Dear Jiawei Fan, I understand that you want to know if the organic matter is a result of a nitrifying, ammonifying and denitrifying process, but you are not exactly interested in the genom of the bacteria ?  In this case you could use 14/15N isotope analysis.

Dear Prof Schneider, thank you very much for your suggestion. We have tried the stable nitrogen isotope analysis, however, it sometimes cannot be used to separate the signals from the above three biological processes, based on the overlapped values of 15N/14N fractionation of these three processes. Actually, I am very interested in the relative importance of these three biological process on the nitrogen cycling within the lake during the geological past.

Dear Jiawei Fan

If such bacterial activities existed in the geological past, there had been a possibility of substrate exhaustion by denitrification which is the only process of the three processes you identified would have resulted in gaseous stripping of N. The absence of substrate would have resulted in bacterial death or dormancy. On the other hand, if O2 was limiting, the ammonfied organic-N (i.e ammoniacal-N) would not have been able to be nitrified, in which case with the absence of denitrification, the geological material was likely have retained much of the organic and ammoniacal-N despite the geological age. I suspect the latter hypothesis is more possible than substrate exhaustion.

I suggest you carefully sample your lacustrine sediments ensuring no cross contamination or infection, and analyse for organic-N (both dissolved and total), ammoniacal-N and enumerate the respective bacteria by incubating with substrates such as NO3 (for denitrifiers) and NH4 (for nitrifiers) and ammonifiers. I assume the sample has mineralisable organic-N hence there is no need to introduce any organic-N or amino acid substrates. I also suggest you perform C dating of the sediment organic matter to assess the geological age of the material to ensure the material you have been interested in had not been from the recent anthropogenic activity.

Kind regards

Selva Selvarajah