Acid deposition :: Bogs

[Acid deposition: For Acid Deposition processes see overview link]

Effects and implications

• Changes in vegetation composition affecting bryophytes, lichens and species diversity of higher plant communities
• Disappearance of Sphagnum species: most disappeared from large areas of the southern Pennines and Peak District, probably a result of S (SO₂, H₂SO₄ and SO₄^2-) deposition (Ferguson et al 1978) over 200 years of industrialization. Losses were particularly evident from the mid 19^th century.
• Where acid deposition is high or concentrated, biodiversity will reduce reflecting the absence of acid sensitive epiphytic species.
• Acidification of bogs could release potentially toxic metal cations in areas subject to past deposition: the significance of this for the environment will depend on the level of deposition of these elements in the past, where water draining from peatlands ends up and what the water is used for. Such elements could subsequently accumulate in the food chain, pollute drinking water, or affect macrophytes growing downstream.

Overview: evidence, processes and main impacts

Bogs are naturally acidic, being rich in organic acids. Sphagnum mosses synthesize polygalacturonic acid (Clymo 1963) and decomposition leads to release of complex humic acid substances (Thurman & Malcolm 1981, 1983). These organic acids contrast the strong mineral acids that form acid deposition.

However, acid deposition plays an important part in the acidification of peat soils (Skiba 1989).  Whether or not acid deposition acidifies a bog depends on the net change in balance of cations versus anions in precipitation as it passes through the profile. It is possible that nutrient uptake and microbial reduction processes, e.g. sulphate reduction, can help mitigate acidification, at least below the water-table (Gorham et al 1984). Unlike mineral soils, ombrotrophic bogs are isolated from underlying mineral soil by several metres of peat. Thus they are relatively protected from problems associated with acidity increasing metal cation solubility ie Al³⁺..

Previous acid sulphate loading to peat bogs has caused significant acidification: with pool water pHs’ around 3.2 near large sulphur sources compared to 4.5 in remote areas (Gorham 1958). The cation exchange capacity (CEC) helps buffer peat bogs against acidification. Examples of mineral acids dominating acidity have only been reported for severely impacted bogs e.g. the southern Pennines in the 1970s (Gorham et al 1984).

Plant community composition is partly determined by the acidity of peat bogs and can change in response to increasing levels of mineral acidity. In the absence of a protective cuticle, lichens readily absorb rainfall of any pH making them highly vulnerable to acid deposition. Vascular plant roots may also be sensitive to high H⁺ ion concentrations so that if acidification is severe, sensitive vascular plants will be excluded. Physiological processes are also pH sensitive.

Since the existing H⁺ pool in peats is so large, it may take years for sustained acid inputs to influence acidity. Establishing the nature and extent of anthropogenic impact relative to the natural acidity of these ecosystems can be challenging (see Wilson et al, 1994).  Acid deposition effects on bog vegetation and the underlying peat are very closely coupled - the characteristics of the vegetation predetermine those of the peat and should always be considered together. Impacts have been observed on the functioning of peat ecosystems, e.g. decomposition, sulphate reduction, nitrate uptake, organic acid production (Wilson et al., 1995). Acid deposition has caused a decline in peat pH and drainage waters. The natural pH of Scottish peats has declined by about 0.5 pH units as a result of acid deposition, leading to a drop of about 1.1 pH units in drainage waters. Peats with highest acidity and lowest base saturation tend to occur in areas where atmospheric deposition is highest (Skiba et al., 1989; Cresser et al., 1993).

Soil pH affects directly the kind, density and the activity of fungi, bacteria and actinomycetes involved in decomposition. But, decomposition does not depend on a few species, rather, a consortium of organisms are involved, with complementary enzymatic activities (Andersen et al 2013). However, whether or not acidity restricts decomposition or slows it down is more difficult to establish. Also apart from locking up essential growth nutrients retarding decomposition on bogs would be a benefit in terms of carbon sequestration. What is known is that the composition of the decomposer community will change however, with acidity favouring fungi over bacteria (Rousk et al 2009). The bacterial community composition and diversity reflects and responds strongly to soil pH (Hartman et al 2008), especially the methanogenic community.

Rain-fed bogs acquire all their nutrients and water from the atmosphere, making them extremely vulnerable to the effects of atmospheric pollutants (Thompson and Bottrell, 1998). Hydrology is the most important determinant of the quality of peat bog ecosystems and the level of the water-table also affects acidity, via effects on the redox potential and the form in which ions occur in solution: this can influence the toxicity of metal cations e.g. Al³⁺ where the trivalent state is the most phytotoxic.

Pollutant deposition type and risk

Indicators of Acid deposition

• Fall in soil pH
• Increase in mineral acids
• Change in composition of epiphytic lichens, absence of acid sensitive species.
• Reduced growth, presence of acid sensitive vascular plants.

Examples of species specific responses

What factors modify acid deposition impacts?

• Low rainfall and falling or low water table will concentrate acidity and exacerbate the effects.

Evidence of recovery

• Increases in soil pH have been recorded over the UK.
• Where SO₂ has been the main driver of acid deposition changes, there is good evidence of acid sensitive Sphagnum returning in response to the decline in SO₂ concentrations. Also, efforts to artificially restore degraded areas of the Pennines with Sphagnum are proving successful.
• Sites in the Peak District, which had been heavily impacted by atmospheric deposition over the last 100 years, were resurveyed in 2005/06 using the same locations and observers as in the early 1980s. The number of moss
• species increased significantly at both sites, and the liverwort species, which were completely lost in the early 1980s, have recovered. However, species numbers are still well below those expected for pristine conditions; this may reflect the long timescale of recovery or the continued impact of nitrogen deposition in the area (RoTAP, 2012).