I don't know the answer, but in a similar environment, the greater the clay content the better protected the C

This might be of some help: https://repository.upenn.edu/cgi/viewcontent.cgi?article=1665&context=edissertations

Sunil Ghimire The soil carbon saturation theory shows some promise in understanding the carbon dynamics of certain soils, but probably not for many soils like those with low clay and silt content but contain a very high amount of organic matter. In many soils, the organic carbon dynamics are so complex and are more influenced by relief and drainage, climate, vegetation, and land use rather than by the physico-chemical and mineralogical properties. Since your question is about the determination of the C saturation, you can consult the original (Six et al. 2002) and subsequent publications by the proponents of the theory (e.g., Stewart et al., 2007).