Sourabh I would suggest you to use specific ion analyzer for the same. As far as your query is concerned, i can only say that the total conversion of ammonia to nitrate takes 4.6 parts oxygen and 7.1 parts alkalinity to convert 1 part ammonia.
What I understand from your question is you are fertilizing soil with nitrogen source and now you want that how much nitrogen is reached to the soil and with what rate, if it is this then I think the simple kinetic equation
r=K [A] [B]
can be used to calculate the r.
Just make the chemical equation and calculate the rate
Hope I understand your question, if not please can you explain a little bit...:)