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 combined effects of grazing and atmospheric nitrogen deposition on water and solute fluxes in dry coastal dune grasslands. Two vegetation types were studied: (a) a short, species-rich stand on calcareous sand (foredune site) and (b) a short, species-poor stand on partly decalcified soil on calcareous sand (innerdune site). In each stand four experimental plots were created: (1) control, (2) fertilized with nitrogen, (3) excluded from grazing by rabbits and (4) combination of fertilization and exclusion of grazing by rabbits.

Due to the large spatial variability of the soil water content, no differences between the treatments could be measured. Average soil water content at 10 cm depth is very small (3-5%) from May until October and does not increase after rainfall. However, measured soil water content at 20 cm and 50 cm depth increased after rainfall. In winter, nearly all measured soil water contents increase upon rainfall, although sometimes one soil water content remained dry till the end of the next summer.

In summer it was impossible to sample soil water for the estimation of the solute concentrations due to the very small soil water content. Therefore, only solute concentrations of the winter period could be evaluated.

Without fertilization, fluxes of nitrogen out of the soil system are below the incoming flux, due to storage in the biomass and in the soil compartment. When fertilized, 70% of the added NH4 +-N was leached from the foredune soil profile as NO3 --N, due to nitrification. Conversely, at the grazed innerdune site most of the added nitrogen remained in the system. Here, nitrification rates will be small due to the decalcified topsoil and NH4 +-N is not easily leached out of the soil compartment. At the exclosures of the innerdune site, about 15% of the amount of the added fertilizer N was leached, after added NH4 + is taken up by the plants and partly washed out as nitrate after mineralization and nitrification of dead biomass.