N-15-AMMONIUM AND N-15-NITRATE UPTAKE OF A 15-YEAR-OLD PICEA-ABIES PLANTATION

Throughfall nitrogen of a 15-year-old Picea abies (L.) Karst. (Norway spruce) stand in the Fichtelgebirge, Germany, was labeled with either N-15- ammonium or N-15-nitrate and uptake of these two tracers was followed during two successive growing seasons (1991 and 1992). N-15-labeling (62 mg N-15 m(-2) under conditions of 1.5 g N m(-2) atmospheric nitrogen deposition) did not increase N concentrations in plant tissues. The N-15 recovery within the entire stand (including soils) was 94% +/- 6% of the applied N-15- ammonium tracer and 100% +/- 6% of the applied N-15- nitrate tracer during the Ist year of investigation. This decreased to 80% +/- 24% and 83% +/- 20%, respectively, during the 2nd year. After 11 days, the N-15 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 the N-15 taken up by spruce was assimilated in needles, and more than 20% in twigs. The relative distribution of recovered tracer of both N-15-ammonium and N-15-nitrate was similar within the different foliage age classes (recent to ii-year-old) and other compartments of the trees. N-15 enrichment generally decreased with increasing tissue age. Roots accounted for up to 20% of the recovered N-15 in spruce; no enrichment could be detected in stem wood. Although N-15-ammonium and N-15-nitrate were applied in the same molar quantities ((NH4+)-N-15:(NO3-)-N-15 =1:1), the tracers were diluted differently in the inorganic soil N pools ((NH4+)-N-15/NH4+: (NO3-)-N-15/NO3- = 1:9). Therefore the measured N-15 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 estimate N-15 uptake from inorganic N pools resulted in a 2-4 times higher ammonium than nitrate uptake by P. abies.