Soil sustainability – Critical loads

What are Critical Loads?

A critical load has been defined by Nilsson and Grennfelt as ‘a quantitative estimate of the 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’.

In other words, the level of input below which damage to the tree, soil or ecosystem is thought unlikely is known as the critical load. Where a critical load is exceeded, it does not always mean that damage will occur. Critical loads are set to protect ecosystems in the long term; the effects of pollutants can take many decades to appear, and thus if damage does occur, it is may not be immediately apparent.

Critical loads are set for:

• Acidity
• Nutrient nitrogen
• Ozone
• Some heavy metals.

Forest critical loads are set to protect specific receptors. We are working to develop critical loads for woodland that are appropriate to both the semi-natural woodland and forest management practices adopted in the UK.

Policy relevance

The critical load approach is a key element of emissions reduction policy. It has proved to be very useful in policy development since its ‘effect based’ approach allows the environmental benefits of emission reductions to be evaluated. The critical loads approach has been adopted by the UNECE Convention on Long Range Transboundary Air Pollution (CLRTAP) to target pollutant reduction targets and it is now one of the main drivers of emission control agreements under the UNECE CLRTAP, the EC European Acidification Strategy, and Clean Air for Europe.