|Langusch, J; Matzner, E: N fluxes in two nitrogen saturated forested catchments in Germany: Dynamics and modelling with INCA, Hydrology and Earth System Sciences, 6, 383-394 (2002)|
The N cycle in forests of the temperate zone in Europe has changed substantially by the impact of atmospheric N deposition. Here we investigate the fluxes and concentrations of mineral N in throughfall, soil solution and runoff in two German watersheds, receiving high N inputs. Our aim was to test the applicability of the INCA model (Integrated Nitrogen Model for European Watersheds) to small forested watersheds. The Lehstenbach watershed (419 ha) is located in the German Fichtelgebirge (NO Bavaria, 690-871 m asl.) and is stocked with Norway spruce (Picea abies (L.) Karst.) of different age. The Steinkreuz watershed (55 ha) with European beech (Fagus sylvatica L.) as the dominating tree species is located in the Steigerwald (NW Bavaria, 400-460 m asl.). The mean annual N fluxes with throughfall were slightly higher at the Lehstenbach (24.6 kg N ha-1) than at the Steinkreuz (20.4 kg N ha-1). In both watersheds the N fluxes in the soil are dominated by NO3. At Lehstenbach the N output with seepage in 90 cm soil depth was similar to the N flux with throughfall. At Steinkreuz more than 50 % of the deposited N are retained in the upper soil horizons. In both watersheds the NO3 fluxes with runoff were lower than those with seepage. The average annual NO3 concentrations in runoff which were in both watersheds between 0.7 to 1.4 mg NO3-N L-1 and no temporal trend was observed. The N budgets at the watershed scale indicated similar amounts of N retention (Lehstenbach: 19 kg N ha-1 a-1; Steinkreuz: 17 kg N ha-1 a-1). The parameter settings of the INCA model were simplified to reduce the model complexity. In both watersheds the NO3 concentrations and fluxes in runoff were matched well by the model. The seasonal patterns with lower NO3 runoff concentrations in summer at the Lehstenbach watershed was replicated. INCA underestimated the increased NO3 concentrations during short periods of rewetting in late autumn at the Steinkreuz watershed. The model will be a helpful tool for the calculation of "critical loads" for the N deposition in Central European forests including different hydrological regimes.