Critical Limits for Heavy Metals in Soil

The heavy metal critical load depends on the acceptable total load from anthropogenic heavy metal inputs (deposition, fertilizers, other anthropogenic sources), below which ecosystem damage is unlikely.

Methods for calculating critical loads for toxic metals are currently being developed within the United Nations Economic Commission for Europe (UNECE) Convention on Long-Range Transboundary Air Pollution (LRTAP) (De Vries and Bakker 1996; Slootweg et al. 2005). In the UK, a research consortium currently contributes to the development and improvement of methods of calculating critical loads for application within the UNECE, and to the development of improved tools for assessing the effects of changing rates of atmospheric deposition on pools of metals in soils and freshwaters. Forest research is a subcontractor to this consortium and has contributed to the calculation, evaluation, and updating of heavy metal critical loads for forest ecosystems in the UK.

Both effect-based steady-state and standstill critical load approaches have been used to calculate and map heavy metal critical loads. In brief, an "effect-based" methodology identifies atmospheric depositions (critical loads) that will not lead to concentrations of heavy metals above critical limits in defined compartments at steady state.

The "critical limit" defines an acceptable maximum concentration of a metal below which long-term deleterious effects to an ecosystem should not occur. Thus, defining the critical limit is crucial to the critical load approach. For lead (Pb) and cadmium (Cd), critical limits of 8 mg Pb m-3 and 0.8 mg Cd m-3, respectively, have been adopted. Critical loads can be derived using:

• Critical limits of heavy metal concentrations in the soil solution that will not harm microbiota and plants, and/or

• Critical limits of (reactive) soil metal concentrations that will not lead to adverse impacts on soil functioning, such as soil invertebrates that ingest soil.

The "standstill" critical load is the atmospheric deposition that will not lead to any further accumulation of heavy metals in the soil. Standstill critical loads should also include inputs other than atmospheric deposition.

Effect-based steady-state and standstill critical load approaches are described further below.

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