We need to talk about nitrogen

Nitrogen in the air is one of the greatest threats to our wild plants, lichens and fungi, yet few people have even heard about it.

Nearly two thirds of the UK’s most sensitive wildlife habitats are affected by excessive nitrogen deposition, according to a new report by the British conservation charity Plantlife that raises the alarm about the devastating impacts of nitrogen pollution.

Emissions into the air of nitrogen oxides and ammonia increased sharply in the second half of the 20th century – the main sources being transport, power stations, industry, farm fertilisers and livestock manure. Between 1960 and 2000, global nitrogen emissions more than doubled and for ammonia emissions this trend is expected to continue up to 2050 due to continued use of artificial fertilisers and increasing meat consumption (Figure 1).

Figure 1. Trends and future projections of  global emissions of  nitrogen oxides and ammonia.

The term “reactive nitrogen” describes all forms of oxidised nitrogen (e.g. nitrogen dioxide) or reduced nitrogen (e.g. ammonia). In most natural or semi-natural ecosystems, naturally occurring forms of reactive nitrogen are usually scarce and limit growth.

Once released into the atmosphere, reactive nitrogen gases undergo chemical and physical transformation. After dispersion by winds, the nitrogen compounds may be deposited on vegetation, soils and waters, where it can cause acidification and/or over-fertilisation of sensitive ecosystems. Moreover, reactive nitrogen damages human health by contributing to increased levels of fine particulate matter (PM_2.5) in the air, ground-level ozone and nitrogen dioxide. It can also be re-emitted as nitrous oxide, which contributes to climate change and stratospheric ozone depletion.

The effects on ecosystems of nitrogen deposition are significant, with observable species loss, changes in soil chemistry and habitat degradation resulting from nutrient enrichment (eutrophication), acidification (lower pH), or direct damage (toxicity).
Increased concentrations of ammonia in the air can often be found in agricultural areas, especially in areas with intensive rearing of livestock. Critical levels of ammonia for sensitive species such as lichens and bryophytes are set at 1 microgram per cubic metre (µg/m3) and for herbaceous species at 3 µg/m³. In the UK, 64 per cent of the land area has ammonia concentrations above the critical level for lichen and bryophyte species, of which there are internationally important communities in the UK. The critical level for herbaceous species is exceeded on 3.9 per cent of the land area.

Scientific studies have demonstrated clear correlations between rates of nitrogen deposition and species richness in a range of ecosystems, both in the UK and western Europe (Figure 2). Low species richness is typically found at high levels of nitrogen deposition, but some ecosystems – those that are naturally rich in nutrients – do not show this correlation. In a characteristic nutrient-poor environment, enrichment with nitrogen provides a competitive advantage to those species most able to use the additional nutrients. This promotes some species, such as grasses, over slower growing species adapted to low nutrient conditions, thus reducing overall species richness and contributing towards the homogenisation of plant communities.

Figure 2.  The role of  nitrogen deposition on widespread plant community change across semi-natural habitats

Vascular plants that are sensitive to elevated nitrogen input include harebell (Campanula rotundifolia), fairy flax (Linum catharticum) and bird’s-foot trefoil (Lotus corniculatus). Certain fungal groups may also be very sensitive to increased nitrogen deposition, as has been seen in the decline of grassland fungi in north-west Europe and major changes in ectomycorrhizal fungi in forests in central Europe. Some flowering plants, such as orchids, which are dependent on ectomycorrhizal fungal associations, may be particularly affected by such declines.

The sensitivity of an ecosystem to pollution input, in this case deposition of nutrient nitrogen, is called a “critical load”. This represents a tolerance threshold for a specific type of ecosystem, above which species loss or other types of degradation is expected either immediately or in the long term. Some ecosystems are much more sensitive than others – i.e. they have a lower critical load.

Habitats such as woodlands, grasslands, heaths and bogs have all been affected by nitrogen deposition that exceeds their critical loads. In Special Areas of Conservation (SACs) in England and Wales, 90 per cent of the land was exposed to excessive levels of nitrogen in 2014, while for the UK as a whole it was 63 per cent.

Looking at policy initiatives, the report notes that the problem of reactive nitrogen is a complex one that requires co-ordinated approaches to address both its causes and consequences, and that effective solutions will need to be sufficiently integrated between different emitting sectors in order to drive reductions in overall emissions.

Protecting habitats from atmospheric nitrogen deposition will need action on three levels: international and national action to reduce long-range deposition; local action that reduces or intercepts emissions close to sensitive designated nature conservation sites; and on-site restoration to mitigate the impact of past or present deposition.

Plantlife is calling for:

• Nitrogen emissions to be tackled through government strategies on climate change, air quality, water quality and natural capital accounting.
• Nitrogen levels to be taken into account in monitoring and management of wildlife habitats – particularly on Areas and Sites of Special Scientific Interest.
• Statutory action plans in severely affected areas to reduce local emissions and restore damaged habitats.
• A coordinated UK framework providing effective regulation, incentives, advice and support to enable farmers to reduce nitrogen emissions.
• Greater public awareness of the impacts of air pollution on plants and ecosystems, putting pressure on governments and others to take urgent action.

Regarding recovery, the report concludes that a reduction in nitrogen deposition is necessary if habitats are to be protected but, even when this has been achieved, reversing the effects can be very slow. Over time, impacted habitats can sometimes, although not always, return to a state close to similar areas where no nitrogen has been added. However, this may take decades and lowering nitrogen deposition is often not enough – measures such as physically removing the accumulated nitrogen may also be required.

Christer Ågren

The report “We need to talk about nitrogen”: http://www.plantlife.org.uk/uk/our-work/policy/nitrogen