Nitrogen deposition threat to 60 per cent of European ecosystems

Most affected are regions with intensive livestock rearing. Scenarios show that both technical measures and a reduction in animal numbers are needed to tackle the problem.

Excess nitrogen, from air pollution, is still a serious threat to ecosystems in Europe. This is evident from a status report by the Coordination Centre for Effects (CCE)that was published last autumn¹.

Critical load is a term used to describe how much deposition an ecosystem can withstand before it is damaged or altered (see box). In 2020, critical loads for eutrophication were exceeded in 61.2 per cent of all ecosystem areas (see figure and table). This is a slight decline from 2000, when critical loads were exceeded in 74 per cent of the surveyed area.

The impact on ecosystems is also determined by how much critical loads are exceeded. It is calculated as the average exceedance in each grid cell and is known as the Average Accumulated Exceedance (AAE). In 2020 the average AAE in Europe was 235 284 eq ha-1 yr-1 and 2020, which is a significant reduction compared to 434 538 eq ha-1 yr-1 in 2000.

Considering these two indicators, i.e. area where critical loads are exceeded and AAE, together, it can be concluded that the former was only reduced by 17 per cent, while the latter was reduced by almost 46 47 per cent. Here it can be assumed that measures have been taken and have to some extent been successful in the areas that have the most severe problems with excess nitrogen. But much effort it still needed to achieve zero exceedance of critical loads, the long-term aim stated in the Seventh EU Environment Action Programme.

If you take a closer look at the map, you can see that there are still several areas in dark red, where critical loads are exceeded by more than 1200 eq ha-1 yr-1. These are all areas with intensive livestock production: the Po Valley in Italy, the Dutch-German and German-Danish border areas and north-eastern Spain. The impact by farming is also confirmed by that fact that in areas with the highest exceedances, reduced nitrogen (such as ammonia, ammonium and amines, mainly from agriculture) accounts for a higher proportion of nitrogen deposition. While in areas with low exceedance, reduced and oxidised nitrogen (mainly NOx from industry and traffic) are of equal magnitude.

The report also notes that protected nature areas, such as nature reserves and Natura 2000 areas, are affected to the same extent as other nature areas.

The CCE has investigated five different scenarios, two for 2030 and three for 2050: Current legislation emissions (CLE), most feasible reduction (MFR) and in the latter case a low-emissions scenario (LOW). MFR includes the most efficient end-of-pipe technologies, but there is no difference in the scope of activities such as traffic and agriculture, compared to CLE. The LOW scenario includes changes due to global climate mitigation policy, including a significant transformation in the agricultural sector leading to a sharp reduction in livestock numbers, especially cattle and pigs.

The area of critical load exceedance in the scenarios shows great variation. In the CLE scenario the affected area is predicted to be 53% by 2030 and 49% by 2050. For MFR, the area of exceedance decreases to 44% by 2030 and 31% by 2050. In the most ambitious LOW scenario, it decreases to 22% by 2050. None of the scenarios succeed in achieving the long-term aim of zero exceedance. This does not mean that it is unachievable, but that it will require even greater structural change.

The report also covers critical loads for acidification. Here the situation is completely different. Exceedances of critical loads for acidity occurred on 14.1% of the European ecosystem area in 2000 and had shrunk to 3.6% in 2020. The European average AAE was about 145 211 eq ha-1 yr-1 in 2000, and 20 years later it had fallen to 22 40 eq ha-1 yr-1 (2020). This development is mainly due to the fact that sulphur emissions in Europe have almost ceased. NOx also contributes to acidification and is the main culprit in the acidification hotspots that still remain, i.e. the Netherlands and its border areas with Germany and Belgium,

In the five future scenarios the affected area will decrease to 1–2%. Average AAE is projected to fall to 26–13 27 eq ha-1 yr-1 by 2050. It is only in the Netherlands where AAE values are not projected to fall below 200 eq ha-1 yr-1 even under the most ambitious 2050 LOW scenario.

Kajsa Pira

1 CCE Status Report 2022 https://www.umweltbundesamt.de/en/publikationen/cce-status-report-2022

Critical loads

The concept of critical loads was introduced in the 1980s and has been used in Europe to develop cost-effective air pollution abatement strategies. Critical loads are scientific estimates of the amounts of pollutants that various ecosystems can tolerate without being harmed. They are sometimes referred to as the limits on what “nature can tolerate”. If pollutant depositions exceed the critical load limits, damage to sensitive ecosystems will by definition occur sooner or later.
The sensitivity of various ecosystems to exposure to acidifying and eutrophying air pollutants varies. Critical loads have therefore been assigned to different habitat classes based on empirical evidence, mainly observations from experiments and gradient studies. For example, the most recent assessment1, which was published by the CCE in 2022, established that tundra can only tolerate 3–5 kg N ha-1 yr-1 (i.e. roughly 200 - 350 eq ha-1 yr-1) without significant change in biomass and species composition. On the other hand, broadleaved deciduous woodland can tolerate 10–20 kg N ha-1 yr-1 (i.e. roughly 700 - 1400 eq ha-1 yr-1). Europe has been monitored and mapped in this way for more than 30 years, and European countries have coordinated this work through the CCE under the Convention on Long-range Transboundary Air Pollution. The CCE since 2018 is hosted at the German Environment Agency.

1 Review and revision of empirical critical loads of nitrogen for Europe | Umweltbundesamt

Figure: Areas where critical loads for eutrophication are exceeded in 2000 (top) and 2020 (bottom).

Figure: Areas where critical loads for acidification are exceeded by acid depositions in 2000 (top) and 2020 (bottom).

Table: Percentage area of ecosystems exposed to excess deposition of eutrophying and acidifying air pollutants in 2000, 2015 and 2020.

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