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 (NH4)(2)SO4 at three levels (40, 160 and 520 kg N . ha(-1). yr(-1)) to replicate plots in monthly increments for two years, and measured NH4+ and NO3- 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; NH4+ accounted for 77% of the leaching loss. The treatments caused an accumulation of exchangeable NH4+ in the soil, but concentrations of exchangeable NO3- remained small, even after samples were incubated at room temperature. Leaching, uptake by plants, and the increase exchangeable NH4+ 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.