VEGETATION AND SOIL BIOTA RESPONSE TO EXPERIMENTALLY-CHANGED NITROGENINPUTS IN CONIFEROUS FOREST ECOSYSTEMS OF THE NITREX PROJECT

Enhancement of the atmospheric N deposition is a serious threat for th e structure and function of ecosystems. Here we evaluate the ecologica l effects of excess N with respect to changes in vegetation and soil b iota in a series of experiments along a N gradient across Europe. The aim of this project (NITREX: N saturation EXperiments) is to assess th e risk of N saturation and the reversibility of N saturation. At the e xperimental sites with a low-to-moderate input, N was added (n = 3), w hile at sites with a high input, N was removed by means of a transpare nt roof (n = 4). The experiments started between 1989 and 1991. Across the N gradient a positive correlation was found between the N concent ration in deposition or soil solution with the N concentration in the needles and in general a negative correlation with the base cations K and Mg. In the N-addition plots there was a tendency towards a decreas ing nutrient status of the needles, whereas at one site N-removal led to an improvement. Addition of N hardly affected fine-root biomass pro duction, whereas signs of growth increase were recorded when the input was reduced. Tree growth was accelerated upon input reduction at two of three sites. Manipulation of N input did not alter the decompositio n rate, although significant differences between sites were noted. Man ipulation of the N input hardly affected the biomass of fungi and bact eria, but a negative relation between the N-addition and part of the s oil fauna may be present among sites. (C) 1998 Elsevier Science B.V.