Nitrogen Retention Capacity of a Northern Hardwood Forest Soil Under Ammonium Sulfate Additions

To determine the N‐retaining capacity of a beech‐dominated stand in the Hubbard Brook Experimental Forest, we added (NH 4 ) 2 SO 4 at three levels (40, 160 and 520 kg N°ha ‐ 1 °yr ‐ 1 ) to replicate plots in monthly increments for two years, and measured NH 4 + and NO 3 ‐ concentrations in monthly samples from zero‐tension lysimeters. The two lower treatment levels failed to increase N loss through the deepest soils horizon when compared to controls. Plots under the highest treatment level retained >95% of the added N; NH 4 + accounted for 77% of the leaching loss. The treatments caused an accumulation of exchangeable NH 4 + in the soil, but concentrations of exchangeable NO 3 ‐ remained small, even after samples were incubated at room temperature. Leaching, uptake by plants, and the increase exchangeable NH 4 + accounted for 3‐25% of the added N. Accumulation of available N without nitrification contradicts common expectations concerning N saturation. It was not clear why excess N was not nitrified.