Nitrogen deposition

What is it?

Nitrogen (N) deposition describes the input of reactive nitrogen from the atmosphere to the biosphere both as gases, dry deposition and in precipitation as wet deposition. Enhanced reactive nitrogen deposition is a consequence of  global emissions of oxidised nitrogen (NO, HNO₃ and NO₂ – often referred to as NO_y) from fossil fuel combustion (Dignon and Hameed, 1989), and reduced N (NH_x) from agricultural sources. Effects of ammonia gas and dry deposition of ammonia (NH₃) is addressed in a separate overview.

Emissions and Deposition

Oxidised Nitrogen

Dry deposition of nitrogen oxides is greatest within large conurbations and close to major highways, where annual mean concentrations can exceed 10 ppb. Nitrogen dioxide (NO₂) is mostly emitted from motor vehicles and heating sources. Atmospheric oxidation produces nitric acid and particulate and aqueous NO₃^-, the main NO_y components of wet deposition. Since 1986, UK emissions of NOx have declined by 50% (RoTAP 2012). However, annual average nitrate concentrations in rainfall have fallen by only 22% over the period 1986 to 2008. The relatively smaller fall in concentrations / deposition is thought to be due to a significant reduction in NO₂ export (62%) (RoTAP 2012).

Reduced Nitrogen

Reduced nitrogen comprises mainly gaseous NH₃, aerosol NH₄⁺ and wet deposited NH₄⁺ which are monitored throughout the UK in the wet and dry deposition networks (see UKEAP ). Wet deposition of reduced N comprises fine particulate ammonium (NH₄⁺) salts or aerosols of acidic gases. These components have a relatively long atmospheric residence time, 4 to 15 days, and when removed by precipitation contribute to N deposition in remote ecosystems after long-range transport (Asman et al. 1998). Dry deposition of reduced N shows strong local-scale variability and is closely correlated to emissions from livestock emissions. The concentration of ammonium in wet deposition has fallen by around 35% over the period 1986 to 2005 (RoTAP 2012). Emissions of NH₃ in the UK peaked in the 1980s and have decreased by about 15% since then. Relative to trends in emissions of SO₂ or NOx, the decline in NH₃ emissions is small (RoTAP 2012).

In 2008 the amounts of reduced (175 Gg-N yr^-1) and oxidised (178 Gg-N yr^-1) forms of N in deposition were approximately equal. However, proportions of NH₄⁺ and NO₃^- ions in precipitation, like the ecosystems they deposit to, are not equally distributed spatially, reflecting in part the non-uniform distribution of their sources (RoTAP 2012).

In addition, the loading of N in wet deposition depends not only on the amount of N but also the amount of precipitation. In the east, N concentrations can be quite high due to the low rainfall, whereas in the west the rainfall is much higher but the concentrations tend to be lower. Reduced N will dominate deposition in rural areas while oxidised N dominates in urban conurbations.

Nitrogen Deposition effects on habitats and species 

Nitrogen is a major growth nutrient: all plants need N in order to grow. It is a major constituent of assimilatory and structural tissue, facilitating conversion of CO₂ to carbohydrate and combining with carbon (C) to form amino acids (Marschner 2005).

Vascular plants take up most of their N through their roots but some can be absorbed above ground via stomata (gases) or the cuticle. Non vascular plants can absorb N through their entire surface (e.g. lichens and bryophytes). Most plants use reactive N, but some can use organic N, e.g. amino acids. If carbon (C) assimilation is restricted, e.g. by insufficient phosphorous (P), light or water, then N can potentially accumulate to excess and become toxic. In other words, N no longer acts as a nutrient rather it becomes a pollutant. Too much N is accepted as one of the main drivers of biodiversity change across the globe (Sala et al 2000).

Communities most at risk from N eutrophication are those rich in bryophytes and where species richness is comprised of slow growing species. Many semi-natural plants do not have the capacity to assimilate nitrogen in the presence of increased N availability (from N deposition) and can be outcompeted by plants that can, e.g. many graminoids (grass) species. This species loss is caused by shading or an inability to compete for other limiting resources. Low growing species such as forbs and non vascular plants are especially at risk. Such species replacements can lead to loss of specialised communities and ecosystems, e.g. heathland transformed into grassland in the Netherlands (Bobbink and Heil, 1993).

Steven et al, 2011 found that changes in both species occurrence and three ecosystem function indices (canopy height, specific leaf area and Ellenberg N) often occur at low levels of N deposition (<10 kgN/ha/yr). This is sometimes lower than established critical loads indicating that they are not set at a level which prevents an impact to all species or ecosystem functions. Changes in species occurrence and indices of ecosystem function also progressively continue as N deposition levels continue rising above the current critical load values. This indicates that ongoing damage occurs above the critical load and there are some benefits from reductions in deposition even if it remains above the critical load.

N deposition can also increase the risk of damage from abiotic factors, e.g. drought (summer and winter) and frost. Where N deposition leads to enhanced foliar N concentrations there is increased risk of damage from pests and pathogens both above and below ground. Detrimental impacts of N below-ground include loss of species diversity with respect to ectomycorrhiza and reductions in decomposer populations, e.g. enchytraeid worms. Nitrogen can also increase litter fall, reducing the amount of light passing through to ground dwelling species.

Habitat management is often used to maintain the balance between slow and fast growing species in some systems. However, getting the balance right between management intervention and N deposition is a complex issue of optimising positive and negative outcomes.  Give link and reference to this recent review.... Review of the effectiveness of on-site habitat management to reduce atmospheric nitrogen deposition impacts on terrestrial habitats 

N deposition can also cause acidification of soils. More details on the effects of acidification can be read here.

While direct toxicity from wet N deposition is less common, except among non-vascular plants, dry deposition of NH₃ can cause toxicity (Marschner 1995). Plants can only control uptake through the stomata. However, mechanisms can be damaged by NH₃ which facilitate greater uptake (see Ammonia overview).

Does the form of N make a difference?

For assessment of effects it used to be assumed that nitrogen originating from NH_x or NO_y has the same ecological effect (Sutton and Fowler 1993, Hornung et al. 1995). Currently, critical loads for N deposition do not distinguish between reduced and oxidised N.

UK wide survey work and manipulation studies have shown that the form of N deposition does affect outcomes, but the situation is complex; responses may be species specific or not apparent (i.e. species are insensitive to form and only the N dose matters). Furthermore, both forms can be damaging to species, but the mechanisms underpinning the damage may be quite different, e.g. Sphagnum capillifolium is sensitive to both reduced and oxidised N, the former probably mediated by accumulation and toxicity and the latter via increasing cellular pH (Sheppard et al 2014). In addition, both above and below ground transformations of the deposited N form can occur (Stevens et al 2010).

Smart et al. (2004) applied model estimates of NO_y and NH_x deposition across the UK for 1996 to explain national scale fine-grained changes in species composition between paired fixed plots from two Countryside Surveys, 1990 and 1998 (Haines-Young et al. 2000). These surveys established a significant positive relationship between NH_x and cover weighted Ellenberg fertility scores for semi-natural grasslands, heaths and bogs. Stevens et al. (2006) concluded that, among NVC U4 grasslands, species richness and cover of forbs declined significantly, particularly in response to NH₄⁺-N deposition. However, it should be pointed out that such surveys cannot distinguish between wet and dry NH_x. It is predicted that dry deposition presents the greatest threat (Krupa, 2003; Sheppard et al 2011).

Dose Effects

There is global acceptance that once N deposition exceeds the demands of sensitive plants, semi-natural ecosystems are at risk of species loss and changes in structure and function (Bobbink et al 2011). However, what is not always clear is what drives the changes; is the vegetation responding to dose or concentration, and is it the annual or cumulative load that is important, is the response linear or does it show a threshold? Several studies have shown that the concentration of ions in wet deposition, together with application frequency, can exacerbate the effect of dose (Pearce and van der Wal 2008: Sheppard et al 1998).  Non-vascular plants are particularly sensitive to high concentrations and repeated exposures as these challenge their ability to both repair and recover from the damage. Thus N damage must be considered in terms of more than just the load. The role of cumulative N load has been discussed by Phoenix et al (2012) using data from long-term N manipulation experiments undertaken in the UK (UKREATE, 2010). The logic underpinning relationships with cumulative deposition are that N can remain in the soil and soil chemical reactions can respond like a bucket that becomes full and starts to overflow, but different systems may be represented by different size buckets and start at different levels. Factors determining the response to N are thus many and varied.

Modifiers of N responses: climate, other nutrients

Several manipulation studies have shown that N effects exacerbate sensitivities to climate extremes, e.g. drought, freezing stress and winter desiccation. Calluna appears to be particularly sensitive to abiotic stress and winter desiccation following enhanced N deposition (Sheppard et al 2008, Carroll et al 1999). Sensitivity to biotic stress from increased pest and pathogen activities is also much more common, e.g heather beetle (Berdowski and Zeilinga 1987; Berdowski 1993).

Sources of evidence showing or predicting N impacts

There is a wealth of information and reviews on nitrogen deposition impacts.  Some examples of these are listed in the table below.