Utilization and leaching of nitrate from two Deschampsia-dominated heathland sites: a lysimeter study using intact soil columns*

High deposition levels of atmospheric ammonia in the Netherlands have led to the major replacement of dwarf shrubs by grasses and to elevated nitrification rates in acid heathland soils. In order to study the efficacy of a naturally established grass-heath of Deschampsia flexuosa at capturing NO^-₃, an outdoor 'mesocosm' lysimeter experiment was set up with relatively large and undisturbed soil columns from two Deschampsia-dominated heathland sites. One of the sites (Ede) had a relatively high rate of nitrate production, whereas the other (Hoorneboeg [HB]) showed practically no nitrate formation. For part of the Ede columns, the fate of labeled nitrate, split-applied at two rates (30 or 150 kg ha^-1 yr^-1) during two seasons, was studied. D. flexuosa was highly effective in acquiring fertilizer nitrate, as demonstrated by (1) distinct rises in foliar NRA, especially at high N; (2) increased ¹⁵N enrichments in all plant components with N rate; (3) significant increases in organic-N and carboxylate concentration in several plant compartments; and (4) clear shifts in biomass allocation in favour of the aboveground tissues.After 18 months at low N, an average 39 and 23% of the applied N was immobilized in the plant and soil compartments, respectively; at high N rate, corresponding recoveries were 33 and 20%. Total leaching of nitrate (beyond a depth of 35 cm) from the unfertilized Ede columns corresponded to an annual loss of 1.9 kmol N ha^-1, whereas leaching was virtually zero from HB columns. Relatively high amounts of N leached from the fertilized columns with apparent fertilizer recovery in the leachate reaching an average 60% at high N. However,¹⁵ N analyses revealed only recoveries of 2.0% (low N) and 7.2% (high N) of the applied N in the leachate. From columns where the plant cover had been removed, apparent and real leaching losses reached values of >100 and 10% of the applied N, respectively. Hence, soil-derived N appeared by far the major source of leaching. Unplanted and unfertilized HB columns displayed high rates of nitrification and leached high amounts of nitrate, suggesting a plant-induced repression of the in situ nitrification at this site.On average, planted columns had lost 37% (low N) and 40% (high N) of the applied N, whereas unplanted lysimeters had lost 89% (for both low and high N). The N not recovered was presumed lost by denitrification due to favourable conditions with respect to nitrate concentration, moisture, carbon supply, and temperature.