PARTITIONING OF N-15-LABELED AMMONIUM AND NITRATE AMONG SOIL, LITTER,BELOWGROUND AND ABOVEGROUND BIOMASS OF TREES AND UNDERSTORY IN A 15-YEAR-OLD PICEA-ABIES PLANTATION

The partitioning of nitrogen deposition among soil, litter, below- and above-ground biomass of trees and understory vegetation was investiga ted in a 15-year-old Picea abies (L.) Karst. plantation in the Fichtel gebirge, Germany, by labeling with 62 mg of N-15 tracer per square met er in March 1991. Ammonium and nitrate depositions were simulated on f ive plots each, by labeling with either N-15-NH4+ or N-15-NO3-, and th e N-15 pulse was followed during two successive growing seasons (1991 and 1999). Total recovery rates of the N-15 tracer in the entire stand ranged between 93 and 102% for both nitrogen forms in 1991, and 82% i n June 1992. delta(15)N ratios increased rapidly in all compartments o f the ecosystem. Roots and soils (to 65 cm depth) showed significant N -15 enrichments for both N-15-treatments compared to reference plots. Newly grown spruce tissues were more enriched than older ones, but the most enriched delta(15)N values were found in the understory vegetati on. Although spruce trees were a much larger pool (1860 g biomass/m(2) ) than understory vegetation (Vaccinium myrtillus 333 g/m(2), Calluna vulgaris 142 g/m(2), Deschampsia flexuosa 22 g/m(2)), the ericaceous s hrubs and the perennial grass were a much greater sink for the N-15 la bel. Eight months after labeling, 9% of the ammonium and 15% of the ni trate label were found in the understory. P. abies retained only 3% of the N-15-ammonium and 7% of the N-15-nitrate. The main sink for both N-15 tracers was the soil, where 87% of the ammonium and 79% of the ni trate tracer were found. The organic soil horizon (5-0 cm depth) conta ined 63% of the N-15-ammonium and 46% of the N-15-nitrate suggesting s trong immobilization by microorganisms of both N forms. Eight months a fter tracer application, about 16% of both N-15-tracers was found belo w 25 cm soil depth. This 16% corresponds well to a 20% decrease in the recovery of both N-15 tracers after 15 months and indicates a total l oss out of the ecosystem. Highly enriched delta(15)N values were found in fruit bodies of fungi growing in reference plots (no N addition), although soils did not show increased delta(15)N ratios. NO transfer o f N-15-tracer between fungi and spruce or understory vegetation was ap parent yet.