Peatlands are found throughout Europe where a suitable cool, moist climate exists; they are particularly widespread in Russia, the Scandinavian countries, the British Isles, and north-eastern Europe (Fig. 1). Partially decomposed organic material accumulates as carbon-rich peat due to an imbalance between the rates of primary production and decomposition, usually because of restrictions on soil oxygen availability, temperature and nutrients. The low oxygen and nutrient levels, waterlogged conditions and cool climate support a unique, characteristic vegetation community dominated by Sphagnum moss and including specialised vegetation such as dwarf shrubs and carnivorous plants.
Air pollution can change both the species composition and the functioning of peatlands. The primary atmospheric pollutant from the Industrial Revolution to the mid 1970s was sulfur deposition, but levels have since greatly declined. Reactive nitrogen (N) deposition (primarily NO3- and NH4+), which can both acidify and eutrophy, became significantly elevated over a widespread area in the early to mid-20th century(3) and is now the major pollutant in atmospheric deposition across most of Europe(4).
Nitrogen is commonly a limiting terrestrial nutrient, and in unimpacted peatlands it is tightly cycled. With long-term elevated N deposition, vegetation composition typically shifts toward species adapted to higher nutrient levels, with an overall loss of diversity(5). In peatlands, field experiments with N additions within the current European range have shown significant declines in bryophyte species richness and productivity, and shifts in composition toward vascular plants (6,7).Community shifts toward more nitrophilous bryophytes in N-enriched regions such as parts of the Netherlands are also well documented (8). In the UK, both a general survey of peatlands across the country(9), and a targeted study of Calluna moorland(10) (Fig. 2) showed significant inverse relationships between levels of nitrogen deposition and species richness, with bryophytes particularly impacted.
|Fig 1 Distribution of peatlands in Europe. Dashed line shows the approximate location of the 5 kg N ha-1 y-1 isocline. From (1,2).||Fig 2 Relationship between bryophyte species richness and N deposition at 36 Calluna moorland sites in northern Britain 2005-6. From (10) .|
Changes in the vegetation also impact below-ground communities and biogeochemical processes. In central UK peatlands, for example, microbial biomass was found to be several orders of magnitude higher in diversely-vegetated sites as compared to degraded locations(11). Research in prairie grasslands also shows that soil microbial biomass increases in proportion to above ground vegetation species richness, even if the biomass of the vegetation does not change(12).
Moderate increases in N deposition from a low level may increase Sphagnum and vascular plant productivity without an equal increase in decomposition rates, leading to enhanced carbon accumulation(13). However, shifts in species composition from bryophytes to vascular plants may increase the production of easily-decomposable plant material, leading to higher rates of decomposition, and reduced carbon accumulation(7,14) If the capacity of the ecosystem to assimilate nitrogen is exceeded, continuing elevated N input can lead to an uncoupling of the nitrogen cycle, with elevated losses in runoff of NO3- and NH4+ (dissolved inorganic N, DIN), and as gaseous NO and N2O(14-16).The particular sensitivity of nutrient-poor ombrotrophic peatlands to nitrogen enrichment is reflected in the low critical load threshold of 5 kg N ha-1y-1 for these ecosystems(17), a level which is exceeded over a significant portion of their range (Fig 1).
Superimposed upon the impact of N deposition is the changing climate in northern Europe (18).Although it is not clear whether effects of nitrogen and a warming climate are additive, synergistic, or independent, there is reason to believe that elevated N deposition predisposes a peatland to sensitivity to other stressors. An N-addition study on the biogeochemistry of a peat bog in the Italian Alps that coincided with the 2003 heat wave showed a more pronounced impact, and longer delay in recovery, in sites receiving the highest N treatments(19). There may be several mechanisms for this: for Sphagnum, cell structure and function, particularly water-retention capacity, can be damaged at chronically elevated N deposition, leading to a reduction in resilience to further environmental stress such as drought(20). For vascular plants, nitrogen fertilisation can lead to nutrient imbalances as well as a reduced and shallower root mass. These vegetation impacts lead to higher peatland vulnerability to pathogens, drought and fire, and loss of carbon through erosion(21).
In some peatlands, damage due to a combination of pollution and climate stress is already occurring: in the southern Pennines of the UK, for example, with high N pollution, a legacy of S pollution and a distribution at the southern edge of the European peat-forming zone, a dramatic decline in bryophyte species richness has been recorded(22), together with a loss of microbial diversity and extremely high levels of soil solution NH4+ and NO3-(23). Catastrophic peat loss, including gullying, slumping, and wildfires after dry summers, is now documented in these areas(24). Without intervention it is likely that these degraded peatlands will ultimately convert to grassland or shrubland (25).
(1) Montanarella et al. (2006) Mires and Peat 1: 1-10. (2) www. emep.int (3) Fowler et al. (2004) Water, Air, Soil, Pollution:Focus, 4, 9-23(4) Fowler et al. (2005) Environmental Pollution, 137, 15-25. (5) Malmer & Wallén (2005) Oikos 109, 539-554 (6) Bobbink et al. (1998) Journal of Ecology 86: 717–738 (7) Bubier et al. (2007) Global Change Biology 13, 1168–1186 (8) Greven, H.C. 1992. Biological Conservation 59: 133-137 (9) Smart et al., (2003) Global Change Biology 9, 1763–1774 (10) Caporn et al. (2007) Report to Defra, London (11) Sen et al. (2007) unpub. data (12) Zak et al. 2003. Ecology 84: 2042-2050 (13) Turunen, et al. Global Biogeochemical Cycles 18, GB3002. (14)Lamers et al. (2000) Global Change Biology, 6, 583-586 (15)Helliwell et al. (2007) Water, Air, Soil, Pollution, 185, 3-19 (16)Skiba et al. (1998) Environmental Pollution, 102 (S1) 457-461 (17) Achermann, B. & Bobbink, R. (2003) Proceedings Empirical Critical Loads for Nitrogen. Expert Workshop. November 2002. SAEFL, Berne. (18) Christensen, J.H. et al. In: Fourth Assessment Report of the Intergovernmental Panel on Climate Change CUP Cambridge.(19) Gerdol et al. Global Change Biology 2007 13: 1810-1821. (20)Van Der Heijden et al. (2000) Global Change Biology 6: 201-212.(21) Davison & Barnes (2002) In: Air Pollution and Plant Life (eds. Bell, J.M. & Treshow, M.) John Wiley & Sons Ltd. London. (22) Lee (1998) Journal of Ecology 86, 1-12 (23) Caporn, Sen, et al. unpublished data; (24) Evans et al. (2006) Geomorphology 79: 45-57 (25) Dise (2009) Science 326: 810-811.