A FOREST SITE NITROGEN DYNAMICS MODEL FOR LAND APPLICATION OF SLUDGE

The application of municipal sewage sludge to forests may raise nitrat e-nitrogen concentrations in percolating groundwater. In agricultural systems, it is usually assumed that the processes governing nitrogen c oncentrations in leachate are relatively short-term, and that other co ntaminants, such as heavy metals, limit long-term application rates. T hese assumptions may not be appropriate for forests because the nitrog en levels in these systems change over time and harvests in most fores ts are relatively infrequent. We have modified a computer model from t he ecology literature to investigate the long-term impact of nitrogen additions on groundwater quality in sludge amended forests. The model is descended from previous models of forest dynamics. It was tested wi th data from natural and sludge amended northeastern forests, and was used to design long-term loading rates for a northern hardwood forest in New Hampshire. Higher loading rates are possible if applications ar e made at multiple-year intervals as added nitrogen is immobilized in the soil and in accumulated litter. If 4.5 Mg/ha (dry weight) of anaer obically digested sludge (225 kg/ha total nitrogen) is applied at thre e-year intervals to a 31-year-old site, the model predicts that leachi ng nitrate-nitrogen concentrations will respect the 10 mg/L drinking w ater standard for nitrogen-N in 99% of all years.