In the environmental context, reactive nitrogen compounds include the following classes:
*oxide gases: nitric oxide, nitrogen dioxide, nitrous oxide.  Containing oxidized nitrogen, mainly the result of industrial processes and internal combustion engines.
*anions: nitrate, nitrite.  Nitrate is a common component of fertilizers, e.g. ammonium nitrate. 
*amine derivatives: ammonia and ammonium salts, urea.  Containing reduced nitrogen, these compounds are components of fertilizers.
All of these compounds enter into the nitrogen cycle.
As a consequence, an excess of Nr can affect the environment relatively quickly. This also means that nitrogen-related problems need to be looked at in an integrated manner.