NITRIFICATION IN SLUDGE-AMENDED MICHIGAN FOREST SOILS

Net nitrification following liquid sludge application to four Michigan forest types was studied by aerobically incubating intact cores containing the forest floor and upper 10 cm of mineral soil. Significant net nitrification occurred in cores receiving surface applications of anaerobically digested municipal sludge (22.3 g N m-2). Core NO3-N contents at 8 wk were 4.7, 5.3, 4.9, and 2.0 g m-2 for aspen (Populus spp.), northern hardwoods, oak (Quercus spp.), and pine (Pinus spp.) forest types, respectively. Net nitrification did not occur during 8-wk incubations of untreated control cores or cores receiving surface applications of sludge sterilized by freeze-drying (24.5 g N m-2) or liming (25.7 g N m-2), suggesting that the nitrifiers responsible for net nitrification in cores receiving the anaerobically digested sludge were added with the sludge. Nitrate content at 8 wk in cores from an oak field site receiving anaerobically digested sludge (43.0 g N m-2) 33 mo prior to incubation was 72% of that in oak site soil cores treated immediately prior to incubation (22.3 kg N m-2), indicating that nitrifying populations introduced with sludge 33 mo prior were still viable. Nitrate content at 8 wk in oak cores was 77% lower when sludge was incorporated. Nitrate contents in incubated soil cores could not be used to directly predict NO3 concentrations in soil leachate and groundwater beneath sludge-treated plots at the four field sites. Results indicate that nitrification and potential for NO3 leaching following sludge application to acid forest soils are influenced by sludge type, forest type, and history of prior sludge applications.