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Most regional watersheds in Europe constitute managed human territories importing large amounts of new reactive nitrogen.
As a consequence, groundwater, surface freshwater and coastal seawater are undergoing severe nitrogen contamination and/or eutrophication problems.
A comprehensive evaluation of net anthropogenic inputs of reactive nitrogen (NANI) through atmospheric deposition, crop N fixation, fertiliser use and import of food and feed has been carried out for all European watersheds. A database on N, P and Si fluxes delivered at the basin outlets has been assembled.
A number of modelling approaches based on either statistical regression analysis or mechanistic description of the processes involved in nitrogen transfer and transformations have been developed for relating N inputs to watersheds to outputs into coastal marine ecosystems.
Key findings/state of knowledge
Throughout Europe, NANI represents 3700 kgN/km²/yr (range, 0–8400 depending on the watershed), i.e. five times the background rate of natural N2 fixation.
A mean of approximately 78% of NANI does not reach the basin outlet, but instead is stored (in soils, sediments or ground water) or eliminated to the atmosphere as reactive N forms or as N2.
N delivery to the European marine coastal zone totals 810 kgN/km²/yr (range, 200–4000 depending on the watershed), about four times the natural background. In areas of limited availability of silica, these inputs cause harmful algal blooms.
Nitrogen (N) inputs from human activities have led to ecological deteriorations in large parts of the coastal oceans along European coastlines, including harmful algae blooms and anoxia.
Riverine N-loads are the most pronounced nitrogen sources to coasts and estuaries. Other significant sources are nitrogen in atmospheric deposition and fixation.
This chapter describes all major N-turnover processes which are important for the understanding of the complexity of marine nitrogen cycling, including information on biodiversity.
Linkages to other major elemental cycles like carbon, oxygen, phosphorus and silica are briefly described in this chapter.
A tentative budget of all major sources and sinks of nitrogen integrated for global coasts is presented, indicating uncertainties where present, especially the N-loss capacity of ocean shelf sediments.
Finally, specific nitrogen problems in the European Regional Seas, including the Baltic Sea, Black Sea, North Sea, and Mediterranean Sea are described.
Key findings/state of knowledge
Today, human activity delivers several times more nitrogen to the coasts compared to the natural background of nitrogen delivery. The source of this is the land drained by the rivers. Therefore, the major European estuaries (e.g. Rhine, Scheldt, Danube and the coastlines receiving the outflow), North Sea, Baltic Sea, and Black Sea as well as some parts of the Mediterranean coastlines are affected by excess nutrient inputs.
Biodiversity is reduced under high nutrient loadings and oxygen deficiency. This process has led to changes in the nutrient recycling in sediments, because mature communities of benthic animals are lacking in disturbed coastal sediments. The recovery of communities may not be possible if high productivity and anoxia persist for longer time periods.
Environmental problems related to nitrogen concern all economic sectors and impact all media: atmosphere, pedosphere, hydrosphere and anthroposphere.
Therefore, the integration of fluxes allows an overall coverage of problems related to reactive nitrogen (Nr) in the environment, which is not accessible from sectoral approaches or by focusing on specific media.
This chapter presents a set of high resolution maps showing key elements of the N flux budget across Europe, including N2 and Nr fluxes.
Comparative nitrogen budgets are also presented for a range of European countries, highlighting the most efficient strategies for mitigating Nr problems at a national scale. A new European Nitrogen Budget (EU-27) is presented on the basis of state-of-the-art Europe-wide models and databases focusing on different segments of Europe's society.
From c. 18 Tg Nr yr−1 input to agriculture in the EU-27, only about 7 Tg Nr yr−1 find their way to the consumer or are further processed by industry.
Some 3.7 Tg Nr yr−1 is released by the burning of fossil fuels in the EU-27, whereby the contribution of the industry and energy sectors is equal to that of the transport sector. More than 8 Tg Nr yr−1 are disposed of to the hydrosphere, while the EU-27 is a net exporter of reactive nitrogen through atmospheric transport of c. 2.3 Tg Nr yr−1.
The largest single sink for Nr appears to be denitrification to N2 in European coastal shelf regions (potentially as large as the input of mineral fertilizer, about 11 Tg N yr–1 for the EU-27); however, this sink is also the most uncertain, because of the uncertainty of Nr import from the open ocean.
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