I think it is much more complex ; it is the total sum of what is growing (or rather died) at the site (all vegetation), plus depending on the activity of decomposers such as fungi and bacteria.
Don't overlook the carbonate minerals, and soluble carbonate and bicarbonate in pore waters. As Vivi points, the contribution of the microflora, microfauna, and macro living organisms also provide a largely overlooked pool of carbon.