THE FATE OF N-15 ENRICHED THROUGHFALL IN 2 CONIFEROUS FOREST STANDS AT DIFFERENT NITROGEN DEPOSITION LEVELS

The stable isotope N-15 was added as ((NH4)-N-15)(2)SO4 to throughfall water for one year, to study the fate of the deposited nitrogen at di fferent levels of N deposition in two ''N saturated'' coniferous fores ts ecosystems in the Netherlands. The fate of the N-15 was followed at high-N (44-55 kg N ha(-1) yr(-1)) and low-N (4-6 kg N ha(-1) yr(-1)) deposition in plots established under transparent roofs build under th e canopy in a Douglas fir (Pseudotsuga menziesii (Mirb.) Franco.) and Scots pine (Pinus sylvestris L.) forest. The applied N-15 was detectab le in needles and twigs, the soil and soil water leaching below the ro oting zone (90 cm depth). Total N-15 recovery in major ecosystem compa rtments was 71-100% during two successive growing seasons after the st art of a year-round N-15 application to throughfall-N. Nine months aft er the year-round N-15 application, the N-15 assimilated into tree bio mass was 29-33% of the N-15 added in the Douglas fir stand and less th an 17% in the Scots pine stand. At the same time total N-15 retention in the soil (down to 70 cm) of the high-N plots was about 37% of the d eposited (NH4)-N-15-N, whereas 46% and 65% of the N-15 was found in th e soil of the low-N deposition plots at the Douglas fir and Scots pine stand, respectively. The organic layers accounted for 60% of the N-15 retained in the soil. The total N deposition exceeded the demand of t he vegetation and microbial immobilization. Total N-15 leaching losses within a year (below 90 cm) were 10-20% in the high-N deposition plot s in comparison to 2-6% in the lowered nitrogen input plots. Relative retention in the soil and vegetation increased at lower N-input levels . Species differences in uptake and tree health seem to contribute to lower N-15 recoveries in the Scots pine trees compared to the Douglas fir trees. The excessive N deposition and resulting ''N saturation'' l ead to conditions were the health and functioning of biota were negati vely influenced. At decreased N deposition, lower leaching losses toge ther with increased soil and plant retention indicated a change in the fate of the N deposited. This may have resulted from changes in ecosy stem processes, and thus a shift along the continuum of N saturation t o N limitation.