Transformation of nutrients and heavy metals during vermicomposting of the invasive green weed Salvinia natans using Eisenia fetida

Abstract

Purpose Effective vermicomposting of Salvinia natans is a good alternative for protecting wetlands damaged by the weed due to the vast potential for use of the product vermicompost in agriculture, but the presence of toxic metals in the weeds may deter the usage. Methods Research was carried out on the physico-chemical and biological characteristics as well as bioavailability and leachability of nutrients and heavy metals (Zn, Cu, Mn, Fe, Ni, Pb, Cd and Cr) during vermicomposting of S. natans mixed with cattle manure and sawdust in five different combinations [trial 1 (eight S. natans: one cattle manure: one sawdust), trial 2 (seven S. natans: two cattle manure: one sawdust), trial 3 (six S. natans: three cattle manure: one sawdust), trial 4 (five S. natans: four cattle manure: one sawdust) and trial 5 (ten S. natans: 0 cattle manure: 0 sawdust)] for 45 days with Eisenia fetida earthworm. Results The highest growth of earthworms was in trial 4, having the highest (40 %) cattle manure. Trial 4 also indicated the highest reduction of volatile solids (38.6 %) and soluble BOD (82.3 %). The water-soluble forms of all nutrients were increased significantly. The highly bioavailable water-soluble and DTPA-extractable forms of heavy metals were reduced favourably after the process. The toxicity characteristic leaching procedure (TCLP) test indicated that leachable heavy metals of the vermicomposts were also reduced and were within the threshold limits for agricultural applications. Conclusions Eisenia fetida was very effective for reduction of bioavailable and leachable forms of selected heavy metals, and the TCLP test confirmed that the vermicompost was not hazardous for soil applications. The potential of the earthworms to increase the available nutrients, but mitigate the metal toxicity during vermicomposting of S. natans will be useful for sustainable land renovation practices.