NITROGEN MOVEMENT UNDER A HARDWOOD FOREST AMENDED WITH LIQUID WASTE-WATER SLUDGE

Owing to the limited quantity of agricultural land available for sludge disposal near urban centers, forests are being considered as potential land application sites. A major environmental concern regarding forest sludge application is the potential for nitrate leaching and subsequent groundwater contamination. This study was conducted to determine the effect of different sludge application rates on soil nitrogen (N) movement in a mature. mixed hardwood forest in Maryland. Liquid municipal sewage sludge was applied to a forest al dry rate equivalents of 0, 3, 6, and 12 Mg ha-1 and loading rates of 0, 200, 400, or 800 kg total N ha-1 (one time application ). Soil percolate samples were collected from November 1986 to June 1988 using suction lysimeters placed at 80 cm soil depth and analyzed for NO3-N. Soil was also sampled to a depth of 180 cm and analyzed for NO3-N each spring and fall from 1986 to 1988. One year after application, NO3-N concentrations in the soil percolate from plots receiving 200 kg N ha-1 had returned to baseline levels. For the 400 and 800 kg N ha-1 sludge treatments, mean NO3-N concentrations in soil percolate fluctuated around 40 mg l-1 for the first year after sludge application. Two years after sludge application, mean NO3-N concentrations in soil percolate from plots receiving 800 kg N ha-1 sludge application, exceeded the control concentrations. A comparison of NO3-N and Cl leaching patterns indicated that net nitrification was greater under the medium than under the high sludge treatment. Two years after sludge application, NO3-N continued to be significantly increased by the sludge application to a soil depth of 120 cm. Based on nitrate levels observed in the soil profile and soil water, applications of sludge to a similar hardwood forest at rates of 200 kg ha-1 of nitrogen should not increase nitrates in the groundwater.