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6. Eutrophication On land and in the sea, however, it is nitrogen that is the limiting factor in the majority of cases. The deposition of nitrogen – originating from emissions of nitrogen oxides and ammonia – therefore acts as a fertilizer in nature. While this favours some species of plants that can easily make use of the extra nitrogen, it does so at the expense of others. It also affects the growth of mycorrhizal fungi. The impoverishment of ecosystems that results from the deposition of nitrogen is a real and very serious problem in large parts of Europe. The increased growth rate that results from nitrogen deposition also increases biological acidification, see factfile 5.2 (chapter 5). Another important effect of nitrogen deposition, at least in those parts of Europe where it is most extensive, is that nitrate ends up in the groundwater, where it causes problems in the production of potable water. This chapter describes the emissions into the air that cause these problems, the areas that are affected, how much nitrogen deposition nature can "tolerate", and gives an idea of the extent to which recovery is possible. The biological effects of nitrogen deposition are described in chapter 2. Note that it is not just airborne nitrogen that ends up in nature. In many environments nitrogen is also added in the form of fertilizer. Large amounts are spread on fields, and sometimes also on natural grazing land, which leads to impoverishment of the natural flora. Fertilizer is also spread on forest land to increase forestry yield. In addition to direct deposition, nitrogen also reaches the sea through leaching from the land and discharges from wastewater treatment plants and individual households. The amounts involved are not small by any means. In local areas of the Netherlands, for instance, annual deposition on forest soil can exceed 100 kilograms of nitrogen per hectare. This is roughly the same amount as is spread as a fertilizer dose on intensively farmed fields. In southern Scandinavia the annual atmospheric fallout on forest soil is 10–20 kilograms per hectare, and in the far north just a few kilograms. Calculations of mass balance show that Swedish forest soils can take between 3 and 20 kilograms of nitrogen per hectare each year, for areas of low and high productivity respectively, without increasing nitrogen leakage from the system. In virgin forest, where no nitrogen is removed through the harvesting of biomass, the critical load has been calculated as 1–3 kilograms per hectare each year. Figure 6.1 shows the critical load limits calculated using the mass balance method for the whole of Europe. It is also possible that the models underestimate the capacity of soils to lock up nitrogen. This capacity has been calculated using historical data, but without making allowance for the fact that a large amount of nitrogen has probably disappeared from the system as a result of recurring forest fires. From experience it seems that the risk of nitrogen leaking from forest soil is small if annual deposition is less than 10 kilograms per hectare. In the range 10–25 kilograms the degree of leaching increases in certain locations. When annual deposition exceeds 25 kilograms per hectare there is significant leaching out, and many soils become saturated with nitrogen, i.e. the rate of loss equals the rate of supply. Calculations based on mass balances show that in 1990 the critical load for nitrogen was exceeded over an area of roughly 213 million hectares of European ecosystems. Emissions have fallen since then however. Preliminary calculations indicate that the exceeded area had shrunk to 140 million hectares by 2000. If all signatory countries do as promised under the Gothenburg Protocol (see table 9.2, chapter 9) and if emissions in non-signatory countries develop as officially projected the area will however increase to 159 million hectares in 2010. This increase is mainly due to projected increased emissions of nitrogen compounds in some large eastern European countries. The four maps in figure 6.2 illustrate the progress. Knowledge in this area is however incomplete, since it is difficult to establish which changes are due to nitrogen deposition and which are caused by other changes, such as the way that land is used. Moreover, changes only appear in the flora after the critical limit has been exceeded, and in some cases only after it has been exceeded for an extended period of time. A further obstacle to establishing critical limits in this way is the lack of reference material. It is unusual to find comparative data that stretches back more than fifty years, and it is probable that significant effects had already taken place in many areas by the 1950s. The critical load is also affected by a number of chemical and physical factors. Researchers recommend that the lower values in table 6.1 are used when the weather is cold or dry, during long periods of frost and/or when there is a limited supply of base cations. Conversely, the high values can be used when the weather is warm, humid, free from frost and when there is a good supply of base cations, or if nitrogen is removed from the system through use of the land, for haymaking or grazing for example.
Most nitrogen reaches the sea as run-off from the surrounding land. Some of this is airborne nitrogen that is deposited on land and then leaches into surface water and is carried to the sea. In the case of the Baltic Sea it is estimated that the nitrogen that is deposited directly on the water surface accounts for around a third of the total supply. There is some risk that the transport of nitrogen from the land to the sea will increase in the future. This is because deposition on many types of land leads to a gradual build-up of nitrogen that could be released as a result of human actions, see below. Another phpect worth mentioning is that a warmer climate leads to faster decomposition of organic matter, with a risk of increased leaching of nitrogen from the land into the sea.No critical load limit has been established for nitrogen in the sea. In a number of places in Europe researchers have built roofs over small areas of forest to shield them from pollutant deposition. In all these trial areas it has been found that the amount of nitrogen leaking from the system falls considerably soon after the supply is stopped, even though the load has been high for a long time and a large reserve of nitrogen has therefore built up in the soil. The same effect has been observed when forest fertilization trials lasting several years have come to an end. The store of nitrogen in the soil does not decrease as quickly as the leaching out of nitrogen. Large amounts may be bound up in vegetation and humus layers, and in many cases nitrogen is conserved very effectively; little of it disappears. Interference, such as forestry felling or liming of the soil can accelerate decomposition, however, and lead to the risk of increased leakage. This risk may remain for a long time. Published 2004-04-06. |
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