Nitrate and ammonium are the major sources of inorganic nitrogen taken up by the roots of higher plants. Most of the ammonium has to be incorporated into organic compounds in the roots, whereas nitrate is readily mobile in the xylem and can also be stored in the vacuoles of roots, shoots, and storage organs. However, in order to be incorporated into organic structures and to fulfill its essential functions as a plant nutrient, nitrate has to be reduced to ammonia. The importance of the reduction and assimilation of nitrate for plant life is similar to that of the reduction and assimilation of CO2 in photosynthesis. In general, permissible limit of nitrogen and ammonia in the sewage outlet should not exceed 2-5%, The tolerance limit of C3 plants is more than C4 plants. However, soils can tolerate  ammonia amounts to 10-90 mg/ Kg in course textured soils, 60-270 mg /kg in medium textured soils  and 90-260 mg/kg in fine textured soils.

please see these design  of Effluent Standards:

http://www.span.gov.my/files/MSIG/MSIGVol4/04_Sec._3_Sewage_Characteristics_and_Effluent_Discharge_Requirements.pdf

Actually, the nitrogen element can not be alone in wastewater,  but can be found in the forms of Ammonia (NH4), Organic Nitrogen (org-N), Nitrate (NO3), and Nitrite (NO2).

Total Nitrogen (t-N),  and Total Kejeldahl Nitrogen (TKN) ,  Ammonia (NH3 and/or NH4), and Nitrate (NO3)  laboratory tests can be used for the nitrogen measurements in an effluent sample. 

The standard limit depends on the use purpose and the country recommended limits, however, if your country does not have such a guideline you can see the FAO guidelines in the following links.  

1- FAO : http://www.fao.org/docrep/t0551e/t0551e05.htm

2- Germany: https://cgi.tu-harburg.de/~awwweb/wbt/emwater/documents/lesson_d1.pdf

For Irrigation and as the inorganic nitrogen in the form of nitrate NO3 is considered as a fertiliser so it is difficult to find a limit. but for the ammonia, the limit is important.