Groundwater and Soil Leachate Inorganic Nitrogen in a Wisconsin Red Pine Plantation Amended with Paper Industry Sludge

In late August 1984, a 2.5:1 primary/secondary paper industry sludge was applied to a row‐thinned, 27‐yr‐old red pine (Pinus resinosa Ait.) plantation in central Wisconsin at levels of 32, 63, and 94 dry Mg ha−1 (550, 1120, and 1670 kg N ha−1, respectively). There were positive treatment effects on NO3‐N and NH4‐N in soil leachates and groundwater, but because of sample variability, few of these differences were significant at the p ≤ 0.05 level. Nitrate‐N concentrations exceeded 10 mg L−1 for all treatments at the 10‐, 45‐, and 75‐cm depths, but the potability standard was exceeded at the 165‐cm depth and in groundwater only on plots receiving the medium and heavy applications. Nitrate‐N concentrations generally were greater in the upper 30 cm of the saturated zone than at 60 and 90 cm, suggesting a lack of vertical mixing. Whereas the maximum NO3‐N concentration decreased with depth, the time required for NO3‐N concentrations to exceed 10 mg L−1 for a given treatment and the duration of the period when NO3‐N concentrations exceeded 10 mg L−1 increased with depth. The seasonal trends in inorganic N concentrations in soil water collected at the 10‐ and 45‐cm depths could be explained on the basis of N mineralization as derived from field incubation studies. A regression equation relating maximum NO3‐N in groundwater to N content of the applied sludge indicates that little groundwater degradation will occur at depths of ≥ 5.2 m in central Wisconsin when the N application to pole‐sized red pine is ≤800 kg ha−1 (≤46 dry Mg sludge ha−1).