Bioaccumulation of cadmium in soil organisms - With focus on wood ash application

Harvesting whole-tree biomass for biofuel combustion intensifies removal of nutrients from the ecosystem. This can be partly amended by applying ash from the combustion back to the system and thus recycle the nutrients. However, besides being rich in inorganic nutrients, ash also contains trace amounts of heavy metals. Due to the risk of toxic effects and trophic transfer of heavy metals, especially cadmium, legislation usually restricts the use of ash as a soil amendment. In order to provide researchers and governmental agencies with a tool to assess the risk of cadmium bioaccumulation in specific soil systems after ash application, we review: 1) the properties of ash; 2) the chemical and toxic properties of cadmium; 3) the key factors affecting cadmium bioavailability, cadmium uptake-, storage- and elimination-abilities in soil organisms and the risk of cadmium accumulation and biomagnification in the soil food web; 4) how ash impact on soil can change the risk of cadmium bioaccumulation. We conclude that for assessing the risk of cadmium bioaccumulation for specific sites, it is necessary to consider both the type and composition of ash, the soil conditions and organism composition on the site. On a general basis, we conclude that granulated ashes low in cadmium content, applied to low pH soils with high organic matter content, in systems with low abundances of earthworms, isopods and gastropods, will have a low risk of cadmium accumulation.