A Forest Site Nitrogen Dynamics Model for Land Application of Sludge

Abstract

The application of municipal sewage sludge to forests may raise nitrate-nitrogen concentrations in percolating groundwater. In agricultural systems, it is usually assumed that the processes governing nitrogen concentrations in leachate are relatively short-term, and that other contaminants, such as heavy metals, limit long-term application rates. These assumptions may not be appropriate for forests because the nitrogen levels in these systems change over time and harvests in most forests are relatively infrequent. We have modified a computer model from the 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 with 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 are 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 anaerobically digested sludge (225 kg/ha total nitrogen) is applied at three-year intervals to a 31-year-old site, the model predicts that leaching nitrate-nitrogen concentrations will respect the 10 mg/L drinking water standard for nitrogen-N in 99% of all years.