NO3 under extreme reductive conditions can be reduced into NH4 and assimilated into plant tissue, but it's much more likely that the plants will get the NO3 first.

We do not keep such conditions that are that reductive in our tanks, least I hope no one does...........

Most of the NO3=> N2 gas is what occurs, not NO3=> NH4.
NO3-> N2 gas is a moderately/slightly anaerobic condition.
These conditions can and do exist in most aquarium sediments and are the basis for DBS's in marine systems for export of NO3 out of the system.

Regards,
Tom Barr