I have linked to descriptions of the complexities of the soil carbon pools at other times and on this page.
You assume that carbon inputs – and primarily by plant carbon fixation – balance the removal through cropping and grazing. It is not true – there are a number of other processes in agricultural systems that determine soil carbon pools. And again – the information is in links that I provide. But equally – the internet facilitates research on this or any other topic it seems. Not like the days when years were spent photocopying articles on biogeochemical cycling in libraries.
This link – provided above – provides an excellent introduction by the global leader in the field.