Controls on N retention and exports in a forested watershed

We conducted a N-15-tracer study in a fertilized, forested catchment at the Bear Brook Watersheds in Maine (BBWM), USA, in order to characterize N cyc ling processes, identify sinks for ammonium-N additions, and determine the contribution of the experimental ammonium additions to nitrate exports fi o m the treated catchment. Distributions of N-15 in plant tissues, soils, pre cipitation and streamwater collected before adding tracers showed that nitr ate-N (the dominant form of inorganic N deposition at the st) inputs under ambient conditions were depleted in N-15 relative to plants and that soil w as enriched in N-15 relative to plants. The N-15 content of streamwater nit rate was within the range of N-15 contents in natural plant tissues, sugges ting that nitrate deposited from the atmosphere is reduced and assimilated into soil and plant N pools before being leached as nitrate from the catchm ent Variations in N-15 natural abundances also suggested that most N uptake by trees is from the forest floor and that nitrification occurs in soils a t this catchment under ambient conditions. Changes in N-15 contents of plan t tissues, soil and streamwater after adding a N-15 tracer to the ammonium sulfate fertilizer applied to the treated catchment showed that soils were the dominant sink for the labeled ammonium. Surface soils (Oea horizon pins my underlying mineral soil to 5cm depth) assimilated 19 to 31 percent of t he 42 kg ha(-1) of N-15-labelled ammonium-N during the tracer study. Aboveg round biomass assimilated 8 to 17 percent of the labeled ammonium-N additio ns. Of the three forest types on the catchment, the soil:biomass assimilati on ratio of labeled-N was highest in the spruce forest intermediate in the beech-dominated hardwood forest and lowest in the mixed hardwood-spruce for est Although ammonium sulfate additions led to increases in streamwater nit rate, only 2 of the 13 kg ha(-1) of nitrate-N exported from the catchment d uring the 2 years of tracer additions was derived from the 42 kg ha(-1) of labeled ammonium-N additions.