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The South Pennines, an area of acid-sensitive geology at the center of a major industrial region, have undergone perhaps the most severe historic exposure to sulphur and nitrogen deposition in the UK. This study addresses a lack of existing research on the region by presenting the findings of a survey of 62 surface waters sampled during a 1-week period in April 1998. Results indicate that acidification in the region is acute; 27 of the sampled surface waters had a negative acidneutralizing capacity (ANC) and 28 had a pH below 5.0. Minimum recorded pH values were below 4.0. Non-marine sulphate levels were extremely high (median 222 mueq/1), and widespread high nitrate concentrations (median 41 mueq/1) suggest that soils in the region as a whole maybe at an advanced stage of nitrogen saturation. A consistent relationship was identified between site acidity and the balance between themajor weathering-derived cations, calcium and magnesium, and sulphate. This could in turn be linked to catchment soil type and land use, with the most acidic conditions occurring in peat-dominated catchments, where weathering is minimal and the influence of atmospheric deposition most pronounced. Percentage of peat in each catchment was the single best predictor of surface water acidity. Nitrate concentrations, although not a dominant control on acidity, varied significantly according to land use. Elevated concentrations were observed in catchments containing forestry, due to enhanced deposition inputs, and in catchments containing improved land, linked to fertilizer use. Ammoniumconcentrations, although low at most catchments, were a significant component of the inorganic nitrogen total in a number of surface waters draining waterlogged peat catchments. [ABSTRACT FROM AUTHOR]

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]

The effect of changes in dissolved inorganic nitrogen (N) depositionon ecosystem functioning was investigated in the NITREX (NITRogen saturation EXperiments) project. Field-scale manipulation experiments were carried out over four to six years in seven coniferous forest ecosystems in northwestern Europe. At sites with low or moderate ambientN deposition, N was experimentally added to throughfall. At sites with high N deposition, N was removed from throughfall. We found that the capacity of the ecosystem to retain N was correlated to its internal N status. Some of the components of this N status like the N concentrations in foliage and forest floor are relatively easy to measure.The C/N ratio of the forest floor is especially closely related to the onset of nitrate leaching. Changes in N input may, in the long run, change the N status of an ecosystem due to for instance a decrease in C/N ratio in the forest floor. Decreased N input resulted in a rapid and large reduction in N concentration in drainage water. Significant improvement in tree nutritional status, tree growth, fine root biomass and diversity of ground vegetation and mycorrhizal fungi population were observed in one site only. The time period of four to six years of manipulated N deposition may have been too short for changes to be manifested in the other sites. [ABSTRACT FROM AUTHOR]

Since 1991 nitrogen deposition to a small forested catchment has been experimentally increased from ambient 11 to a total of 50 kg N ha-1 year-1 by sprinkling small portions of dissolved ammonium nitrate on about 30 occasions annually. Concentrations ofinorganic N in runoff (streamwater) were measured by flow-proportional sampling and during 6 shorter periods by frequent sampling and continuous monitoring. Shortly after the onset of the N additions, nitrate (NO3) began to appear in the runoff (streamwater), after 2 years in soil solution at shallow depths and after 4 years at alldepths. N concentrations in runoff were highly variable, and most ofthe N lost from the catchment occurred during a few episodes. Flux of N from the catchment in runoff was much larger than that calculatedfrom concentrations in lysimeter samples and discharge from groundwater levels. The annual inorganic N loss was only about 5% of the incoming N. Episodic behavior of NO3 in runoff was studied by monitoring discharge episodes after 2, 3, 4 and 6 years of N addition. The nature of NO3 concentrations in runoff during these episodes changed as the treatment proceeded. Factors such as season, hydrology, timing and size of N additions govern the loss of NO3 in runoff during episodes. Nitrogen saturation at this catchment may proceed first in the discharge areas near the outlet to uphillsoils on the slopes. [ABSTRACT FROM AUTHOR]