The effect of heavy metals on microbial respiration in organic materials used as soil amendments was evaluated to assess the stability of the materials. Solutions of Pb (II), Cu (II) and Zn (II) at rates of 5, 10 and 50 mg metal g(-1) were added to green waste compost, peat, coir and wood bark. Metal toxicity led to a significant decrease in carbon dioxide evolved by the contaminated materials, up to 80% less at the highest rate of addition compared to the untreated material. There was a significant negative correlation between the organic carbon content of an amendment and the inhibition of CO2 evolution by all three heavy metals. There was also a significant negative correlation between an amendment's cation exchange capacity and the inhibition of CO2 evolution caused by Cu and Zn. The ability of the organic materials to enhance respiration in a soil from the vicinity of a Pb/Zn mine was also evaluated, by applying them to the soil at rates of 1, 10 and 20%. CO2 evolution from the contaminated soil was enhanced significantly by the addition of all of the amendments, with coir causing up to 90% enhancement at high levels of addition. (C) 2010 Elsevier B.V. All rights reserved.