Guidance on applying the critical load range for atmospheric nitrogen deposition to bog habitats in the UK

1.    Background

1.1.  Blanket bog and raised bog are some of the rarest wildlife habitats in the world.  Their importance is recognised in Europe by their inclusion in the EU Habitats Directive as a priority habitat. In addition to its importance as a wildlife habitat, the role of blanket/active bog in the provision of a number of ecosystem services is significant. This includes the ability to capture and store large amounts of carbon, its role in securing high water quality and its ability to reduce flood risk downstream through slowing hydrological pathways.

1.2.  Much of the blanket bog and raised bog in the UK is degraded to some degree and its condition is greatly influenced by activities such as peat extraction, drainage, managed burning and heavy grazing. These can damage the hydrology and properties of the peat, which in turn lead to loss of structural and floristic diversity (especially peat-forming species) of the bog.  However, atmospheric pollution can also damage raised/blanket bog.  These habitats receive all their nutrients from the atmosphere and the characteristic species are naturally adapted to very low nutrient environments.  National modelling suggests that many of our peatlands are subject to significant critical load exceedence as shown in the UK Air Pollution Information System.

1.3.  Restoration of peat-forming vegetation is a clear priority for UK peatlands. The success of restoration may be challenged by ongoing nutrient pollution of sensitive bog habitats.  This provides advice on how to apply a critical load to an individual bog site for use in impact assessments.  

2.    Introduction to the nutrient nitrogen critical load range for bogs

2.1.  The nitrogen critical load for raised and blanket bog, as set by the expert group under the UNECE Convention on Long Range Transboundary Air Pollution, is 5-10kgN/ha/yr.  The range reflects “real intra-ecosystem variation between different regions where the ecosystem has been studied (Achermann & Bobbink, 2003, Bobbink & Hettelingh, 2011). 

2.2.  The expert group identified water table height and precipitation as ‘modifying factors’ affecting the choice of which part of the critical load range to apply:

  • Precipitation - “apply the high end of the range to areas with high levels of precipitation and the low end of the range to those with low precipitation levels”
  • Water level - “apply the low end of the CL range to systems with a low water table, and the upper end of range to those with a high water table”.

2.3.  The Statutory Nature Conservation Bodies1 advise that once the hydrology of a bog has been restored there is a significant time lag before the typical micro-topography of the bog (hummocks and hollows) and Sphagnum bog moss diversity develops.  Many of the Sphagnum species that drive the development of bog micro-topography are highly sensitive to nutrient enrichment.  Critical load application needs to consider this.

3.    Guidance on selecting a critical load within the range

3.1.  To take into account the above information, a three step approach to determining the appropriate critical load (within the UNECE critical load range) for an individual bog site/habitat is advised, as illustrated in Figure 1.  It is recommended that this approach is applied at the detailed assessment stage and that the minimum of the critical load range (5kgN/ha/yr) continues to be used for screening (Stage 2 assessment).

three step approach Bogs critical loads

Figure 1: Recommended approach to determining the critical load for an individual raised/blanket bog site.

3.2.  The first step is consider the status/condition of the bog and whether long-term restoration is needed (and is a conservation aim/objective).

3.3.  The next step (step 2) is to consider the water table modifying factor.  Evidence shows that bogs with low water tables are more vulnerable to N deposition (e.g. Limpens et al., 2003).  All bogs in the UK have suffered to a greater or lesser extent from drainage, peat extraction and nutrient enrichment, and consequently there are no intact and fully functioning raised bogs in the UK. This has left a legacy of disrupted hydrology, and on most sites results in a significantly lowered water table within the bog.  The critical part of any bog restoration involves restoration of the water table to the bog surface (+/- 10cm) throughout the year.  Bogs which experience periods of a month or more/year where the water table is lower than this will be more vulnerable, so sites where hydrology has not yet been fully restored and those where climate results in regular drought should be given greater protection.

3.4.  On this basis, where bog hydrology has not been fully restored and the bog is not active, the lower end of the critical load range (5kg/ha N) should be applied. This lower critical load should also be used on fully restored sites that have seasonally lower water tables for climatic reasons.

3.5.  Where the bog hydrology has been fully restored and there is active growth of bog Sphagna (i.e. Sphagnum papillosum, Sphagnum magellanicum and Sphagnum capillifolium) over the majority of the site, a higher critical load within the range may be applied if appropriate based on precipitation level (as described in section 2.2 above and Figure 1 step 3 below).

3.6.  The third step is to consider the precipitation modifier, using the approach developed in the UK status report (July 2011). 

The Air Pollution Information Systems includes a tool to identify the relevant rainfall band.

4.    References

Joint Nature Conservation Committee (2011). Towards an assessment of the state of UK

Peatlands, JNCC report No. 445.

Natural England (2011).  England's peatlands: carbon storage and greenhouse gases (NE257)

RoTAP (2012). Review of Transboundary Air Pollution: AcidificationEutrophication and Ground-level Ozone. Report for Defra and the Devolved Administrations.

Achermann, B. & Bobbink, R. (2003).  Empirical Critical Loads for Nitrogen.  Proceedings of an Expert Workshop, Berne, 11-13 November 2002.  SAEFL.

Bobbink, R. & Hettelingh, J.P. (eds).  (2011).  Review and revision of empirical critical loads and dose-response relationships.  Proceedings of an expert workshop, Noordwijkerhout, 23-25 June 2010.  Published by RIVM.

Limpens, J., Berendse, F. & Klees, H. (2003).  N deposition affects N availability in interstitial water, growth of Sphagnum and invasion of vascular plants in bog vegetation. New Phytologist, Vol 157, Issue 2; 339-347.

Hall, J., Emmett, B., Garbutt, A., Jones, L., Rowe, E., Sheppard, L., Vanguelova, E., Pitman, R., Britton, A., Hester, A., Ashmore, A., Power, S., Caporn, S. (2011).  UK Status Report July 2011:  Update to empirical critical loads of nitrogen Report to Defra under contract AQ801 Critical Loads and Dynamic Modelling.

1 Natural England, Natural Resources Wales, Scottish Natural Heritage and Northern Ireland Environment Agency.   

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