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