Vegetation and soil biota response to experimentally-changed nitrogen inputs in coniferous forest ecosystems of the NITREX project.

摘要

Enhancement of the atmospheric N deposition is a serious threat for the structure and function of ecosystems. Here the ecological effects are evaluated of excess N addition with respect to changes in vegetation and soil biota in a series of experiments along a N gradient across Europe as part of the NITREX project. The aim of this project is to assess the risk of N saturation and its reversibility. At the experimental sites with a low-to-moderate input (Sweden, Denmark, UK (Wales)) N was added, while at the sites with a high input (Germany, Netherlands (2)) N was removed by means of a transparent roof. The experiments started between 1989 and 1991. Across the N gradient a positive correlation was found between the N concentration in deposition orsoil 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 decreasing nutrient status of the needles, whereas at one site N-removal led to an improvement. Addition of N hardly affected fine-root biomass production, whereas signs of growth increase were recorded when the input was reduced. Tree growth was accelerated upon input reduction at 2 of 3 sites. Manipulation of N input did notalter the litter decomposition rate, although significant differences between sites were noted. Manipulation of the N input hardly affected the biomass of soil fungi and bacteria, but a negative relation between the N-addition and part of the soil faunamay be present among sites. In contrast mycorrhizal fungi were sensitive to N input, with fine-root growth and mycorrhizal growth and diversity lower in high-input sites, and responsive to a reduction in deposition rates.