MODELING SEASONAL NITROGEN, CARBON, WATER AND HEAT DYNAMICS OF THE SOLLING SPRUCE STAND

Daily flows of water, heat, carbon and nitrogen from 1976 until 1991 were simulated with the one-dimensional models SOIL-SOILN for a 105-year-old spruce stand on an acid brown forest soil at Selling (central Germany). Nitrogen deposition in the area exceeded 40 kg N ha(-1) yr(-1). Comparisons were made with measured soil water tensions, chloride and mineral nitrogen concentrations in soil water, and tree biomass components. Down to GO-cm depth, the simulated soil water tensions agreed well with measured data when a summer surface resistance of 125 s m-1 was used to calculate potential transpiration. Modelling efficiency (an indicator of agreement between simulations and measurements) for pF-values of soil water tensions ranged from 0.55 to 0.87. At 100-cm depth, the modelling efficiency was only 0.15. The mean value of the chloride concentrations were, at the same time, underestimated. The average deep percolation was 38% of the precipitation. A lower summer surface resistance of 80 s m(-1) was necessary to increase the agreement between simulated and measured chloride concentrations, which decreased the deep percolation to 31% of the precipitation. On average, simulation resulted in an accumulation of N in trees (21 kg N ha(-1) yr(-1)) but a decrease of N in soil (7 kg N ha(-1) yr(-1)). Simulated leaching was high (27 kg N ha(-1) yr(-1)) and probably overestimated due to overestimated soil nitrate concentrations below the root zone. To evaluate the carbon and nitrogen model, too few detailed measurements were available on, especially, plant N uptake and amounts of soil mineral N.