DESIGN OF LONG-TERM SLUDGE-LOADING RATES FOR FORESTS UNDER UNCERTAINTY

A simple time series describing nitrate-nitrogen concentrations percol ating from a sludge-amended forest is presented for the case where app lications are made at several-year intervals. The time series converge s to a quasi-steady-state solution that can be solved for an applicati on rate limited by percolating nitrate-nitrogen concentrations. A chan ce constraint incorporates uncertainty associated with precipitation a nd evapotranspiration, the most important factors in determining the e xcess water available for leaching. Design loading rates for eight New York state forest regions are discussed. If applications occur at 3-y ear intervals, rates range from 0.2 to 5.3 Mg/ha dry weight depending on the design confidence level, local excess water patterns, forest ni trogen uptake, sludge type, and atmospheric nitrogen deposition rates. Results are compared to predictions made with FORSENTO, a comprehensi ve model for simulating sludge applications to northern hardwood fores ts. FORSENTO simulations suggest that mature hardwoods need only 12 kg /ha to support annually perennial material growth and that atmospheric nitrogen deposition may eventually meet or exceed needs of trees so t hat landspreading may not be sustainable indefinitely in some areas.