One of the things that has been pushed to sequester carbon is no-till. Ken Olson, a researcher who I respect a lot, has data that shows that organic carbon declines even in no-tilled soils. His data shows a hold your own situation at best with a decline in a lot of his studies.
I see carbon enrichment in the surface of notilled soils, so I am not sure why the rest of the profile would decline. This was interesting data to view in any case.
One hypothesis is this: easily decomposed soil organic matter from the surface is mixed to depth year after year during tillage. That stuff decomposes pretty rapidly and if there is not replacement from tillage, there will be a loss at depth when tillage stops. Fresh organic inputs can also induce a loss of stable SOM by "priming" the soil microbes. So there might be a carbon buildup at surface and a loss at depth that yields a small overall loss over the profile. That should reverse over time, though, so that eventually the soil could recover (some of) the carbon it lost during initial cultivation.
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