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

摘要

The N cycle in forests ofthe temperate zone in Europe has been changed substantially by the impact ofatmospheric N deposition. Here, the fluxes and concentrations of mineral N inthroughfall, soil solution and runoff in two German catchments, receiving high Ninputs are investigated to test the applicability of an Integrated NitrogenModel for European Catchments (INCA) to small forested catchments. TheLehstenbach catchment (419 ha) is located in the German Fichtelgebirge (NOBavaria, 690-871 m asl.) and is stocked with Norway spruce (Picea abies (L.) Karst.) of different ages. The Steinkreuzcatchment (55 ha) with European beech (Fagus sylvatica L.) as thedominant tree species is located in the Steigerwald (NW Bavaria, 400-460 m asl.).The mean annual N fluxes with throughfall were slightly higher at theLehstenbach (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. AtLehstenbach, the N output with seepage at 90 cm soil depth was similar to the Nflux with throughfall. At Steinkreuz more than 50 % of the N deposited wasretained in the upper soil horizons. In both catchments, the NO₃fluxes with runoff were lower than those with seepage. The average annual NO₃concentrations 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 modelcomplexity. In both catchments, the NO₃ concentrations and fluxes inrunoff were matched well by the model. The seasonal patterns with lower NO₃runoff concentrations in summer at the Lehstenbach catchment were replicated.INCA underestimated the increased N₃ concentrations during shortperiods of rewetting in late autumn at the Steinkreuz catchment. The model willbe a helpful tool for the calculation of “critical loads?? for the Ndeposition in Central European forests including different hydrological regimes. style="line-height: 20px;">Keywords: forest ecosystem, modelling, N budgets, N saturation, NO₃ leaching, water quality, INCA