Ammonia :: Bogs

Effects and implications

 [Ammonia is one of the key pollutants that contribute to nitrogen deposition. Please read the Nitrogen deposition - Broadleaved, Mixed and Yew Woodland record to understand the full impacts effects of nitrogen deposition including ammonia.]

• Ammonia exposure will predispose sensitive plants to stress much faster than wet N deposition ie. at lower N loads.
• Effects will vary depending on the exposure concentrations, the length of time of exposure and whether PK deposition has also been increased, e.g. dust from point sources.
• Direct damage to sensitive species, e.g. bleaching and leaf discoloration,  observed in Sphagnum species and lichen Cladonia portentosa at high >20 µg m^-3 concentrations. Bleaching is a particularly likely consequence of NH₃ exposure. Breakdown of Sphagnum hummocks and increase in bare peat which can increase the likelihood of erosion and surface oxidation.
• Increase in algal growth over Sphagnum especially where PK also enriched.
• Reduced ability of stomata to close under drought conditions, leading to plant water stress (Van Hove et al. 1991, Erisman and Draaijers 1995) highly visible as greatly increased amount of grey foliage in Calluna a consequence of winter desiccation(Sheppard et al 2008; 2011).
• Changes in the composition of ground-flora, bryophyte and lichen communities.
• There may also be subtle changes in plant morphology, physiology and biochemistry which not only increases growth, but also increases sensitivity to environmental factors such as wind, frost, drought and pests (e.g. increased tissue N concentrations can predispose plants to insect attack).

Overview: evidence, processes and main impacts

Ombrotrophic bog ecosystems represent one of the few ecosystems where the effects of ammonia have been studied in situ, both through manipulation (Leith et al 2004; Sheppard et al 2011) and in transects from point sources (Sutton et al 2009).

They are particularly vulnerable to ammonia deposition. Ammonia is an alkaline gas and these bogs, which comprise largely Sphagnum moss and peat, provide a large, wet and acidic sink for ammonia. Furthermore, many bog species have very high surface areas to mass (Jones 2006), which together with their low N compensation point, increases their capacity for uptake (Dragosits et al 2008). Both these factors increase the likelihood of ammonia uptake, especially at low concentrations. The compensation point ‘controls’ the amount of ammonia taken up: because these systems are generally N limited their internal N status is low, when internal concentrations are below those in air NH₃ will deposit, until the system saturates.  Effects of ammonia are likely to be mediated in the gaseous form as a concentration, for which critical levels apply, or in ionic form dissolved in precipitation which can be taken up as NH₄⁺ directly into shoots or washed off onto the soil and taken up via the roots. The revised Critical Levels for ammonia (Cape et al 2009) provide the most protection for the ecosystem.

A long-term field experiment at Whim bog in the Scottish Borders where ammonia effects were compared with those of wet deposition (Sheppard et al., 2011) showed that ammonia was far more damaging than wet deposition per unit N applied. Significant changes in species composition and loss of the hummock forming keystone Sphagnum, S. capillifolium were observed   Reductions in the cover of keystone species, absence of characteristic species, changes in species composition, proportions and abundance, expansion of nitrophilous species e.g. graminoids, at the expense of lower plants are the main causes for concern. A transect study in Ireland, extending from a point source to Moninea bog (Sutton et al 2011) found very similar types of highly visible damage to hummock Sphagnum, eradication of some Cladonias by excessive amounts of algae.

Ammonia can cause stomatal malfunction, ie. disrupts stomatal closure: enhancing uptake and also increasing the likelihood of drought stress, through uncontrolled water loss. This has been demonstrated for Calluna although other ericoids e.g. Vaccinium myrtillus, Empetrum nigrum and Erica tetralix appear to be less sensitive (Sheppard et al 2008). Ammonia deposition also leads to high foliar N status which can increase the risk of damage from fungal pathogens e.g. on Calluna, and other abiotic stresses e.g winter desiccation (Sheppard et al 2008).

Detrimental effects on Calluna, which helps structure Sphagnum hummock formation and can protect the lower storey plants from exposure to higher concentrations (filtering), can increase the likelihood of damage to under storey lower plants. High ammonia concentrations promote rapid uptake and accumulation of toxic ammonium ions in plant tissue (Krupa 2003), and can cause rapid loss of large areas of vegetation. This loss of vegetation leads to exposure of the peat, facilitating oxidation and accelerating decomposition and mineralisation, e.g loss of CO₂, until recolonisation, expansion or invasion re-vegetates the surface. In the absence of competition from Calluna, the expansion of both Eriophorum vaginatum and E. angustifolium is likely. Likewise, nitrophilous (nitrogen-liking) mosses e.g. Campylopus spp. are more likely to colonise the bare peat. Exposure to high NH₃ concentrations facilitate invasion by non characteristic and ruderal species e.g. Epilobium, the nitrophilous fern Dryopteris dilatata, Birch seedlings and Digitalis purpurea (Sheppard et al 2011; Greven 1992). Ericoids and other typical climax species along with co-habiting species are mostly characterised by relatively slow growth rates and mycorrhizal symbioses and are less able to up-regulate their C assimilation in response to increased N supply. However, the graminoids within these communities e.g Molinia and Eriophorum do have the ability to up regulate their C assimilation and are likely to expand into any gaps in the canopy left by damaged Calluna. Thus bogs that are surrounded by sources of fast growing ruderals and nitrophilous grasses are vulnerable to seeing their species composition change as ammonia sensitive plants are ’replaced’ by nitrophiles, which will alter community structure and function and threaten ecosystem sustainability.

Vulnerability to ammonia deposition will depend on location, because the gas is chemically reactive concentrations fall rapidly and exponentially from its source (Dragosits et al 2008). However, the majority of bogs are not close to ammonia sources. Evidence from Whim bog, where the bog has been exposed to ammonia for nearly 11 years has shown that threshold concentrations for visible damage and loss of cover decrease with time, indicating a cumulative NH₃ effect. There also appeared to be a lag in the response time of under storey species, most likely reflecting the increase in level of exposure once the over storey canopy has gone, e.g. speed of damage to Calluna compared with Sphagnum. This study also flagged up the importance of rainfall in ameliorating the detrimental impacts of ammonia, which may translate into species swapping e.g. species of Sphagnum associated with wetter environments, which appear to be less NH₃ sensitive, to expand.

Because ammonia deposits as a gas, Critical levels have been derived to protect ecosystems, although, once deposited NH₃ gas represents part of the N load to that ecosystem.  

Pollutant Type and Risk Areas

Most ammonia sources have an agricultural association (animals, manure and fertiliser and senescing vegetation), but all animals in large numbers can generate significant emissions. Because of the fundamental role of hydrology in bogs and the link with species composition, rainfall and water table have an overriding influence on the species composition of bogs. Where bogs are subject to hydrological stress the risk of deleterious N effects and species change will be exacerbated e.g. Eastern Britain. However, ammonia is less likely to deposit to dry surfaces so whereas effects may be higher the risk in the east may be lower.

Most of the blanket bogs in the Flow country and on the Scottish islands are too far away from agricultural sources to be affected by these, however, wintering geese, gull and seabird colonies and seals are also significant ammonia sources which if they are within 1-3 km may affect these bogs. Bogs in other areas e.g. Wales, Cheshire, the Pennines and Northern Ireland may be at risk from elevated background ammonia concentrations from agricultural emissions. Thus most bogs will escape the high NH₃ concentrations associated with acute toxicity, but probably contain some sensitive species where effects of cumulative exposure are an issue.

Indicators of NH₃ impact

These depend on the extent of the deposition and level of NH₃ exposure, but relatively robust examples include:

• Bleaching on sensitive species, usually occurs first on lichens e.g. C. portentosa.
• Damage to hummock species and disruption and physical breakdown of the hummock.
• Damage, greying of Calluna foliage, breakdown of Calluna canopy.
• Increase in nitrophilic species, including mosses able to capitalize on the increase in bare peat e.g. Campylopus introflexus.
• Which nitrophilic species are present will depend on other factors e.g. peat pH and which other nutrients are potentially limiting.
• Absence of habitat constants ie. those species which are considered to be integral components of an ecosystem e.g. Sphagnum.
• Increase in Ellenberg N.
• Increases in foliar N
• More pathogen damage

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 ammonia impacts?

• Hydrology and water table height mostly determine species composition on bogs, and NH₃ exposure can upset and modify the relationship.
• Managementcan influence water table and cause effects that may mimic those from excess N deposition e.g. lowering the water table will favour many fast growing vascular plants but be detrimental for Sphagnum, the consequences of which will also favour vascular plants. Water table also influences how Sphagnum utilises N, low water tables increase the likelihood of detrimental effects (Williams et al 1999).
• Droughts will exacerbate NH₃ impacts on vascular plants. However, dry conditions also restrict ammonia deposition.