Uptake of [15N] Ammonium and [15N]Nitrate in a 140-Year-Old Spruce Stand (Picea abies) in the Fichtelgebirge (NE Bavaria)

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Abstract
In April 1994 a 15N tracer pulse study was started in a 140-year-old spruce stand (Picea abies [L.] Karst.) located at the Fichtelgebirge (NE Bavaria). Highly enriched (98%) [15N]ammonium and [15N]nitrate were applied simulating wet deposition. For two growing seasons the pathways and dynamics of the tracer were followed in all compartments of spruce (needles and twigs of all age classes, stem wood and bark, roots) and understorey vegetation and in soils of the organic (L/Of and Oh) and mineral horizons (A0–5 and A5–10). By variations of the application time on different plots within the growing season (spring, summer and autumn) a seasonal effect of labelling on uptake and distribution patterns was tested. First results of this tracer study indicate that young and old spruce stands do not differ basically in pattern of uptake and distribution of mineral nitrogen. There are indications that spruce uses preferentially ammonium versus nitrate and that the ratio of ammonium/nitrate which is being consumed depends on the ammonium/nitrate ratio in the soil solution. The uptake rates decrease within the growing season.