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{{OR/14/047}}
Critical loads are a quantitative estimate of exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge (Nilsson & Grennfelt, 1988). In the UK critical loads are applied to broad habitats sensitive to acidification and/or eutrophication; this report considers only empirical critical loads for eutrophication (nutrient nitrogen) from atmospheric nitrogen.

Critical loads for nitrogen are published as a range (Bobbink & Hettelingh, 2011)<ref name="Bobbink 2011">BOBBINK, R, HETTELINGH, J P. (Eds.). 2011. Review and revision of empirical critical loads and dose-response relationships. Proceedings of an expert workshop, Noordwijkerhout, 23e25 June 2010, ISBN 978-90-6960-251-6. http://www.rivm.nl/bibliotheek/rapporten/680359002.pdf</ref> to encompass the variability in response of habitats to nitrogen. In the UK, a single value within these ranges has been chosen for the calculation of critical load exceedances; this ‘mapping value’ is based on UK evidence of nitrogen impacts (Hall et al. 2011<ref name="Hall 2011">HALL, J, EMMETT, B, GARBUTT1, A, JONES, L, ROWE, E, SHEPPAR, L, VANGUELOVA, E, PITMAN, R, BRITTON, A, HESTER, A, ASHMORE, M, POWER, S, and 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. 5th July 2011. http://cldm.defra.gov.uk/PDFs/UK_status_report_2011_finalversion_July2011_v2.pdf</ref>; 2014 in press). Nitrogen Critical loads are reviewed and updated under the Convention on Long Range Transboundary Air Pollution (CLRTAP), the last review being in 2010 (Bobbink & Hettelingh, 2011)<ref name="Bobbink 2011"></ref>. Critical loads have been derived for seven ecosystem types: mire, bog and fen; grasslands; heathland, scrub and tundra; woodland and forest; inland surface waters; coastal; marine. In the UK critical loads have been mapped for N habitat types: acid grassland, calcareous grassland, dwarf shrub heath, montane, bog, managed and unmanaged woodlands, dune grassland and saltmarsh (Table 3). The critical loads are applied to each 1 x 1 km square of national-scale habitat distribution maps (Hall et al. 2011<ref name="Hall 2011"></ref>; 2014 in press) and then compared with national 5 x 5 km resolution atmospheric N deposition maps. Where the deposition is greater than the critical load (ie, the critical load is 'exceeded') the habitat is considered to be at risk from adverse impacts from excess nitrogen deposition (Figure 4a).

The latest analysis (based on CBED annual mean deposition for 2010–12) shows that:

* '''N deposition exceeds the critical loads across 65% of the total area of UK habitats sensitive to eutrophication'''

The above national-scale analysis is based on the areas of all nitrogen sensitive broad habitats mapped in the UK. The current study in relation to GWDTEs considers all designated habitat features (ie, not just ‘wetland’ habitats) found within SSSIs in England and Wales that are sensitive to nitrogen, and for which nitrogen critical loads are available. Critical loads are not available for some habitat feature types due to a lack of sufficient published data and evidence of nitrogen impacts. In addition, some SSSIs may contain habitat features that are not sensitive to nitrogen deposition. Consequently, in this study, critical loads were applied to 2355 of the 3320 sites, with no critical loads available for 965 sites.

The critical load values applied to the sensitive habitat features of SSSIs may differ from those applied to the broad habitats mapped nationally (Table 3). The Statutory Nature Conservation Bodies have set ‘recommended’ values from within the published ranges, for use in Article 17 reporting for the Habitats Directive, and these are the critical load values that have been used in this study. In many cases these ‘recommended’ values are the same as the ‘mapping values’, but for some habitats they may be the minima of the published range; in particular for habitats where there is less UK evidence available, or for habitat types not mapped nationally. Critical loads have been applied (where available) to each nitrogen-sensitive feature habitat within each site; these critical loads may vary from one habitat to another (Table 3). This also means that critical load exceedance may vary from one habitat to another. For national scale work the exceedance metric ‘Average Accumulated Exceedance’ is frequently mapped (see Figure 4b); in this study a precautionary approach has been taken, by using the maximum exceedance (rather than AAE) per site (SSSI). It should also be noted that historically due to a lack of digital data on the spatial location of feature habitats within each site, it is assumed in this data analysis that all feature habitats can occur anywhere and everywhere within each site.

As with many other target values not all sites may follow the rules and in some cases impacts (on designated sites) can be seen below the critical loading value for nitrogen (Stevens et al. 2011)<ref name="Stevens 2011">STEVENS, C J, SMART, S M, HENRYS, P, MASKELL, L C, WALKER, K J, PRESTON, C D, CROWE, A, ROWE, E, GOWING, D J, and EMMETT, B A. 2011. Collation of evidence of nitrogen impacts on vegetation in relation to UK biodiversity objectives. ''JNCC Report'', No. 447. http://jncc.defra.gov.uk/pdf/jncc447_web.pdf</ref>.

<center>
{| class="wikitable"
|+ Table 3    Critical loads of nutrient nitrogen showing published ranges (Bobbink & Hettlingh, 2011<ref name="Bobbink 2011"></ref>) and values applied in the UK (Hall et al. 2011<ref name="Hall 2011"></ref>)
| '''Habitat type'''
| '''EUNIS code'''<ref name="EUNIS code">Habitat class of the European Nature Information System (EUNIS); these are the habitat classes for which the nutrient nitrogen critical load ranges have been set within Europe (Bobbink & Hettelingh, 2011).</ref><ref name="Bobbink 2011"></ref>
| '''Critical load range (kg N ha-1 year-1)'''
| '''UK Mapping Value'''<ref name="UK Mapping Value">The single value from within the range used for national-scale critical loads mapping for UK broad habitats, based on UK evidence of nitrogen impacts (Hall, 2011).</ref><ref name="Hall 2011"></ref>. '''(kg N ha-1 year-1)'''
| '''Recommended Value<ref name="Recommended Value">The single value from within the range set by UK SNCBs for site-specific applications to habitat features of designated sites and used for Article 17 Reporting under the Habitats Directive.</ref> (kg N ha-1 year-1)'''
|- style="vertical-align:top;"
| '''Marine habitats'''
|
|
|
|
|- style="vertical-align:top;"
| Mid-upper saltmarshes
| A2.53
| 20–30<ref name="20–30">Expert judgement: when no empirical data were available for the ecosystem; critical load based upon expert judgement and knowledge of ecosystems which were likely to be comparable with this ecosystem.</ref>
| 25
| N/A
|- style="vertical-align:top;"
| Pioneer & low saltmarshes
| A2.54/55
| 20–30<ref name="20–30"></ref>
| 25
| N/A
|- style="vertical-align:top;"
| '''Coastal habitats'''
|
|
|
|
|- style="vertical-align:top;"
| Shifting coastal dunes
| B1.3
| 10–20<ref name="10–20">Expert judgement: when no empirical data were available for the ecosystem; critical load based upon expert judgement and knowledge of ecosystems which were likely to be comparable with this ecosystem.</ref>
| not mapped
| 10
|- style="vertical-align:top;"
| Coastal stable dune grasslands
| B1.4<ref name="B1.4">For acidic dunes, the 8–10 kg N ha-1 year-1 range should be applied; for calcareous dunes the range 10–15 kg N ha-1 year-1 should be applied.</ref>
| 8–15 <ref name="8–15">Quite reliable: when the results of some studies were comparable.</ref>
| 9 acid dunes
12 non-acid dunes
| 8
|- style="vertical-align:top;"
| Coastal dune heaths
| B1.5
| 10–20<ref name="10–20"></ref>
| not mapped
| 10
|- style="vertical-align:top;"
| Moist to wet dune slacks
| B1.8<ref name="B1.8">Apply the lower end of the range to habitats with low base availability, and the higher end to those with high base availability.</ref>
| 10–20<ref name="10–20"></ref>
| not mapped
| 10
|- style="vertical-align:top;"
| '''Inland surface water habitats'''
|
|
|
|
|- style="vertical-align:top;"
| Softwater lakes (permanent oligotrophic)
| C1.1<ref name="C1.1">This critical load should only be applied to oligotrophic waters with low alkalinity with no significant agricultural or other human inputs.</ref>
| 3–10<ref name="3–10">Reliable: when a number of published papers of various studies showed comparable results. </ref>
| not mapped
| 3
|- style="vertical-align:top;"
| Permanent dystrophic lakes, ponds, pools
| C1.4<ref name="C1.4">This critical load should only be applied to waters with low alkalinity with no significant agricultural or other direct human inputs.</ref>
| 3–10<ref name="3–10">Expert judgement: when no empirical data were available for the ecosystem; critical load based upon expert judgement and knowledge of ecosystems which were likely to be comparable with this ecosystem. </ref>
| not mapped
| 3
|- style="vertical-align:top;"
| '''Mire, bog & fen habitats'''
|
|
|
|
|- style="vertical-align:top;"
| Raised & blanket bogs
| D1<ref name="D1">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. Apply the low end of the range to systems with a low water table and the high end of the range to those with a high water table.</ref>
| 5–10<ref name="5–10">Reliable: when a number of published papers of various studies showed comparable results.</ref>
| 8,9,10 (rainfall dependent)
| 5
|- style="vertical-align:top;"
| Valley mires, poor fens & transition mires
| D2<ref name="D2">For EUNIS category D2.1 (valley mires) use the lower end of the range.</ref>
| 10–15<ref name="10–15">Quite reliable: when the results of some studies were comparable.</ref>
| not mapped
| 10
|- style="vertical-align:top;"
| Rich fens
| D4.1<ref name="D4.1">For high latitude systems apply the lower end of the range.</ref>
| 15–30<ref name="15–30">Expert judgement: when no empirical data were available for the ecosystem; critical load based upon expert judgement and knowledge of ecosystems which were likely to be comparable with this ecosystem.</ref>
| not mapped
| 15
|- style="vertical-align:top;"
| Montane rich fens
| D4.2<ref name="D4.2">For high latitude systems apply the lower end of the range.</ref>
| 15.25<ref name="15.25">Expert judgement: when no empirical data were available for the ecosystem; critical load based upon expert judgement and knowledge of ecosystems which were likely to be comparable with this ecosystem.</ref>
| not mapped
| 15
|- style="vertical-align:top;"
| '''Grassland & tall forb habitats'''
|
|
|
|
|- style="vertical-align:top;"
| Semi-dry calcareous grassland
| E1.26
| 15–25<ref name="15–25">Reliable: when a number of published papers of various studies showed comparable results.</ref>
| 15
| 15
|- style="vertical-align:top;"
| Dry acid & neutral closed grassland
| E1.7<ref name="E1.7">Apply the lower end of the range to habitats with low base availability, and the higher end to those with high base availability.</ref>
| 10–15<ref name="10–15">Reliable: when a number of published papers of various studies showed comparable results.</ref>
| 10
| 10
|- style="vertical-align:top;"
| Inland dune pioneer grassland
| E1.94<ref name="E1.94">Apply the lower end of the range to habitats with low base availability, and the higher end to those with high base availability.</ref>
| 8–15<ref name="8–15"> Expert judgement: when no empirical data were available for the ecosystem; critical load based upon expert judgement and knowledge of ecosystems which were likely to be comparable with this ecosystem. </ref>
| not mapped
| 8
|- style="vertical-align:top;"
| Inland dune siliceous grassland
| E1.95<ref name="E1.95">Apply the lower end of the range to habitats with low base availability, and the higher end to those with high base availability.</ref>
| 8–15<ref name="8–15">Expert judgement: when no empirical data were available for the ecosystem; critical load based upon expert judgement and knowledge of ecosystems which were likely to be comparable with this ecosystem.</ref>
| not mapped
| 8
|- style="vertical-align:top;"
| Low & medium altitude hay meadows
| E2.2
| 20–30<ref name="20–30">Expert judgement: when no empirical data were available for the ecosystem; critical load based upon expert judgement and knowledge of ecosystems which were likely to be comparable with this ecosystem.</ref>
| not mapped
| 20
|- style="vertical-align:top;"
| Mountain hay meadows
| E2.3
| 10–20<ref name="10–20">Expert judgement: when no empirical data were available for the ecosystem; critical load based upon expert judgement and knowledge of ecosystems which were likely to be comparable with this ecosystem.</ref>
| not mapped
| 10
|- style="vertical-align:top;"
| Molinia caerulea meadows
| E3.51
| 15–25<ref name="15–25">Expert judgement: when no empirical data were available for the ecosystem; critical load based upon expert judgement and knowledge of ecosystems which were likely to be comparable with this ecosystem.</ref>
| not mapped
| 15
|- style="vertical-align:top;"
| Juncus meadows & Nardus stricta swards
| E3.52
| 10–20<ref name="10–20">Quite reliable: when the results of some studies were comparable.</ref>
| 15
| 10
|- style="vertical-align:top;"
| Moss & lichen dominated mountain summits
| E4.2
| 5–10<ref name="5–10">Quite reliable: when the results of some studies were comparable.</ref>
| 7
| 7
|- style="vertical-align:top;"
| Alpine & subalpine acid grassland
| E4.3
| 5–10<ref name="5–10">Quite reliable: when the results of some studies were comparable.</ref>
| not mapped
| 5
|- style="vertical-align:top;"
| Alpine & subalpine calcareous grassland
| E4.4
| 5–10<ref name="5–10">Quite reliable: when the results of some studies were comparable.</ref>
| not mapped
| 5
|- style="vertical-align:top;"
| '''Heathland, scrub & tundra habitats'''
|
|
|
|
|- style="vertical-align:top;"
| Arctic, alpine & subalpine scrub
| F2
| 5–15<ref name="5–15">Quite reliable: when the results of some studies were comparable.</ref>
| not mapped
| 5
|- style="vertical-align:top;"
| Calluna dominated upland wet heaths
| F4.11<ref name="F4.11">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. Apply the low end of the range to systems with a low water table and the high end of the range to those with a high water table. 
Apply the high end of the range to areas where sod cutting has been practiced; apply the lower end of the range to areas with low-intensity management.</ref>
| 10–20<ref name="10–20">Quite reliable: when the results of some studies were comparable.</ref>
| 10
| 10
|- style="vertical-align:top;"
| Erica tetralix dominated lowland wet heaths
| F4.11<ref name="F4.11">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. Apply the low end of the range to systems with a low water table and the high end of the range to those with a high water table. 
Apply the high end of the range to areas where sod cutting has been practiced; apply the lower end of the range to areas with low-intensity management.</ref>
| 10–20<ref name="10–20">Expert judgement: when no empirical data were available for the ecosystem; critical load based upon expert judgement and knowledge of ecosystems which were likely to be comparable with this ecosystem.</ref>
| 10
| 10
|- style="vertical-align:top;"
| Dry heaths
| F4.2<ref name="F4.2">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. Apply the low end of the range to systems with a low water table and the high end of the range to those with a high water table. 
Apply the high end of the range to areas where sod cutting has been practiced; apply the lower end of the range to areas with low-intensity management.</ref>
| 10–20<ref name="10–20">Reliable: when a number of published papers of various studies showed comparable results.</ref>
| 10
| 10
|- style="vertical-align:top;"
| '''Forest habitats'''
|
|
|
|
|- style="vertical-align:top;"
| Broadleaved woodland
| G1
| 10–20<ref name="10–20">Reliable: when a number of published papers of various studies showed comparable results.</ref>
| 12
| 10
|- style="vertical-align:top;"
| Beech woodland
| G1.6
| 10–20<ref name="10–20">Expert judgement: when no empirical data were available for the ecosystem; critical load based upon expert judgement and knowledge of ecosystems which were likely to be comparable with this ecosystem.</ref>
| 15
| 15
|- style="vertical-align:top;"
| Acidophilous oak dominated woodland
| G1.8
| 10–15<ref name="10–15">Expert judgement: when no empirical data were available for the ecosystem; critical load based upon expert judgement and knowledge of ecosystems which were likely to be comparable with this ecosystem.</ref>
| 10
| 10
|- style="vertical-align:top;"
| Coniferous woodland
| G3
| 5–15<ref name="5–15">Reliable: when a number of published papers of various studies showed comparable results.</ref>
| 12
| 10
|- style="vertical-align:top;"
| Scots Pine woodland
| G3.4
| 5–15<ref name="5–15">Quite reliable: when the results of some studies were comparable.</ref>
| 12
| 12
|}
</center>

[[Image:OR14047fig4.jpg|thumb|center|650px| '''Figure 4'''    Examples of national-scale critical load exceedance maps generated under Defra contract AQ0826: (a) Average Accumulated Exceedance (AAE) of nitrogen critical loads for sensitive UK habitats by CBED total nitrogen deposition for 2010–12; (b) Maximum AAE of nitrogen critical loads per SSSI by CBED total nitrogen deposition (area-weighted value per SSSI) for 2010–12. AAE is an exceedance metric that averages exceedance over the entire sensitive habitat area; it is calculated as: AAE=(∑exceedance*exceeded area) ÷ total sensitive habitat area. ''Contains OS data © Crown Copyright and database right [2015]''. ]]
==References and footnotes==
<References/>
[[Category:OR/14/047 Atmospheric deposition at groundwater dependent wetlands: implications for effective catchment management and Water Framework Directive groundwater classification in England and Wales | 07]]

OR/14/047 Critical loads - Revision history

Dbk: 1 revision imported

Geosource>Ajhil at 08:25, 25 February 2016