WATER AND SOLUTE FLUXES IN DRY COASTAL DUNE GRASSLANDS - THE EFFECTS OF GRAZING AND INCREASED NITROGEN DEPOSITION

A five-year monitoring study has been carried out to examine the combi ned effects of grazing and atmospheric nitrogen deposition on water an d solute fluxes in dry coastal dune grasslands. Two vegetation types w ere studied: (a) a short, species-rich stand on calcareous sand (fored une site) and (b) a short, species-poor stand on partly decalcified so il on calcareous sand (innerdune site). In each stand four experimenta l plots were created: (1) control, (2) fertilized with nitrogen, (3) e xcluded from grazing by rabbits and (4) combination of fertilization a nd exclusion of grazing by rabbits. Due to the large spatial variabili ty of the soil water content, no differences between the treatments co uld be measured. Average soil water content at 10 cm depth is very sma ll (3-5%) from May until October and does not increase after rainfall. However, measured soil water content at 20 cm and 50 cm depth increas ed after rainfall. In winter, nearly all measured soil water contents increase upon rainfall, although sometimes one soil water content rema ined dry till the end of the next summer. In summer it was impossible to sample soil water for the estimation of the solute concentrations d ue to the very small soil water content. Therefore, only solute concen trations of the winter period could be evaluated. Without fertilizatio n, fluxes of nitrogen out of the soil system are below the incoming fl ux, due to storage in the biomass and in the soil compartment. When fe rtilized, 70% of the added NH4+-N was leached from the foredune soil p rofile as NO3--N, due to nitrification. Conversely, at the grazed inne rdune site most of the added nitrogen remained in the system. Here, ni trification rates will be small due to the decalcified topsoil and NH4 +-N is not easily leached out of the soil compartment. At the exclosur es of the innerdune site, about 15% of the amount of the added fertili zer N was leached, after added NH4+ is taken up by the plants and part ly washed out as nitrate after mineralization and nitrification of dea d biomass.