Ecosystem recovery after a decrease in nitrogen input to a Scots pine stand at Ysselsteyn, the Netherlands

In a highly N-saturated Scots pine stand atmospheric N input to the forest floor was reduced to pre-industrial levels by means of a transparent roof and application of clean, artificial throughfall water. The N input was reduced from approximate to 60 kg N ha(-1) yr(-1) to < 5 kg N ha(-1) yr(-1). The experiment began in 1989 and is part of the NITREX? project. The aim is to assess the reversibility of N saturation. The results showed a close correlation between the input and output of N. The timing of this response is fast in contrast to the vegetation response which lags for some years. After this lag-phase signs of ecosystem recovery were noticed. The N concentration in the needles declined and the nutritional balance improved. Free arginine-N concentrations in the needles may be indicative of tree response to changes in N input. Although the total-N concentration in the needles is still high, the rapid drop in arginine-N concentrations may be the first sign of recovery. The trees have reacted with a growth improvement which is significantly correlated to decreasing arginine-N concentrations. The above-ground biomass of the nitrophilous ground vegetation decreased markedly during the experimental period, whilst there was a re-colonization and increase of fruitbodies of mycorrhizal fungi. (C) 1998 Elsevier Science B.V.