Determining Sludge Fertilization Rates for Forests from Nitrate‐N in Leachate and Groundwater

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Abstract
Municipal and papermill wastewater sludges were applied to conifer and hardwood forests growing on sand soils (Entic Haplorthods, Spodic Udipsamments, and Alfic Haplorthods), in northwestern Lower Michigan where annual precipitation averages 765 mm/y, to investigate the impact of sludge on nitrate‐N concentrations in soil water and groundwater. During the first growing season after treatment, all forms of N in groundwater and soil water remained near background levels except under aspen (Populus grandidentata Michx.) plots treated with 46 dry Mg municipal sludge/ha. Following the first spring snowmelt period after sludge was applied (spring 1977), nitrate‐N concentrations exceeded the 10 mg/L potable water standard in groundwater under plots treated with 16 or more dry Mg/ha undigested papermill sludge. Soil leachate exceeded 10 mg/L under pine plantations receiving 19.3 dry Mg/ha and aspen sprout stands receiving 23 dry Mg/ha or more anaerobically digested municipal sludge.Sludge application rates which would not degrade water quality in water‐table aquifers were estimated by regression analysis of nitrate (NO3−) concentrations in relation to dry solids loading. Applying raw papermill sludge at rates not exceeding 9.5 dry Mg/ha (670 kg total N/ha) to a red pine (Pinus resinosa Ait.) plantation was found to be consistent with levels that met the 10 mg/L potability standard in groundwater. These analyses showed anaerobically digested municipal sludge could be applied to a red pine and white pine (Pinus strobus L.) plantation at 16.5 dry Mg/ha (990 kg total N/ha) or less and to aspen sprouts at rates up to 19 dry Mg/ha (1140 kg total N/ha) with the same water quality limits.

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