Soil organic matter (SOM) is the largest reservoir of terrestrial carbon and plays an important role in the global carbon cycle. Carbon isotope systematics of SOM have been widely used to constrain the dynamics of this important carbon reservoir though interpretation of carbon isotope data remains controversial. It has been widely observed that the 13C/12C ratio of SOM increases systematically with soil depth though there is little consensus as to which of several mechanisms may be responsible for this pattern. Here we present a process-based theoretical model of steady state δ13CSOM versus depth profiles, which is coupled with a carbon concentration model. We show that widely observed δ13CSOM versus depth profiles can be modeled using a reasonable range of model parameters. More importantly, we show that coupling carbon isotope data with carbon concentrations in soils allows for tighter constraints on model parameters that have biological and environmental significance.