Usually total nitrogen includes organic forms: urea, proteins, and so on. These are usually analyzed together after the sample is digested.

Do you not expect to find organic nitrogen in your samples ?

Only if they are the only N-forms present, which seems unlikely

I assume in the title you meant "nitrate", not "nitrite". As suggested by other responders, it seems likely that there will be organic nitrogen present, either in the synthetic feed or formed by the microorganisms present. Some will certainly be present in particulate form (the biomass, if there is any), but some likely will also be released in soluble form. Also, nitrite-N is usually much lower than the other forms, but this is not always the case, so it should also be included. In some cases (in the absence of any microbial growth), it may be possible to assume that organic- and nitrite-N are low, in which case most of the total-N will be ammonium-N + nitrate-N.

Best of luck with your work!

- Peter