DOI: 10.5285/299e2779-0787-46bb-9482-03ad474eae27
title: Critical loads of acidity and nutrient nitrogen for UK habitats

description: The data provide critical loads of acidity, and of nutrient nitrogen for the distributions of UK habitats sensitive to acidification and/or eutrophication. Critical loads have been calculated and applied to UK natural and semi-natural habitats sensitive to acidification and/or eutrophication (excess nitrogen as a nutrient).  Critical loads data are available for these habitat types at 1x1 km resolution for the UK.  In addition, acidity critical loads are available for 1752 selected freshwater sites throughout the UK. Critical loads are defined as the maximum pollutant load (of acid or nitrogen deposition) that a sensitive element of the environment (e.g., soils, vegetation) can tolerate without adverse harmful effects occurring, according to present knowledge. 

Habitat distributions are defined from a combination of CEH Land Cover Map 2000 and a number of ancillary data sets (e.g., species distributions, altitude, soils), used to further refine their distributions.  It should be noted that the habitat distributions maps and areas provided here and used for UK critical loads research for Defra (a) only include areas where data exist for the calculation or derivation of critical loads; (b) may differ from other national habitat distribution maps or estimates of habitat areas.  This may also result in a difference in the total habitat areas mapped for acidity and for nutrient nitrogen critical loads.

The data have been generated under numerous Defra-funded contracts that brought together UK experts on the impacts of air pollution on UK habitats.  The acidity critical loads data were last revised in 2004 and the nutrient nitrogen critical loads data were updated in 2011.  The data are based on methods agreed at national and international meetings and workshops held under the UNECE Convention on Long-Range Transboundary Air Pollution (CLRTAP).
description: The methods used to calculate UK critical loads are based on internationally agreed approaches and the best available national-scale data sets.  Acidity critical loads for terrestrial habitats are based on the mineralogy and chemistry of the dominant soil type in each 1x1 km grid square (with the exception of the bog habitat which is assumed to be on peat), together with habitat-specific data.  For woodland habitats simple mass balance equations, based on balancing the acidic inputs to, and outputs from a system, are used to derive a critical load that ensures the selected chemical criterion is not exceeded.  Acidity critical loads are mapped for the following habitats: acid grassland, calcareous grassland, dwarf shrub heath, bog, montane, managed (productive) coniferous woodland, managed (productive) broadleaved woodland, and unmanaged woodland.  Acidity critical loads for 1752 surface waters in acid-sensitive regions of the UK are calculated using the catchment-based First-order Acidity Balance (FAB) model.  The acidity critical loads consist of three separate values for each habitat:
 *	CLmaxS = maximum critical load of sulphur: the acidity critical load expressed in terms of sulphur only, when nitrogen deposition is zero
 *	CLmaxN = maximum critical load of nitrogen: the acidity critical load expressed in terms of nitrogen only, when sulphur deposition is zero
 *	CLminN = minimum critical load of nitrogen: representing the long term nitrogen removal processes in the soil (e.g. nitrogen uptake and immobilisation) and harvesting of vegetation.

Critical loads of nutrient nitrogen for natural and semi-natural habitats, and unmanaged (non-productive) woodlands are empirically derived values based on observed changes in the structure or function of ecosystems.  For managed (productive) woodlands a nitrogen mass balance approach is used to derive critical loads that will prevent an increase in the leaching of nitrogen compounds and ensure sustainable production.  Nutrient nitrogen critical loads are mapped for the following habitats: acid grassland, calcareous grassland, dwarf shrub heath, bog, montane, managed (productive) coniferous woodland, managed (productive) broadleaved woodland, unmanaged Beech woodland, unmanaged acidophilous oak woodland, unmanaged Scots Pine woodland, other unmanaged woodland, dune grasslands, saltmarsh.  

A full description of the methods and data used to derive and calculate UK critical loads of acidity and nutrient nitrogen can be found in the 'Methods Report' (Hall et al, 2015).

Hall, J., Curtis, C., Dore, T., Smith, R.  2015.  Methods for the calculation of critical loads and their exceedances in the UK.  Report to Defra, prepared under Contract AQ0826. http://www.cldm.ceh.ac.uk/sites/cldm.ceh.ac.uk/files/MethodsReport_Updated_July2015_WEB.pdf