To shed light on the interactions between soil nitrogen (N) and carbon (C), we set up a model “experiment” in silico to test several hypothesized responses of decomposers to N. We found that decomposers were stimulated by N when decomposing high C:N detritus, but inhibited when decomposing low C:N, processed organic C. The consequence is that under exogenous N addition (e.g., contemporary N deposition), forests may accumulate light fraction C predominantly, at the expense of coarse detritus.