Simulation of increased nitrogen deposition to a montane forest ecosystem: partitioning of the added 15N

Schleppi Patrick 1, Bucher-Wallin Inga 2, Siegwolf Rolf 2, Saurer Matthias 2, Muller Nathalie 1, Bucher Jürg B. 1

1 Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), CH-8903 Birmensdorf,
2 Paul Scherrer Institute, CH-5232 Villigen, Switzerland

Water Air Soil Pollut. 116 (1999): 129-134

DOI: 10.1023/A:1005206927764


Nitrogen (N) was added over two years to a spruce-dominated (Picea abies) montane forest at Alptal,central Switzerland. A solution of ammonium nitrate (NH4NO3) was frequently sprinkled on the forest floor (1500 m2) to simulate an additional N deposition of 30 kg N ha-1 year-1 over the ambient 12 kg bulk inorganic N deposition. The added nitrogen was labelled with 15NH415NO3 during the first year. Results are compared to a control plot.
Neither the trees nor the ground vegetation showed any increase in their N content. Only 4.1% of N in the ground vegetation came from the N-addition. Current-year needles contained 1.1 mg N g-1 d.wt.,of which only 2% was from labelled N; older needles had approx. half as much 15N. The uptake from the treatment was therefore very small. Redistribution of N also took place in the trunks: 1 to 2-year-old wood contained 0.7% labelled N,tree rings dating back 3 to 14 years contained 0.5%.
Altogether,the above-ground vegetation took up 12% of the labelled N. Most 15N was recovered in the soil: 13% in litter and roots,63% in the sieved soil. Nitrate leaching accounted for 10%. Factors thought to be influencing N uptake are discussed in relation to plant use of N and soil conditions.

Keywords: nitrogen deposition,montane forest,Picea abies,N-15 isotope,nitrate leaching