA is much less common than denitrification. The amounts of ammonia/ammonium being produced would be much less than nitrites/nitrous oxides that would be produced through the denitrification pathway.
So, let's assume that nitrites are being produced; the amount of it being produced would be dependent on the number of facultative aerobic bacteria, which would be in turn based on the amount of anaerobic space there is in the Matrix biomedia (assuming it exists). This surface area would be greatly outmatched by the surface area that exists in aerobic conditions, so nitrification (production of nitrates) would be much more favoured.
With nitrates being produced, water changes will be required (assuming you do not have a planted aquarium whereby plants are uptaking nitrates...).
To my knowledge, in reef aquariums, the production of nitrates is slowed down through the addition/construction of a plenum.