Nitrogen was added over three years to a spruce-dominated (Picea abies) forest at Alptal, central Switzerland. The site (altitude 1200 m) is moderately affected by atmospheric nitrate and ammonium deposition (12 kg N ha^-1 year^-1). Gleysols occur atop a Flysch substratum and bear different humus types with various botanical associations. This patchy vegetation is related to the micro-topography and to light conditions. An ammonium nitrate solution was chronically sprinkled over a plot (approx. 1500 m²) to simulate a deposition increase of 30 kg N ha^-1 year^-1. This treatment was compared to both a control plot and a year of pre-treatment measurements.
The ground vegetation was monitored in permanent quadrates. Its botanical composition has remained unaffected to date. During the first two years, foliage analyses of the most abundant species showed only inconsistent variations in their nitrogen content. Tree growth was measured by dendrometers installed on trunks and by photographs of crowns. Leaf area index (3.8 in average) was measured with a plant canopy analyser and litterfall collected fortnightly. None of these parameters revealed any effect of nitrogen addition. Current-year spruce needles contained 1.1% N. In spite of this slight N deficiency, no increase was measured during 3 years of N addition.
Two factors probably explain the unchanged N content in plants. First, N taken up from the treatment was diluted into a large biomass, delaying any effect on the trees and on the ground vegetation (all perennial species). Second, the uptake itself was small: 12% of the N addition going into aboveground biomass, as shown by ¹⁵N labelling. This, in turn, is attributed to other factors limiting root growth and nitrogen uptake: short period of vegetation, soil anaeroby, phosphorus deficiency, nitrate leaching, denitrification and effective fixation of ammonium in clay minerals. The relative importance of these factors is discussed for this site.

Nathalie Muller

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