Nitrogen saturation of alpine forest ecosystems: first results of the Swiss NITREX-project at Alptal

Schleppi Patrick 1, Bucher Jürg B. 1

1 Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Zürcherstr. 111, CH-8903 Birmensdorf, Switzerland

ProClim - AlpenForum Symposium The role of the hydrological cycle in mountain ecosystems, Zurich, CH, 16-17/11/1995


Abstract

Introduction
Nitrogen is an essential element for all living organisms but also a potential risk for natural and semi-natural terrestrial ecosystems, as experienced in regions with high civilisation-made N deposition. Potential dangers to forests are in the health of their trees (nutritional imbalances, higher sensitivity to stresses), in their biodiversity (outcompetition by nitrophilous species) and in the quality of the water delivered to streams and groundwater.
Nitrate leaching is considered a sign of nitrogen saturation, i.e. as a result of inorganic N inputs exceeding the plant demand (Aber et al., 1989). The saturation threshold can thus be considered as a critical load. In central and northern Europe, nitrate leaching has been found to occur for depositions above 9 to 25 kgN/ha/year (Dise & Wright, 1995).
The European project NITREX, initiated in 1991, addresses the question of N saturation through experimental manipulation of the deposition to coniferous stands (Wright et al., 1995). Fourteen institutes are involved and work on 8 sites ranging from almost pristine to highly impacted. These sites either receive additional nitrogen (to simulate higher deposition) or they are protected from high local inputs by roofs built under the canopy (removal of contaminated precipitation, replaced by clean irrigation water). Removal experiments all lead to a clear recovery of the soil water chemistry while the nutritional status of the trees takes longer to respond. Addition experiments tend to increase nitrate leaching, but this effect is extremely variable between sites. It probably depends on the dynamics of the soil organic matter.
Site and experiment
The Swiss contribution to NITREX is a collaboration between the Swiss Federal Institute for Forest, Snow and Landscape Research in Birmensdorf and the Institute for Terrestrial Ecology of the ETH Zurich. It is an addition experiment started in 1994 in the Prealps.
Three small plots (approx. 1500 m2) have been delimited by trenches within the headwater catchment of Erlenbach in the valley of Alptal(SZ), at an altitude of 1200m. Because of the impermeable Flysch sub-soil, the plots can be considered as mini-catchments. Two are situated in a forest dominated by Norway spruce (Picea abies), the third one is wetland (an abandoned unfertilised meadow). Local depositions are approximately 16 kgN/ha/year (Fischer-Riedmann, 1995), i.e. within the range of N critical loads for forests. Nitrate and ammonium each account for half of the wet deposition (12 kgN/ha/year), while dry deposition is mainly due to ammonia and nitrogen dioxide.
One of the forested plots is experimentally treated with NH4NO3. Forty kgN/ha/year are added to water and applied by sprinklers. This results in an increase of precipitation by 10%. Water is collected from precipitation on a plastic sheet and also applied, unaltered, to the control plot. The wetland mini-catchment remains untreated and serves mainly as a comparison with the whole Erlenbach catchment. The runoffs are measured continuously by V-notch weirs and proportional samples are taken for weekly analyses. The forest catchments were compared during one year before starting the nitrogen addition. These pre-treatment measurements were necessary because it was not possible to design an experiment with replications. They are the only results available at present.
Results and discussion
The runoff generated by the artificial mini-catchments responds quickly to precipitation. High runoff is associated with high water table along the water paths and with low electrical conductivity. This points to a preferential flow through macropores, and thus to little contact between water and mineral soil during high-runoff events. This may lead to nitrate losses not actually linked to an N saturation of the ecosystem. Intensive process studies are presently starting to clarify this point.
As a prerequisite to a nitrogen balance, we calculated the water balance of the forested mini-catchments. Estimating the evapotranspiration acording to Penman-Montieth, we found a slightly positive balance, with inputs exceeding outputs by 5-7%. This may be due to some infiltration but is actually within the expected range of precision of the estimates.
Both mini-catchments were very similar in their nitrogen discharge which amounted to 4 kgN/ha/year as nitrate, mainly during winter and spring. Important nitrate losses occurred during snowmelt events. These results are in agreement with measurements done for the Erlenbach catchment. Newer analyses, however, have shown that more N can be lost in dissolved organic form (DON) than as nitrate. The DON is therefore now also routinely analysed.
This preliminary monitoring showed that the mini-catchments are a valuable tool to study the water balance and the element cycles on this site. Compared with larger, natural catchments, they are better suited for manipulation experiments.

References:
- Aber J.D., Nadelhoffer K.J., Steudler P. & Melillo J., 1989: Nitrogen saturation in northern forest ecosystems. BioSci. 39: 378-386.
- Dise N.B. & Wright R.F., 1995: Nitrogen leaching from European forests in relation to nitrogen deposition. For. Ecol. Managem. 71: 153-161.
- Fischer-Riedmann A., 1995: Atmosphärische Konzentration und Deposition von N-haltigen Komponenten im Wald des hydrologischen Einzugsgebietes Erlenbach im Apltal. Diss. ETH Zürich 11035.
- Wright R.F., Roelofs J.G.M., Bredemeier M., Blanck K., Boxman A.W., Emmett B.A., Gundersen P., Hultberg H., Kjønaas O.J., Moldan F., Tietema A., van Breemen N. & van Dijk H.F.G., 1995: NITREX: responses of coniferous forest ecosystems to experimentally changed deposition of nitrogen. For. Ecol. Managem. 71: 163-169.