A model representing the nitrogen dynamics of a northern hardwood forest is amended to include a mass balance and to predict percolating nitrate‐nitrogen concentrations. A component for including land application of municipal sewage sludge is added and Monte Carlo simulation used to predict the long‐term impact of land application on stand development, soil humus dynamics, nitrogen saturation, and nitrate losses at Hubbard Brook Experimental Forest in New Hampshire. Simulations suggest that applying 4 Mg/ha (dry mass) of anaerobically digested sludge to a 30‐yr‐old forest at 3‐yr intervals for 50 yr may increase production of harvestable boles and branches by an average of 24.6% over controls. An application rate of 8 Mg/ha results in little additional biomass increase but significantly threatens groundwater and surface water quality. Repeated applications significantly increase the nitrogen in soil humus. Species representation is not affected significantly unless sludge is applied after harvest during forest reorganization. Sustainable loading rates are on the order of 3‐4 Mg/ha if sludge is applied at 3‐yr intervals.