, Volume 33, Issue 1, pp 1-23

First online:

Partitioning of 15N-labeled ammonium and nitrate among soil, litter, below- and above-ground biomass of trees and understory in a 15-year-old Picea abies plantation

  • Nina BuchmannAffiliated withLehrstuhl Pflanzenökologie, Universität BayreuthDepartment of Biology, University of Utah
  • , Gerhard GebauerAffiliated withDepartment of Biology, University of Utah
  • , Ernst-Detlef SchulzeAffiliated withDepartment of Biology, University of Utah

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The partitioning of nitrogen deposition among soil, litter, below- and above-ground biomass of trees and understory vegetation was investigated in a 15-year-old Picea abies (L.) Karst. plantation in the Fichtelgebirge, Germany, by labeling with 62 mg of15N tracer per square meter in March 1991. Ammonium and nitrate depositions were simulated on five plots each, by labeling with either15N-NH4 + or15N-NO3 , and the15N pulse was followed during two successive growing seasons (1991 and 1992). Total recovery rates of the15N tracer in the entire stand ranged between 93 and 102% for both nitrogen forms in 1991, and 82% in June 1992. δ5 N ratios increased rapidly in all compartments of the ecosystem. Roots and soils (to 65 cm depth) showed significant15N enrichments for both15N-treatments compared to reference plots. Newly grown spruce tissues were more enriched than older ones, but the most enriched δ15N values were found in the understory vegetation. Although spruce trees were a much larger pool (1860 g biomass/m2) than understory vegetation (Vaccinium myrtillus 333 g/m2, Calluna vulgaris 142 g/m2, Deschampsia flexuosa 22 g/m2), the ericaceous shrubs and the perennial grass were a much greater sink for the15N label. Eight months after labeling, 9% of the ammonium and 15% of the nitrate label were found in the understory. P.abies retained only 3% of the15N-ammonium and 7% of the15N-nitrate. The main sink for both15N tracers was the soil, where 87% of the ammonium and 79% of the nitrate tracer were found. The organic soil horizon (5-0 cm depth) contained 63% of the15N-ammonium and 46% of the15N -nitrate suggesting strong immobilization by microorganisms of both N forms. Eight months after tracer application, about 16% of both15N-tracers was found below 25 cm soil depth. This 16% corresponds well to a 20% decrease in the recovery of both15N tracers after 15 months and indicates a total loss out of the ecosystem. Highly enriched δ15N values were found in fruit bodies of fungi growing in reference lots (no15N addition), although soils did not show increased δ15N ratios. No transfer of15N-tracer between fungi and spruce or understory vegetation was apparent yet.

Key words

15N-tracers ammonium ecosystem fungi nitrate Picea abies (L.) Karst understory competition