Mobility of heavy metals in sandy soil after application of composts produced from maize straw, sewage sludge and biochar

Studies on the availability of heavy metals in composted organic materials and in soil amended with these materials are of practical significance. They are used in the assessment of the purity of the soil environment and of the biological value of plants intended for human and animal consumption. Composting of organic materials has a significant effect on changes in mobile forms of heavy metals. Therefore, the aim of this study was to determine the effect of the addition of biochar and sewage sludge on (i) the contents of water soluble forms of Cu, Cd, Pb, and Zn in composts; and (ii) the contents of mobile forms of these elements in sandy soil after the addition of composts. Addition of sewage sludge and biochar to maize straw did not increase the heavy metal forms extracted with water in total content of heavy metals. The content of Cd and Cu extracted with water in composts produced from maize straw and sewage sludge, and produced from maize straw, sewage sludge and biochar was higher than the one determined in compost produced from maize straw. The content of Pb and Zn extracted with water in compost produced from maize straw, sewage sludge and biochar was lower than in compost produced from maize straw. The addition of sewage sludge and biochar to maize straw had an immobilizing effect on mobile forms of the studied elements compared to compost produced from maize straw and sewage sludge. The addition of composts to soil decreased the contents of mobile forms of Cu, Cd, and Pb extracted with 1 M NH4NO3 compared to the contents in the control soil. However, the content of Zn extracted with NH4NO3 increased in treatments with 0.5% dose of compost produced from maize straw and sewage sludge and 0.5% dose of compost produced from maize straw, sewage sludge and biochar. In none of the analyzed cases, the application of the composts produced did not exceed the acceptable content of studied elements in the soil.