The MERLIN model was applied on the results of a field-scale manipulation experiment with decreased nitrogen (N) deposition in an N saturated forest ecosystem in the Netherlands. The aim was to investigate the mechanisms that could explain the observed rapid response of nitrate as a result of the decreased N input. Calibrating the model to pre-treatment data revealed that, despite the high atmospheric N input, the trees relied on N mineralised from refractory organic matter (ROM) for their growth. MERLIN could simulate only the fast response of nitrate leaching after decreased input if this ROM mineralisation rate was decreased strongly at the time of the manipulation experiment.