Nitrogen Oxides :: Arable

[See also: Nitrogen deposition :: Improved 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)

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 are driven by its role as an ozone precursor, and NO₂ impacts on human health,  rather than because of its direct effects on vegetation. However, direct effects may occur in the immediate vicinity of major roads and in the centre of cities, caused by high NO_x emissions from vehicles.

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).

Many NOx fumigations have involved crop species, particularly tomato, which is relatively

sensitive. Exposure to 250 ppb (495 µg m³) NO₂ markedly reduces growth (Taylor and Eaton, 1966).

Soybean was exposed to 100 to 500 ppb (~200-990 µg m³) NO₂ for 7 h per day for 5 days. The lowest exposures had favourable effects whilst the highest had inhibitory effects. 100 and 200 ppb (~200-400 µg m³) NO₂ exposure resulted in increased net photosynthesis and foliar nitrogen concentrations whilst 500 ppb NO2 reduced net photosynthesis, and chlorophyll and nitrogen concentrations (Sabaratnam and Gupta, 1988).

NO₂ exposure can also affect plant-insect interactions. For example, a range of aphid-host crop associations were fumigated with 100 ppb NO₂ and the mean relative growth rates of all aphid species except one increased by up to 75 % both during and following exposure (Houlden et al., 1990a).

It should be noted that the above concentrations are very high and would only be found in very close proximity to a source. The average background concentration for NO2 in the UK are around X.

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

None available.

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.