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

Nitrogen (N) was added over two years to a spruce-dominated (Picea abies) montane forest at Alptal, central Switzerland. A solution of ammonium nitrate (NH4NO2) was frequently sprinkled on the forest floor (1500 m(2)) to simulate an additional input of 30 kg N ha(-1) yr(-1) over the ambient 12 kg bulk inorganic N deposition. The added nitrogen was labelled with (NH4NO3)-N-15-N-15 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 11 mg N g(-1) dry weight, of which only 2% was from labelled N; older needles had approximately half as much N-15. 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 0.4%. Altogether, the above-ground vegetation took up 12% of the labelled N. Most N-15 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.