Table 1a provides a summary of the direct effects of reactive nitrogen in aquatic and terrestrial ecosystems, which differ considerably. Reduced nitrogen is highly toxic in both ecosystems but, as its presence in aquatic ecosystems is restricted to areas close to sources or to anoxic conditions, its role in terrestrial ecosystems is more pronounced. In contrast to aquatic ecosystems, oxidized nitrogen is not considered toxic in terrestrial ecosystems. However, indirect effects of reactive nitrogen show considerable similarities between terrestrial and aquatic ecosystems for both oxidized and reduced nitrogen (Table 1b). Both types of ecosystem are indirectly affected by nitrogen oxides in atmospheric deposition causing acidification and an increase in free metal ions. In addition, increased nitrogen availability results in both systems in higher productivity, causing significant species composition shifts, in which highly competitive species start to outcompete less competitive species.

The impact of reactive nitrogen on sensitive aquatic and terrestrial ecosystems has led to the understanding that these ecosystems need protection from the anthropogenic input. In Europe in particular, there has been an emphasis on environmental management based on the long-term maintenance of the ecological status. In the case of deposition from the atmosphere, the convention on long-range trans-boundary air pollution has provided the framework within which ecotoxicological studies have been used to define critical loads of atmospheric nitrogen deposition below which ecosystems retain their biodiversity, functions, and characteristic species. As eco-toxicity strongly depends on specific characteristics of ecosystems, these critical loads vary between different aquatic and terrestrial ecosystems. Typical critical loads for nitrogen for sensitive ecosystems are listed in Table 2.

Table 2 Critical loads for different ecosystems based on empirical research and expert judgment as published by Swiss Agency for the Environment, Forests and Landscape (Environmental documentation no. 164, 2001)

Critical load

Ecosystem (kg N hayr1)

Terrestrial ecosystems

Temperate forests 10-20

Boreal forests 10-20

Tundra 5-10

Arctic, alpine, and subalpine scrub 5-15 habitats

Northern wet heath 10-25

Dry heath 10-20

Semi-dry calcareous grassland 15-25

Dune grassland 10-20

Low altitude hay meadows 20-30

High altitude hay meadows 10-20

Wet oligotrophic grassland 10-25

Aquatic ecosystems

Raised and blanket bogs 5-10

Poor fens 10-20

Rich fens 15-35

Mountain rich fens 15-25

Softwater lakes 5-10

Dune slack pools 10-20

Pioneer and low mid salt marsh 30-40

For aquatic systems, the new water framework directive of the European Union provides a framework within which environmental management needs to ensure the long-term ecological status of different water bodies. This leads to environmental standards for water quality (e.g., for ammonia, total inorganic nitrogen or dissolved oxygen) which vary according to the characteristics of the ecosystem and which specifically address the impact on key groups of plants and animals. In order to protect water ecology and water quality, aquatic ecosystems are required to have 'good ecological status' and 'good chemical status'. The ecological status is defined by biological parameters such as plant and animal species composition depending on the type of water body. Good chemical status is defined in terms of compliance with all the quality standards established for chemical substances at

European level. The chemical status depends strongly on the type of water body, soil parameters, and other abiotic factors. However, plants and animals might respond to values much lower than these. Restrictions of the emissions and input of nitrogen to water bodies improve the nitrogen status of these waters.

Because reactive nitrogen is rapidly and widely dispersed through different ecosystems via the atmosphere and aquatic pathways and because the global anthropogenic input of nitrogen is expected to increase, initiatives such as those described above are needed worldwide to prevent and control ecosystem damage due to the effects of reactive nitrogen.

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