Vermicompost from sewage sludge: effects on heavy metal presence in soil and bioaccumulation in castor bean

Vermicomposting alters the concentrations of heavy metals in sludge and their absorption by plants. Therefore, the study aimed to assess the content of heavy metals in the soil and castor bean leaves in response to mineral fertilization and different doses of sewage sludge vermicompost. The study was conducted in Brazil, in a Cambisol. The treatments were distributed in a randomized block design with four replications and consisted of fertilization solely with mineral fertilizers and five doses of vermicompost applied based on the N content of the fertilizer and the nitrogen requirement of castor beans (0; 14; 28; 42; and 56 t ha-1, on a dry basis). The content of Cu, Fe, Mn, Ni, Cd, Cr, and Pb in the soil and castor bean leaves did not differ between vermicompost and mineral fertilization. The contents of Cu, Ni, Cd, Cr, and Pb in the soil and castor bean leaves were not influenced by the application of sewage sludge vermicompost doses. At the maximum vermicompost application rate (56 t ha-1), elevated concentrations of Zn, Fe, and Mn were observed in the soil, along with increased content of Fe in castor bean leaves. This result was also confirmed after estimating the geo-environmental contamination indices GAI (Geo-accumulation index) and PI (Pollution index). However, both had a low correlation with metal contents in castor beans, indicating the lesser interference of sludge use in metal absorption by plants.