A COMPARATIVE-STUDY OF NITROGEN FLOWS IN 2 SIMILAR MEADOWS AFFECTED BY DIFFERENT GROUNDWATER LEVELS

1. A comparative study was carried out on nitrogen flows in two meadow ecosystems that were similar al the beginning of the experiment. In the wet meadow, groundwater levels had been raised and in the dry meadow no changes in the original groundwater levels had been imposed. 2. Annual nitrogen mineralization and nitrogen accumulation in above-ground biomass were reduced by c. 20% in the wet meadow as compared with the dry meadow. In the dry compartment the peak in nitrogen mineralization was reached in spring, whereas in the wet compartment the peak was reached in summer. 3. In the wet compartment a smaller fraction of the ammonium produced was oxidized to nitrate than in the dry compartment (60% vs. 76%). 4. In both meadows annual nitrogen losses through denitrification were similar (about 17 kg N ha-1 year-1). During the summer, however, denitrification was greater in the wet compartment. During the winter denitrification increased sharply, because of nitrate accumulation in the soil at the end of the growing period and was greater in the dry compartment. 5. The nitrogen balance of the two meadow ecosystems shows that the inputs through atmospheric deposition and the outputs through hay removal are quantitatively the most important components of the nitrogen balance. Raising groundwater levels led to less nitrogen accumulation in the aboveground biomass and, thus, to smaller nitrogen outputs through hay removal.