Fate of heavy metals and importance of influencing factors during vermicomposting

Vermicomposting is a beneficial choice for weakening the ecological threat of various organic wastes. However, the complicated mechanisms of composting processes and the diverse biogeochemical fates of heavy metals (HMs) fractions limits the comprehensive understanding of vermicomposting. The vermicomposting experiment using cow manure, straw and superphosphate as the original materials was set up to clarify the variations of both total contents and occurrence forms of HMs (As, Cd, Co, Cr, Cu, Mn, Ni, V and Zn) and corresponding influencing factors in the composting products-earthworm system. The results showed that vermicomposting resulted in the 71.11% decrease in ammonium nitrogen (AN) and the 63.21% increase in nitrate nitrogen (NN), respectively. Co, Cu, V and Zn were released from earthworms to vermicomposting products. The bioconcentration factor (BCF) results showed that the migration efficiency of Zn (4.35) was much higher than that of other HMs (0.11-0.71). Both bioaccumulation and migration efficiency of HMs depends on the initial contents of HMs in both earthworms and raw composting materials. Vermicomposting altered the fraction redistribution of HMs by increasing the F5 form, F4 form of Co, Cu and V, as well as the F2 and F3 form. The BCF results showed the F1 (6.29), F2 (5.62) and F3 (7.49) form of As, as well as F1(4.46) form of Cr accumulated well by earthworms. Permutational multivariate analysis of variance (PERMANOVA) showed that earthworms significantly influenced the physicochemical properties (R²=11.84%, P<0.001), as well as total contents (R²=38.46%, P<0.001) and occurrence forms (R²=36.79%, P<0.001) of HMs in the vermicomposting products. More importantly, random forest model, mantel-test and partial least squares path model all confirmed that pH and nitrogen had the important effects on HMs migration during vermicomposting, followed by organic carbon and the ratio of carbon to nitrogen. In conclusion, this study has strengthened the understanding of HMs fate during vermicomposting. Moreover, it is the first to rank the importance of factors influencing HMs migration during vermicomposting.