It's obvious by now, that the "previous proofs" are non existant.
And, no, school will not commence. We are of the age, now, where we can choose who we want for a "teacher". And it sure as hell isn't you.
Just admit you were wrong and stop trying to drag this out and avoid the obvious.
We must go to the next step. If we can measure the amount of nitrogen in water and we can measure the amount of water passing a given point then we can determine the amount of nitrogen passing the given point. In lbs per second perhaps.
The next step is to determine the total amount of nitrogen passing that point for the year. That is simply multiplying the lbs per second times 365 days, times 24 hours, times 60 minutes, times 60 seconds.
Because the rate of flow and the concentration of nitrogen varies over the year, for accuracy, one would divide the year into many short finite intervals and sum them. The more intervals the more accurate.
We have now determined the total lbs of nitrogen flowing past that point for a year. Be sure you understand this before we go to the next point because it is important.