title: 15N-ammonium and15N-nitrate uptake of a 15-year-old Picea abies plantation.
abstract: Throughfall nitrogen of a 15-year-old Picea abies (L.) Karst. (Norway spruce) stand in the Fichtelgebirge, Germany, was labeled with either15N-ammonium or15N-nitrate and uptake of these two tracers was followed during two successive growing seasons (1991 and 1992).15N-labeling (62 mg15N m-2under conditions of 1.5 g N m-2atmospheric nitrogen deposition) did not increase N concentrations in plant tissues. The15N recovery within the entire stand (including soils) was 94%±6% of the applied15N-ammonium tracer and 100%±6% of the applied15N-nitrate tracer during the 1st year of investigation. This decreased to 80%±24% and 83%±20%, respectively, during the 2nd year. After 11 days, the15N tracer was detectable in 1-year-old spruce needles and leaves of understory species. After 1 month, tracer was detectable in needle litter fall. At the end of the first growing season, more than 50% of the15N taken up by spruce was assimilated in needles, and more than 20% in twigs. The relative distribution of recovered tracer of both15N-ammonium and15N-nitrate was similar within the different foliage age classes (recent to 11-year-old) and other compartments of the trees.15N enrichment generally decreased with increasing tissue age. Roots accounted for up to 20% of the recovered15N in spruce; no enrichment could be detected in stem wood. Although15N-ammonium and15N-nitrate were applied in the same molar quantities (15NH4+:15NO3-=1:1), the tracers were diluted differently in the inorganic soil N pools (15NH4+/NH4+:15NO3-/NO3-=1:9). Therefore the measured15N amounts retained by the vegetation do not represent the actual fluxes of ammonium and nitrate in the soil solution. Use of the molar ammonium-to-nitrate ratio of 9:1 in the soil water extract to estimate15N uptake from inorganic N pools resulted in a 2-4 times higher ammonium than nitrate uptake by P. abies.
authors.affiliation: Lehrstuhl Pflanzenökologie, Universität Bayreuth, Postfach 10 12 51, D-95440, Bayreuth, Germany.
authors.forename: N
authors.initials: N
authors.lastname: Buchmann
authors.affiliation: Lehrstuhl Pflanzenökologie, Universität Bayreuth, Postfach 10 12 51, D-95440, Bayreuth, Germany.
authors.forename: E-D
authors.initials: ED
authors.lastname: Schulze
authors.affiliation: Lehrstuhl Pflanzenökologie, Universität Bayreuth, Postfach 10 12 51, D-95440, Bayreuth, Germany.
authors.forename: G
authors.initials: G
authors.lastname: Gebauer
keywords: 15N; Ammonium; Nitrate; Picea abies (L.) Karst; Tracer