N fluxes in two nitrogen saturated forested catchments in Germany: dynamics and modelling with INCA

Abstract. The N cycle in forests of the temperate zone in Europe has been changed substantially by the impact of atmospheric N deposition. Here, the fluxes and concentrations of mineral N in throughfall, soil solution and runoff in two German catchments, receiving high N inputs are investigated to test the applicability of an Integrated Nitrogen Model for European Catchments (INCA) to small forested catchments. The Lehstenbach catchment (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 ages. The Steinkreuz catchment (55 ha) with European beech (Fagus sylvatica L.) as the dominant 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 catchments the N fluxes in the soil are dominated by NO₃. At Lehstenbach, the N output with seepage at 90 cm soil depth was similar to the N flux with throughfall. At Steinkreuz more than 50 % of the N deposited was retained in the upper soil horizons. In both catchments, the NO₃ fluxes with runoff were lower than those with seepage. The average annual NO₃ concentrations in runoff in both catchments were between 0.7 to 1.4 mg NO₃-N L^-1 and no temporal trend was observed. The N budgets at the catchment scale indicated similar amounts of N retention (Lehstenbach: 19 kg N ha^-1yr^-1 ; Steinkreuz: 17 kg N ha^-1yr^-1). The parameter settings of the INCA model were simplified to reduce the model complexity. In both catchments, 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 catchment were replicated. INCA underestimated the increased N₃ concentrations during short periods of rewetting in late autumn at the Steinkreuz catchment. 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.

Keywords: forest ecosystem, modelling, N budgets, N saturation, NO₃ leaching, water quality, INCA