Indicator Assessment

Critical load exceedance for nitrogen

Indicator Assessment
Prod-ID: IND-148-en
  Also known as: SEBI 009
Created 17 Sep 2009 Published 21 May 2010 Last modified 26 Oct 2017
9 min read

Nitrogen emissions and deposition of nitrogen compounds have decreased since 1990 but relatively little compared to sulphur emissions. Agriculture and transport are the main sources of nitrogen pollution (EEA, 2007c). In addition, nitrogen components can lead to eutrophication of ecosystems. When this pollution exceeds certain levels ('critical load'), it is damaging to biodiversity. Critical load exceedance is still significant(1).

(1) The critical load of nutrient nitrogen is defined as 'the highest deposition of nitrogen as NOX and/or NHY below which harmful effects in ecosystem structure and function do not occur according to present knowledge' (ICP, M&M, 2004).

This indicator is no longer updated.

Total emissions of acidifying substances (sulphur, nitrogen) and of nitrogen in the EEA-32 from 1990 to 2006

Note: How to read the graph: in 1990, the total of acidifying emissions was around 1 500 Gg, while for nitrogen fractions it was more than 500 Gg.

Data source:


Exceedance of the critical loads for eutrophication in Europe (as average accumulated exceedances), 2004

Note: How to read the map: for Norway, exceedances of the critical load for nutrient nitrogen are in general not a major problem. Size of grid-cell: 50 km x 50 km.

Data source:

CCE IMPACT Database MNP (Netherlands Environmental Assessment Agency); Coordination Centre for Effects (CCE), Data Centre of the International Cooperative Programme on Modelling and Mapping of Critical Levels and Loads and Air Pollution Effects, Risks and Trends (ICP Modelling and Mapping, ICP M&M), Convention on Long-range Transboundary Air Pollution (LRTAP) of the United Nations Economic Commission for Europe (UNECE) of the Convention on Long-range Transboundary Air Pollution. 2007.

CCE/EMEP, 2007. Personal communication, input to Air pollution in Europe 1990–2004. EEA Report No 2/2007. European Environment Agency, Copenhagen, Denmark. Available at:

CCE, 2007: 'Critical Loads of Nitrogen and Dynamic Modelling', Coordination Centre for Effects Progress Report 2007, MNP Report 500090001/2007.

Across the EU-25, approximately 47 % of (semi-) natural ecosystem areas were subject to nutrient nitrogen deposition leading to eutrophication in 2004. A relatively smaller 15 % of the ecosystem area received deposition of acidifying compounds including nitrogen (CCE/EMEP, 2007). Ecosystem types in use by European countries for critical load calculations are forests; marine and coastal habitats; littoral zones; mire, bog and fen habitats; grasslands and tall forb habitats; heathland, scrub and tundra habitats; inland un-vegetated or sparsely vegetated habitats; agricultural habitats; inland and surface water habitats (for details see CCE, 2007). The extent to which critical loads are exceeded varies significantly across Europe.



Supporting information

Indicator definition

Exceedance of critical loads for nitrogen deposition indicating risks for biodiversity loss in (semi)-natural ecosystems.


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Policy context and targets

Context description

The availability of nutrients is one of the most important abiotic factors that determine plant species composition in ecosystems. Nitrogen is the limiting nutrient for plant growth in many natural and semi-natural ecosystems. Most of the plant species from oligotrophic and mesotrophic habitats are adapted to nutrient-poor conditions, and can only survive or compete successfully on soils with low nitrogen availability. High nitrogen deposition causes changes in vegetation composition and vegetation structure. These changes in turn affect the fauna composition (UNECE, 2003).

High variations in sensitivity to atmospheric nitrogen deposition have been observed between and within different natural and semi-natural ecosystems. Critical loads are used to describe this sensitivity. A critical load is defined as 'a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge' (Nilsson and Grennfelt, 1988). Exceedances of critical loads by current or future nitrogen loads indicate risks for adverse effects on biodiversity.

Because of short- and long-range atmospheric transport, nitrogen (N) deposition has increased in many natural and semi-natural ecosystems across the world. The emissions of ammonia (NH3) and nitrogen oxides (NOX) strongly increased in Europe in the second half of the 20th century. Ammonia is volatilised from intensive agricultural systems, whereas nitrogen oxides originate mainly from burning of fossil fuel by traffic and industry (UNEP, 2005).

Significant geographical variability occurs in emissions and deposition of nitrogen compounds across Europe. Historically emission control strategies have focussed on reducing the emission of oxides of nitrogen. However across Europe it is now clear that nitrogen deposition is dominated by agricultural releases, predominantly ammonia. Therefore, while past effort has focussed predominately on reducing the oxides of nitrogen, future effort must also take into account reduced forms of nitrogen.

Relation of the indicator to the focal area

Excess nitrogen is one of the major threats to biodiversity. Excessive levels of reactive forms of nitrogen in the biosphere and atmosphere constitute a major threat to biodiversity in terrestrial, aquatic and coastal ecosystems. On land it causes loss of sensitive species and hence biodiversity by favouring a few nitrogen tolerant species over less tolerant ones. In coastal waters it leads to algal blooms and deoxygenated dead zones in which only a few bacteria may survive.


No targets have been specified

Related policy documents

No related policy documents have been specified



Methodology for indicator calculation

Deposition loads are modelled as part of EMEP; on a European scale, the EMEP Unified Model is used (see Monitoring of nitrogen deposition is used to calibrate the models.

European critical loads are assessed using scientifically reviewed methods and data.
There are various endpoints (protection aims) for setting critical loads. The ICP MandM and CCE have developed methods to derive critical loads for protecting (semi)-natural ecosystems (

1. Critical loads based on empirical data;
2. Critical loads based on dynamic ecosystem models;
3. Critical loads based on steady state modelling.

Methods 1 and 2 are particularly relevant for setting critical loads for protecting biodiversity.

Below are described the methodologies helping to produce the different maps/graphs relevant for this indicator.

(1) European maps of percentage natural area with critical load exceedances

  • Combine recent European deposition map (EMEP) with recent European critical load map (CCE).
  • Sum in each 50 x 50 km EMEP-grid the total natural area where the deposition (in mol/ha/yr) exceeds the critical loads (in mol/ha/yr) and divide this by the total natural area. Use the ecosystem specific deposition rates.
  • Plot the percentage area with exceeded critical loads within each EMEP-grid.

(2) European maps of the percentage natural area protected under the EU Habitat directive With critical load exceedances

  • See steps described above

(3) European maps of the height of the exceedance in natural areas or protected areas

  • Combine recent European deposition map (EMEP) with recent European critical load map (CCE).
  • Sum in each 50 x 50 km EMEP-grid the deposition which exceeds the critical load. Use the ecosystem specific deposition rates.
  • Plot the calculated sum of excess of deposition within each EMEP-grid.

(4) Graphs of changes in percentage natural area with critical load exceedances or height of the exceedances

  • Combine a number of recent European deposition maps with the recent European critical load map (CCE).
  • Sum per year, in all European EMEP-grids the total natural area where the deposition (in mol/ha/yr) exceeds the critical loads (in mol/ha/yr) and divide this by the total natural area in Europe. Use the ecosystem specific deposition rates. Similar calculations can be made for individual countries.
  • Plot the calculated percentage per year in a graph.
  • Similar calculations can be made for the excess of deposition.

Methodology for gap filling

No methodology for gap filling has been specified. Probably this info has been added together with indicator calculation.

Methodology references



Methodology uncertainty

No uncertainty has been specified

Data sets uncertainty

No uncertainty has been specified

Rationale uncertainty


  • Not all critical loads are defined to protect biodiversity. Empirical critical loads (Method 1 above) are often set to protect for changes in species composition and/or vegetation changes. Method 2 (used in some countries) is often based on criteria that should protect biota (plants, fish, trees etc) and yields critical loads comparable to empirical critical loads. Method 3 is more indirectly linked to risks for biota; it is presently based on chemical soil and/or water conditions. However, National Focal Centres often use several methods for calibration and/or validation purposes.
  • Critical load exceedances indicate risks but not immediate effects of air pollution. Nevertheless time delay is often short in respect to effects of nitrogen deposition on biota.
  • The indicator is focusing only on threats to (semi-natural) terrestrial ecosystems. However, excessive levels of nitrogen (and phosphorus) in water bodies, including rivers, coastal zones and other wetlands also cause major damage to biodiversity including fisheries. However, in most aquatic ecosystems in Europe the main source of nitrogen is not atmospheric deposition but run-off of nitrates and other nitrogenous compounds from agricultural lands.

Data sources

Other info

DPSIR: Pressure
Typology: Performance indicator (Type B - Does it matter?)
Indicator codes
  • SEBI 009
Frequency of updates
Updates are scheduled every 5 years
EEA Contact Info


Geographic coverage

Temporal coverage