True, but I think that only occurs in the upper atmosphere and at a very low rate. Atmospheric N2 is also converted by bacteria into ammonia, which is absorbed

True, but I think that only occurs in the upper atmosphere and at a very low rate. Atmospheric N2 is also converted by bacteria into ammonia, which is absorbed by plants. And lightning oxidizes N2, as do combustion engines. I'm not sure if all those different reactions add up to a significant fraction, though. It might be true that most of the N2 molecules from Caesar's time still exist.