An implicit time-stepping model for decomposition, nitrification, denitrification, and plant nutrient uptake is implemented in the land ecosystem component of a coupled Earth System Model. The new approach replaces the original explicit time-stepping decomposition module, and retains the same coupled carbon-nitrogen mechanisms of the explicit scheme. The ability of the new approach to reproduce the dynamics of the original model is demonstrated. The new approach is also shown to be more flexible, enabling evaluation of alternative hypotheses regarding, for example, the patterns of competition between plants and microbes for available soil nutrient resources. Several numerical challenges are identified and explored, with results that provide general guidance for future development and extension of this or other implicit time-stepping soil and plant biogeochemistry schemes. In particular, this work demonstrates robust operation at very low predicted concentrations of reactive species.