Explain why it is difficult to trace elements through biogeochemical cycles?

Question

Explain why it is difficult to trace elements through biogeochemical cycles?

Answer 1

Because a given atom generally cannot be distinguished from others of the same element. So, if you watch an oxygen molecule enter a body there is no way of knowing when it leaves, and in what form. Likewise you cannot identify if a particular carbon atom in a CO2 molecule gets converted into limestone, rather than another.  Some of these questions can be addressed in the short term by using isotopes that can be followed, but even then you don't know what is happening to individual molecules