Search

During the last three decades the experimental manipulations of whole ecosystems have been a useful and widely-used tool for investigation of the effects of air pollution, air pollution reduction strategiesand management practices on the health and productivity of forests and the acidification of catchments and fresh waters. NITREX and EXMANprojects involve whole-ecosystem manipulations of forest ecosystems in Europe. The aims of these ecosystem experiments have been to investigate the impact of a continued or increased load of air pollutants on the ecosystems, and the possibilities of reversing the acidifying effects by soil amelioration, addition of buffer-acting substances orby removal of the air pollutants. Along with the field experiments, models have been used to predict future effects and dynamics in the ecosystems under different air pollution scenarios. The major findingsfrom those projects have been used in political decisions on reduction of sulphur emission in Europe via the sulphur protocol signed in 1994. Today work is going on to formulate a NOx protocol. NITREX and EXMAN results contribute to the scientific information basefor these protocols. Large-scale ecosystem manipulation projects have increased our understanding of ecosystem function and response to external change and created scientific evidence for political environmental decisions and legislation. In the future, controlled-ecosystem experiments clearly play an important role in new research on ecological effects of changes in the global atmosphere and climate. [ABSTRACT FROM AUTHOR]

European concern over the cause and consequences of forest decline, acidification of soils and surface waters, and the nutrient enrichment of terrestrial and aquatic ecosystems, led to the establishment of the NITREX (Nitrogen saturation experiments) and EXMAN (Experimental Manipulation of Forest Ecosystems in Europe) projects, two research networks of large-scale manipulation experiments under the auspices ofthe EU Commission of European Communities. NITREX comprises 10 experiments at 8 sites in 7 countries at which nitrogen is either added toor removed from ambient atmospheric deposition to simulate major changes in nitrogen deposition. EXMAN comprises experiments at 6 sites in 4 countries at which ambient atmospheric deposition is experimentally altered in chemical composition and/or quantity. The ultimate goalof this research is to contribute to the scientific basis required for the refinement of EU policy on atmospheric quality, and the legislation which will emanate from that policy. [ABSTRACT FROM AUTHOR]

The NITREX project entails large-scale manipulation of nitrogen deposition to whole, forested ecosystems at eight sites in Europe. Nitrogen is added at sites with low-to-intermediate ambient N deposition and removed at sites with high deposition. Changes in outputs of dissolved constituents reflect the integrated effects on ecosystem processes and changes in storage. At sites exhibiting clear symptoms of nitrogen saturation prior to treatment, the nitrate flux in leachate and runoff responded rapidly to changes in deposition. Reduced deposition gave immediate improvement in water quality. At sites with low nitrogen losses prior to treatment, the response to increased deposition was small and delayed. Together the results point to significant hysteresis in output response related to the nitrogen status of the ecosystem. The input--output budgets indicate that forest ecosystems requiremany years to adjust to changes in nitrogen deposition. [ABSTRACT FROM AUTHOR]

The deposition of nitrogen from the atmosphere to coniferous forestsposes the risk of N saturation and disruption of the N cycle. Beginning in April 1991, we have conducted a whole-ecosystem manipulation of N deposition at a mature coniferous forested-catchment at Gardsjon,SW Sweden. We have added in weekly portions of about 40 kg NH4NO3-N ha-1 yr-1 in 5% extra water to the ambient 11 kg NH4NO3-N ha-1 yr-1 in throughfall. The Gardsjon experiment is part of the European NITREX project (Nitrogen saturation experiments). Both the annual mean concentrations and peak concentrations of nitrate in runoff increased dramatically in response to the chronic additions of N. The seasonal pattern and yearly amount of N loss changed during the 5 years of treatment. During the first years of treatment, concentrations of NO3- were low during the growing season and high during the winter; during the fourth and fifth years, elevated concentrations were present also during the summer. Ammonium concentrations, although generally much lower than NO3-, followed the same seasonal pattern. The inorganic N lost during the 5 years of treatment was 0.6%, 1.1%, 5.0%, 5.7% and 4.5% respectively, of the annual inorganic N input. In the fifth year, drought and generally very low runoff led to moderate N loss despite the highNO3- and NH4+ concentrations. Sulphate, inorganic Aln+, H+ and Na+ concentrations were significantly lower during the treatment period relative to the untreated control catchment while NO3-, NH4+, K+ and Ca2+ concentrations increased. The N concentrations in the runoff indicate incipient N saturation at the manipulated catchment. But theexperimental dramatic increase in N deposition occurred so rapidly that [ABSTRACT FROM AUTHOR]