SUSTAINABILITY OF SEWAGE-SLUDGE LAND APPLICATION TO NORTHERN HARDWOODFORESTS

A model representing the nitrogen dynamics of a northern hardwood fore st is amended to include a mass balance and to predict percolating nit rate-nitrogen concentrations. A component for including land applicati on of municipal sewage sludge is added and Monte Carlo simulation used to predict the long-term impact of land application on stand developm ent, soil humus dynamics, nitrogen saturation, and nitrate losses at H ubbard 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 control s. An application rate of 8 Mg/ha results in little additional biomass increase but significantly threatens groundwater and surface water qu ality. Repeated applications significantly increase the nitrogen in so il humus. Species representation is not affected significantly unless sludge is applied after harvest during forest reorganization. Sustaina ble loading rates are on the order of 3-4 Mg/ha if sludge is applied a t 3-yr intervals.