Nitrogen Deposition :: Dunes, Shingle & Machair

Effects and implications

• These systems are adapted to low levels of mineral N availability: increasing the availability of N will threaten the competitive balance between species leading to changes in composition and loss of habitat species constants.
• Speeds up succession through the chronosequence, movement between the dune stages.
• Lichens and mosses are particularly sensitive both from direct effects associated with N accumulation and from shading as a consequence of increase growth of overstorey vegetation in response to N deposition
• Species sensitivity to other stresses e.g. grazing pressure, desiccation and pathogens may be enhanced.
• Potentially damaging interaction between N deposition and grazing, but grazing may offset eutrophication effects on graminoids (grasses). 
• Species composition changes likely to be mediated through variability in P acquisition and demand between species.

Overview: evidence, processes and main impacts

Sand dune habitats are one of the most natural remaining vegetation types in the UK, supporting over 70 nationally rare or red-data book species. Pressures threatening their existence include: sea-level rise, climate change, agricultural improvement, recreational use, lack of management, over-stabilisation and N deposition. They are generally infertile and thus sensitive to N deposition. The greatest impact from N is likely to be on early succession communities which include many of the sand dune rarities. In UK dunes, effects of N occur through eutrophication and impacts on soils (Phoenix et al. 2012). In semi-fixed (open) dune habitats, N deposition increased cover of marram grass (Ammophila arenaria) and increased total biomass (Jones et al 2004), while in fixed dune grasslands, plant species diversity decreased and biomass increased. Acid dune systems appear to be more sensitive to N inputs than calcareous dunes. A gradient survey of a UK dune grassland community just starting to acidify showed strong declines in species richness (Hall et al. 2011). On the continent, experimental evidence from sand dune mesocosms in the Netherlands has shown that N deposition increases grass and sedge cover, and decreases herbaceous species cover (van den Berg et al. 2005). Nitrogen increased. over of the sand sedge Carex arenaria  in Baltic areas, and altered soil processes in acidic dune systems (Remke et al. 2009ab). Ammonia deposition, can pose a real risk for the more acidic communities and there are likely to be point sources of wild animal and bird colonies e.g. seals, seabirds, geese etc sufficiently close to affect these systems. Anthropogenic point sources such as poultry units also present a serious risk to dune vegetation, altering plant growth and plant tissue N contents, even several kilometres upwind (Jones et al. in press). The risk of change in species composition, driven by N deposition depends partly on a local source of propagules (seed, spore or fruit) e.g. farmland. The different types of dune system, from the sea moving inland will be differentially at risk

UK sand dune soils are generally well-buffered (high carbonate content), with the exception of the few acidic dune systems, and do not suffer from N led acidification. Soil C:N ratios generally increase (Jones et al. 2004), in contrast to other systems, while available N appears to decrease. Dissolved organic nitrogen (DON) concentrations in groundwater generally increase (Jones et al 2004). Nitrogen retention in sand dune soils is poor, due to the low levels of organic matter, so that excess N will be leached (ten Harkel et al. 1998; Hall et al. 2011), often as DON. Grazing (rabbits) will enhance leaching losses, but least from calcareous soils (Jones et al. 2005). However, sufficient N can be retained to cause adverse effects. Significant correlations between soil development and N deposition have been demonstrated over time (Jones et al. 2008), with implications for rates of successional development and long-term species change in dunes (Rowe et al. 2011). The moss layer is probably responsible for this N retention, retaining N deposition and slowly releasing it into the soil system when basal layers decompose (Plassmann et al. 2009).

Phosphorus limitation is common in many UK dunes and will moderate vegetation responses to excess N deposition. However, as the soil pH approaches 5, P solubility will increase and thus its availability, increasing the likelihood of adverse N stimulated eutrophication. Dunes affected by N deposition where P availability is sufficient to permit a growth response are likely to become dominated by Festuca rubra and to a lesser degree by Poa pratensis at the expense of other grasses, herbs and particularly mosses and lichens. Under P limiting conditions the loss of diversity will be restricted to the N sensitive species. Negative impacts of N most likely result from reduced light due to the expansion of grasses and forbs and increased litter production by these groups (Jones 2002). Dominant dune building species i.e. Ammophila arenaria, Elytrigia juncea and Leymus arenaruis (Harkel et al 1998; Veer & Kooijman 1997) are likely to respond positively to N deposition by increasing their cover.

Fixed dunes have fewer nutrients from sea spray, some soil development, but are generally N and P co- limited. The vegetation of dune slacks reflects soil moisture and pH. Young slacks in the UK tend to be base rich and co-limited by N and P. Eutrophication will favour graminoids (e.g. Agrostis stolonifera, Carex flacca, Holcus lanatus), at the expense of Anagallis tenella, Leontodon saxatalis, Prunella vulgaris and Plantago coronopus. Expansion of Poa pratensis is dependent on P availability. Salix repens is likely to expand as it is able to access P via ectomycorrhiza, but it can become susceptible to pests and enhanced herbivory (e.g. beetle Lochmea capreae and rabbits) (Ranwell 1959). Sedges are also likely to expand as they are able to access P via their dauciform roots (hairy lateral roots). Thus species having a low P demand, or effective P foraging capacity, will expand with N eutrophication at the expense of competitors with higher P requirements. 

Shingle: there is very little known about N effects on these communities. We assume they behave in a similar way to acidic dune communities (shingle is usually acidic), but note that moisture is strongly limiting and may moderate N responses in early successional shingle habitats.

Machair systems represent former beaches standing above the current adjacent beach. Machair sands owe their fertility largely to their high seashell content, up to 90%. Machair systems are unique, supporting rare carpet flowers including Irish Lady's Tresses, orchids and Yellow Rattle, along with a diverse array of bird species including the Corn Crake, Twite, Dunlin, Common Redshank and Ringed Plover, as well as rare insects such as the northern colletes bee. Some Machairs are threatened by erosion caused by rising sea levels as well as by recreational use of adjacent beaches. There are no known studies of atmospheric N deposition impacts on Machair. In principle a critical load may be set by comparison with other similar species-rich calcareous grassland types. In practice, however, N deposition rates in the main areas of Machair on the western Isles of Scotland are rather small due to the absence of orographic enhancement, but sometimes can be affected by bird and seal colonies which are the most likely local source of N as ammonia. Machair sand should be relatively nutrient rich (P, K, Ca, Mg) but are expected to be N limited, and therefore extremely sensitive to N deposition given the potential availability of these other nutrients.

Pollutant deposition type and risk areas

Indicators of N enrichment

• Change in species composition due to eutrophication, and shading of the lower storey vegetation, mosses, lichens and forbs, by taller faster growing species e.g. graminoids and sometimes sedges.
• Increased moss and lichen tissue N content.
• Acceleration of soil development and the natural chronosequence of plants and selective loss and replacement of N fixing forbs.
• Increased pests and grazers.

Below ground

• Increase in soil C:N ratios despite poor N retention. 
• Increased N leaching and increase in groundwater DON concentrations. 

Example evidence of species specific responses

Some examples of specific responses are given in the table below. This does not represent a comprehensive review of all species impacts.

What factors modify N deposition impacts?

• P availability: consequences of responses favouring grass species are worse where P availability is higher.
• Loss of the rich diversity of plants and replacement by graminoids, grazing management may help offset some of the eutrophication effects.
• Grazing will modify amounts of N leached
• pH: acid dunes ecosystems are more sensitive than calcareous ones.
• Dune systems were historically managed by grazing (sheep, horses, cattle) which help remove nutrients, but can accelerate N cycling and cause damage by trampling and overgrazing.