R. van Herwijnen 1, 3, 4 , T. Laverye 2 , S. K. Ouki 1 , A. Al-Tabbaa 3 , M. E. Hodson 2 & T. R. Hutchings 4 1 School of Engineering, University of Surrey, Guildford, UK 2 Department of Soil Science, School of Human and Environmental Science, University of Reading, UK 3 Department of Engineering, University of Cambridge, UK 4 Land Regeneration and Urban Greening Group, Forest Research, Farnham, UK Abstract Compost is often proposed as a suitable material for the remediation of contaminated brownfield sites intended for soft end-use. In addition to vitalising the soil, it is also supposed to immobilise metals and so break contaminant- receptor pathways and reduce the ecotoxicity of the contaminants. However, some research has demonstrated contradictory effects between composts on metal immobilisation. In our study, five different composts were tested to examine both their metal retention and toxicity reduction capabilities on three different metal contaminated soils. Leaching tests, a plant growth test with Greek cress (Lepidium sativum), an earthworm (Eisenia fetida) survival and condition test and a bacterial toxicity test using Vibrio fischeri were carried out. The batch leaching tests results showed that spent mushroom compost caused an increase in metal concentration in the leachates while LimeX70 compost caused a decrease. The variation in behaviour between different composts for each soil was somewhat random, so a generic conclusion could not be drawn. Toxicity tests showed significant reduction of metal bioavailability and toxicity. Our results also suggest that more rigorous tests should be undertaken to understand the mechanisms involved in metal complexation using different compost types, in order to optimise the use of compost for remediation. Keywords: compost, heavy metals, toxicity, leaching, remediation. www.witpress.com, ISSN 1743-3541 (on-line) © 2006 WIT Press WIT Transactions on Ecology and the Environment, Vol 94, Brownfields III 99 doi:10.2495/BF060101 metals from contaminated soils The effect of composts on the leaching of