Nitrogen deposition :: Bogs

Effects and implications

• N can cause visible damage in S. capillifolium (Sheppard et al., 2011) especially the predominantly green pigmented capitula lose their bright sheen, and turn a dull olive brown. In this state these capitula can be easily dislodged.
• N deposition can change Sphagnum hummock integrity through changes in morphology: S. capillifolium stems may elongate with less branches and with lower mass (Manninen et al., 2011) giving rise to looser, less dense hummocks with correspondingly reduced water holding capacity.
• Reduction in Sphagnum cover can occur in response to: N accumulation and ammonium toxicity (Sheppard et al 2011); competition for light, shading when over storey plant growth is stimulated (Bubier et al., 2007) or when growth of more nitrophilous mosses is stimulated, again leading to shading (Greven, 1992).
• Other responses include increased cover of more N demanding nitrophilous graminoids eg Molinia caerulea and Deschampsia flexuosa (Danish ombrotrophic bog survey, Aaby, 1994) and sedges e.g. Eriophorum vaginatum (Sheppard et al. 2011) with the capacity to grow tall.
• The carnivorous Drosera species will be adversely affected if growth of tall species eg Eriophorum or Andromeda is stimulated by nitrogen deposition (Redbo-Torstensson, 1994). 
• Damage and cover reduction in lichens e.g. Cladonias on the drier hummocks: Visible symptoms  include bleaching, loss of apices and thallus greening (Sheppard et al 2011).
• Increased algal growth, especially where P and K supply are also improved, , which can overgrow and shade mosses.
• Species richness may not necessarily decline, and may even increase but the ‘replacing species’ are unlikely to be of functional or conservation value. These species changes can be seen in an increase in Ellenberg N, + 20 % @ 15-20 kg N ha^-1 y^-1 (Stevens et al 2011)
• Increased incidence of pathogens on Sphagnum: a fungal parasite (Limpens et al., 2003).
• Damage to Sphagnum leads to loss of capacity to bind (immobilise) incoming nutrients, making more available for the vascular plant over-story which increases and shades out the lower plants (Tomassen et al 1993; Limpens et al 2003; Sheppard et al 2013).
• Nitrogen accumulation in bogs will promote mineralization, is the conversion of organic N to NH₄⁺, and further increase N availability for vascular plants.
• Where a reduction in Sphagnum cover results, the important functions these species perform e.g. maintaining wet, acidic, anoxic, low nutrient conditions that help exclude plants that are not distinctive to bogs and restrict decomposition, will be compromised

Overview: evidence, processes and main impacts

N deposition effects have been reasonably well studied in bogs although studies have often been rather short-term, up to 3 years. It is clear from longer term investigations that when realistic N deposition rates are used significant effects may take > 5 y to show, e.g. declines in the moss understorey in response to increase in the vascular plant over-storey (Bubier et al 2007;Gunnarsson et al 2004).

Ombrotrophic bogs rely on atmospheric inputs for nutrients, and are thus highly sensitive to increases in N deposition (Thompson and Bottrell, 1998; Berendse et al., 2001).  Reductions in the cover of keystone species, absence of characteristic species, changes in species composition, proportions and abundance, expansion of nitrophilous (nitrogen-liking) species e.g. grasses, at the expense of lower plants, are the main causes for concern.

Bogs have largely been engineered by their keystone genus, Sphagnum, which underpin the acid, wet and anaerobic environment. Some Sphagnum species are highly N sensitive at N loads above the CL. Reductions in growth and declines in Sphagnum cover are often reported in response to enhanced reactive N deposition. Many hummock forming Sphagnum species have been shown to be sensitive to N deposition above the CL, whereas faster growing species found in wetter areas e.g. hollows and pools where ionic N concentrations are lower, tolerate higher N loads e.g. S. fallax. For N sensitive Sphagnum, exposure to N deposition in excess of their growth requirements leads to N accumulation, reduced growth, changes in morphology that lower hummock density and water holding capacity (Carfrae 2007; Manninen et al 2011), reductions in cover and possibly loss of that species from the site. Thus, N deposition may cause a specific species to be absent from a site. Lower plants (mosses, liverworts and lichens) in general are most at risk from enhanced N deposition. However, not all lower plant species are N sensitive, many are relatively tolerant of enhanced N deposition and shading (e.g. species of Hypnum). Some bog plants, even those with relatively low Ellenberg N values e.g Eriophorum vaginatum respond positively to additional N, but expansion of these species can lead to significant declines in the proportion of keystone functional groups e.g. Sphagnum species. Enhanced N deposition can increase the cover of some tall sedges and grasses to the detriment, potentially, of dwarf shrubs, Drosera species and lower plants, through shading. N enrichment disturbs the competitive balance between grasses and dwarf shrubs as the latter can rarely sustain their initial growth stimulation that can accompany N enrichment of bogs. Which species lose out on a bog as result of N enrichment depends to a large degree on the ability of adjacent and neighbouring species to utilise N and for the species that can’t exploit the added N, their ability to tolerate shading and stresses associated with N accumulation. 

Ammonia presents a much bigger threat to bogs per unit N deposited than wet deposited N, reflecting the rapid, large perturbations which cause species eradication.

The likelihood of invasive species being able to exploit the increased N availability on bogs and out compete native species will be greater in bogs surrounded by farmland or major roads which provide a reservoir of seed bearing grasses and ruderal plants.

Pollutant deposition type and risk

Indicators of N enrichment

These depend on the extent of the deposition and form of the N but relatively robust examples include:

• Increase in vascular plant cover and reduction in cover of lower storey plants.
• Reduction in cover of hummock forming Sphagnum e.g. S. capillifolium.
• Increase in overall canopy height
• Increase in Eriophorum cover.
• Increase in soil water mineral N.

Examples 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?

• Hydrology and water table height mostly determine species composition on bogs, but N deposition can upset and modify the relationships. Likewise, the height of the water table can exert a significant effect on the ability of Sphagnum species to use N (Williams et al 1999).
• Management can influence water table and cause effects that may mimic those from excess N deposition eg 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 of nitrogen deposition (Williams et al 1999).
• Bogs have low levels of most available nutrients, not just nitrogen (N). Phosphorus (P), and also potassium (K) availability can co-limit growth. Co-limitation by P or K will modify growth responses to N and affect the likelihood of 'grass' and nitrophile encroachment (Carfrae et al 2007; Sheppard et al. submitted). Long-term studies suggest that although higher PK availability may offset detrimental effects of N deposition in the short-term (few years), in the long-term they can significantly exacerbate N impacts, especially where the surrounding area provides a source of these faster growing species that typically colonise areas of disturbance e.g. some Juncus species, and species of Epilobium (rosebay willow herb) (Sheppard et al submitted).