Ecosystem effects of nitrogen and sulphur deposition in the future

Filip Moldan, IVL Swedish Environmental Research Institute, Göteborg, Sweden and Julian Aherne, School of the Environment, Trent University, Ontario, Canada

Svante Odén’s 1967 article in Dagens Nyheter primary focused on linking distant air pollution sources to the declining pH in precipitation in Sweden, but it also warned of the negative impacts of ‘acid rain’ on soils, waters and living organisms. In the following years both concepts were firmly established and used in international policy efforts, eventually leading to emission reductions of acidifying pollutants.

Today’s situation is very different; European emissions of the main air pollutants have decreased significantly between 1990 and 2015 (sulphur oxides by 83%, nitrogen oxides by 52% and ammonia by 18%). As a consequence, the situation has dramatically improved for the majority of acidified ecosystems. However, nitrogen emissions and deposition have decreased relatively less compared to sulphur. In addition to acidification, nitrogen deposition can lead to ecosystem eutrophication. As such, nitrogen deposition is globally considered as a major threat to ecosystem functioning with important consequences for biodiversity, carbon sequestration and nitrogen retention.

In ecosystem science, the three principle methods of inquiry are experimental research, monitoring and modelling. The long-term experimental and monitoring site at Gårdsjön, Sweden, provides a good example of some resolved and some unresolved questions regarding the ecosystem effects of air pollution. During 1991 a large plastic roof was constructed to cover the entire area of a small experimental catchment at Gårdsjön, and all incoming rainfall was replaced by ‘clean precipitation’ sprinkled underneath the roof. Within three months the catchment started to show signs of recovery from acidification.

After 10 years the roof was dismantled, by then it served its purpose, to experimentally determine how quickly, and to what extent, forests would recovery from acidification once air pollution was eliminated. In a different experiment, parallel to the roof project, nitrogen was added to another small catchment at Gårdsjön, to investigate the effects of continuous high nitrogen deposition.

After the first ten years and 400 kg/ha of nitrogen addition, the transition from a nitrogen-poor to nitrogen-rich ecosystem was far from complete. Today, after 26 years, and a cumulative dose of >1000 kg N/ha, the ecosystem is still capable of retaining most of the added N, suggesting that serious ecosystem damage is still far away and difficult to predict with models and data at hand. Globally, experimental additions of nitrogen have demonstrated soil acidification, loss of soil base cations, and mobilisation of toxic metals.

However, the intensity of impacts and duration, vary depending on the ecosystem under investigation, site quality, and level of treatment (nitrogen addition).

Recovery from acidification to date has been relatively well described by our process-oriented models, arguably giving credibility to model predictions of future ecosystem status with respect to acidification. However, the situation is more complicated for nitrogen. Nitrogen deposition exceeds critical loads across much of Europe. There is consensus that current deposition levels are not sustainable, and will eventually have significant long-term impacts on ecosystem functioning and plant species biodiversity.

However, our current process-oriented models are not adequate to address the timing and extent of ecosystem damage. It is certain that continuous high nitrogen deposition will impact our ecosystems, but from the modelling point of view there are still challenges in predicting future ecosystem status.

This is further complicated by ongoing climate change and changes in land use.


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