NITREX - RESPONSES OF CONIFEROUS FOREST ECOSYSTEMS TO EXPERIMENTALLY CHANGED DEPOSITION OF NITROGEN

In large regions of Europe and eastern North America atmospheric depos ition of inorganic nitrogen compounds has greatly increased the natura l external supply to forest ecosystems. This leads to nitrogen saturat ion, in which availability of inorganic nitrogen is in excess of biolo gical demand and the ecosystem is unable to retain all incoming nitrog en. The large-scale experiments of the NITREX project (nitrogen satura tion experiments) are designed to provide information regarding the pa tterns and rates of responses of coniferous forest ecosystems to incre ases in N deposition and the reversibility and recovery of impacted ec osystems following reductions in N deposition. The nitrogen input-outp ut data from the NITREX sites are consistent with the general pattern of nitrogen fluxes from forest ecosystems in Europe. At annual inputs of less than about 10 kg ha(-1) year(-1), nearly all the nitrogen is r etained and outputs are very small. At inputs above about 25 kg ha(-1) year(-1) outputs are substantial. In the range 10-25 kg ha(-1) year(- 1) these forest ecosystems undergo a transition to nitrogen saturation . The 10 kg ha(-1) year(-1) apparently represents the minimum threshol d for nitrogen saturation. The NITREX experiments indicate that nitrog en outputs respond markedly across the 10-25 kg ha(-1) year(-1) range of inputs. In contrast, the nutrient concentrations in foliage, a meas ure of tree response, is delayed by several years. Nitrogen saturation can apparently be induced or reversed within only a few years, at lea st with respect to the commonly used diagnostic of nitrogen saturation -nitrogen output in leachate or runoff.