Modeling the nitrogen dynamics of sludge-amended forests.

摘要

In recent years the sludge disposal options available to many municipalities in the northeastern United States have been sharply curtailed. Ocean dumping has been banned, landfill space is in short supply, and incinerators have proven expensive to operate and difficult to site. The search for disposal alternatives has led to consideration of forests for landspreading operations. Sewage sludges contain significant levels of nutrients, particularly nitrogen, that may accelerate the growth of some northeastern forests. If sewage sludge is loaded too heavily, however, nitrogen can leach in the form of nitrate to pollute groundwater. This threat may increase as sludge applications are repeated.; This dissertation presents two models for designing sewage sludge loading rates for forests. The first is a detailed nutrient cycling model called FORSENTO which simulates the long-term development of sludge amended forests. FORSENTO uses Monte-Carlo simulation to estimate the nitrate-nitrogen concentrations in leachate from a 10m by 10m forest plot. Parameter values are given for northern hardwood forests. The model is used to examine the significance of organic matter accretion, fine root dynamics, and litter decay rates on groundwater nitrate concentrations. If sludge is applied at three year intervals, loading rates for northern hardwood forests across New York State are found to range between 3.8 Mg/ha and 7.9 Mg/ha (dry solids) for aerobically and anaerobically digested sludges depending on site precipitation and evapotranspiration patterns, and the frequency with which nitrate-nitrogen concentrations in percolating groundwater are allowed to exceed the 10 mg/l drinking water standard.; The second model uses a steady-state assumption to create a relatively simple design tool. Steady-state nitrogen dynamics may occur after many repeated sewage sludge applications are made to a mature forest. Closed form solutions for determining sewage sludge application rates are presented for a number of hydrological conditions. This model shows that a mature northern hardwood forest may receive little or no benefit from repeated landspreading operations so that most or all sludge nitrogen leaches to pollute groundwater. The results suggest that long-term land application systems located in New York State hardwood forests are not sustainable.