Nitrogen availability and uptake by grassland in mesocosms at two water levels and two water qualities

We studied the effect of water table (-5 or -30 cm) and water type (rain-or groundwater) on the above-and below-ground phytomass production, species composition and nitrogen uptake of grassland. Nitrogen mineralization, nitrification, methane production, redox potential and pH at different depths in the profile were measured and used to monitor gradual changes in variables influencing phytomass production. The rise in the water level lowered the nitrogen uptake in the above-ground phytomass from 14.1 to 11.4 g N per m(2), but the DM production did not decrease and varied from 566 to 690 g per m(2). The total root mass increased from 82 to 363 g DM per m(2), with the proportion in the 5 to 10 cm layer increasing the most from 13 to 24%. The high water level lowered the potential N mineralization in the upper 5 cm of the soil from 16.1 to 4.3 g N per m(2) and in the deeper 5 to 30 cm layer from 12.6 to 9.4 g N per m(2) respectively, so the importance of the deeper layer as a source of N increased. The total amount of mineral N that accumulated in the 40 cm deep soil cores decreased from 31.3 to 15.5 g N per m(2). The above-ground vegetation took up 71 to 76% of this amount in the high water level treatment and only 37 to 57% under drier conditions. Redox potential and methane production indicated anaerobic conditions below 5 cm in both level treatments and in the top 5 cm of the high water level treatment. But some nitrification was measured there also, thus aerobic and anaerobic conditions occurred together. The low N mineralization was attributed to low soil respiration. Raising the water level brought about an increase in the above ground biomass of Glyceria fluitans and an increase in root mass, especially deeper in the soil. Both are responsible for the relatively greater fraction of nitrogen that was taken up from the soil, although less N was available. The nitrification indicates that oxygen is transported by the root system to soil microsites and partly compensates for the anaerobic conditions caused by water saturation. The calcareous groundwater raised the pH in the upper soil layer from 5.3 to 5.8 but no effect on N mineralization was measured.