1 Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), CH-8903 Birmensdorf, Switzerland
Increased deposition rates were simulated in a small catchment within a spruce (Picea abies) forest at Alptal (central Switzerland, 1200 m a.s.l.). This treatment was applied by sprinkling rain water enriched with NH4NO3 (+20-25 kg/ha/a N) and compared with a control catchment receiving only rain water (12 kg/ha/a bulk N deposition) and where NO3--N leaching is approximately 3 kg/ha/a.
During rain events, NO3- concentration in runoff water was correlated with the water discharge, both showing simultaneous peaks. Labelling with 15N showed that these peaks partly arise from NO3- deposited during the rain event itself. Preferential and lateral water flow promote very fast leaching in the gleyic soils of the site. NO3- losses thus doubled after only few weeks of the N addition experiment. These losses increased over time to reach roughly one third of the added N after 10-15 years. In the soil, both NO3-, NH4+ ions are rapidly immobilised, less than 2% still being extractable after one week. During the same time, an equilibrium between NH4+ and NO3- is reached, independently of which ion was deposited (or added as 15N tracer). The 15N signal disappears from NO3- leaching within weeks as labelling stops. At the same time, the hydrolysability of recently added N in the soil is comparable to that of native N. N immobilised in the soil thus appears to be stable in the long term and still accounts for most of the N retained in the ecosystem and induces a decrease in the C:N ratio of the top soil by one unit every 2 to 3 years.
The N saturation induced by the experimentally increased deposition also affects the greenhouse gas fluxes of the forest. Three years of measurements with static chambers showed increased emissions of nitrous oxide. A tendency to reduce soil respiration was also observed.