Development, testing, and dynamics of a vertically-resolved C and N model in CLM4

We have added a vertical dimension to the soil biogeochemistry model of CLM4, the land component of CESM, in order to assess the role of subsurface biogeochemical processes on the global carbon cycle. The new model includes full treatment of carbon and nitrogen interactions, including controls on SOM turnover, nitrification, and denitrification by depth-dependent temperature, moisture, and redox state. We present a description of the model structure; baseline simulation results of steady-state C, 14C, and N distributions; and comparison against worldwide data. The model realistically predicts the soil organic matter vertical distribution and the vertical distribution of 14C at the sites tested, indicating that SOM production and decomposition are reasonably represented in the model. We also present NO3- leaching and N2O surface efflux predictions and comparison with observations, and discuss improvements to the SOM model associated with improving N cycle dynamics in the model.