Good responses above. One additional point: you want to insure that your ammonium salts are not above 0.1 % w/v since a range of bacteria are inhibited at higher conc., some even at 0,1% NH4Cl (roughly, pending on the salt 10-20 mM NH4 ions) level. If you use urea, we usually stay below 0.6% w/v. If you are looking for diversity in environmental samples you need to go to very low substrate concentrations but still keep the C/N ratio around 25 -30. If you use nitrate, you need to be careful with the concentration and the CN ratio, since some bacteria (I know some anaerobes) who will at lower Carbon conc only form nitrate which then kill -at least inhibit-- some of your microorganisms. AT high carbon conc they will go all the way to ammonium (see above comment re pH effect)

The C/N ratio is not fixed. It can be dependent on the species in the mixture. As long as the synthetic inorganic nitrogen salt(s) is metabolized by the bacteria it can be used and then accounted for when calculating the C/N ratio. For composting, which is a "very" mixed culture (not only bacteria) the optimal C/N ratio is between 25 and 30. If too low C/N ratio, no biomass formed, if too high C/N ratio ammonia is formed with unwanted raise in pH.

As J. Rokem cited the relation C / N is highly variable depends on your objective, to culture bacteria that grow or what you want to find response.

In the case of biological reactors this is an important parameter and is related to the conditions under which the process develops mainly related biological fermentation. The C / N ratio ideal for an optimal digestion is in the range 20 to 30:1, approximately.

Good responses above. One additional point: you want to insure that your ammonium salts are not above 0.1 % w/v since a range of bacteria are inhibited at higher conc., some even at 0,1% NH4Cl (roughly, pending on the salt 10-20 mM NH4 ions) level. If you use urea, we usually stay below 0.6% w/v. If you are looking for diversity in environmental samples you need to go to very low substrate concentrations but still keep the C/N ratio around 25 -30. If you use nitrate, you need to be careful with the concentration and the CN ratio, since some bacteria (I know some anaerobes) who will at lower Carbon conc only form nitrate which then kill -at least inhibit-- some of your microorganisms. AT high carbon conc they will go all the way to ammonium (see above comment re pH effect)

Thank you J. Rokem, Fabbricio Motteran and Juergen Wiegel sir.

 How is the C/N ratio calculated for mixed bacterial cultures?? 

Ooops, I just saw that in my original response is a typo in the third line from the bottom: " ...nitrate.... should read  "  ..nitrite  ..." and not nitrate.

  Sorry.Juergen