Nitrogen Oxides :: Grasslands

[See also: Nitrogen deposition :: Calcareous grassland; Nitrogen deposition :: Acid grassland]

Effects and Implications

• Effects are mainly on growth, photosynthesis and nitrogen assimilation/metabolism with few species showing visible injury.
• Visible decline symptoms for example, leaf discoloration can occur at very high concentrations (> 400 ug m³)
• Direct damage to mosses, liverworts and lichens, which receive their nutrients from atmospheric deposition, often leads to reductions in species diversity, but also leads to an increase in nitrogen-loving species.
• Species composition changes

Overview: evidence, process and main impacts

Direct effects of NO_x (the sum of nitric oxide NO and nitrogen dioxide NO₂) have been recognised in the setting of a critical level concentration (Ashmore and Wilson 1994, Sanders et al. 1995). The gases are considered together partly because their concentrations in air are inextricably linked through their atmospheric chemistry, and partly because little is known of the direct effects of NO alone. However, rates of NO_x deposition are rather slow, so that local impacts are likely to be less than for other pollutant gases such as ozone or ammonia.

The major role of NO_x is as a transboundary pollutant, and its conversion in the atmosphere to nitric acid (HNO₃) vapour and nitrate particles which are deposited directly or in precipitation many hundreds of km from sources. NO_x is also a key precursor for ozone production in the atmosphere. Emission controls (in addition to those set out to protect human health) are driven by its role as an ozone precursor rather than because of its direct effects. However, direct effects may occur in the immediate vicinity of major roads and in urban areas, caused by high NO_x emissions from vehicles. NO_x may lead to ground flora changes related to eutrophication.

A report prepared for English Nature by Bignal et al 2004 summarised the plant responses to the exposure of the motor vehicle exhaust and motor vehicle pollutants under controlled conditions (Table 1).

Table 1: Summary of plant responses to exposure to motor vehicle exhaust and motor vehicle pollutants under controlled conditions. This table shows the responses to NO₂ and NO.

+ or - is an increase or decrease in the response, 0 is no effect. The results are based on a literature review of studies. (Derived from Bignal et al. 2004).

Work by Angold (1997) in the New Forest, Hampshire, has shown that there is a close correlation with the extent of damage to plant health and the volume of traffic the road is carrying. Most available information is based on work from lowland chalk grasslands in southern England and the Netherlands. Reports of species composition change associated with enhanced N loading have become more widespread in recent years see Calcareous grasslands/N deposition record).

As with ammonia, NO_x may lead to ground flora changes related to eutrophication.  A key concern in this ecosystem is the increase in cover of false brome grass (Brachypodium pinnatum) at the expense of overall species diversity (Baxter and Farmer 1994). Subtle changes in plant morphology, physiology and biochemistry can occur which do not affect growth but increase sensitivity to environmental factors such as wind, frost, drought and pests.

The grass Poa pratensis showed decreased growth in 68 ppb (135 ug m³) NO₂ exposed for 140 days in winter (Ashenden, 1979). Three other grass species tested showed increases in leaf area; one of these species had reduced above and below-ground dry weight, one increased in dry weight and the other increased the above ground biomass whereas the roots were decreased in dry weight.

Pollutant type and risk

The measured NO₂ concentrations across the UK highlight the predominance of traffic and urban sources, with the largest concentrations in the large conurbations and adjacent to the motorway network, with annual mean concentrations in excess of 20 µg m^-3 in these areas. Urban centres are hot spots due to the high density of traffic emissions and the smaller contribution from the residential and commercial sectors, as well as other area sources such as mobile machinery (e.g. excavators, bulldozers, and front and back loaders) (RoTAP, 2012). However, in rural areas, away from industry and roads NOx concentrations are usually low and not a concern to ecosystems.

Indicators of NOx impacts

Biochemical changes have only been used as additional indicators of potentially relevant ecological responses. In an ecological context, growth stimulation and reduction are both potentially negative responses. For instance, NOx (and NH3 and NH4 +) generally cause an increase in the shoot:root ratio, which may or may not be beneficial (WHO, 2000)

Examples of species specific responses

What factors modify NOx impacts?

Responses to nitrogenous pollutants can be further modified and exacerbated by interactions with other environmental factors, including frost, drought and pest organisms. These interactions generally include increased susceptibility to these factors, which may in turn lead to major ecological changes.

Nitrogen oxides are known to have greater adverse effects in the presence of SO₂ or O₃, and hence the critical level should apply where these pollutants are also close to their critical level.