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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.

Composting of water hyacinth, mixed with cattle manure, rice husk and sawdust in four different proportions, was performed in a pilot scale rotary drum composter. The physico-chemical characteristics, i.e., temperature, moisture content, pH, electrical conductivity, total organic matter (OM), nitrogen dynamics and nutrients were evaluated during the 20 days composting process. The stabilities of the composts were also investigated with respirometric analysis, i.e., CO 2 evolution rates and oxygen uptake rate (OUR). Among all trials, trial 1 (6 water hyacinth, 3 cattle manure, 1 rice husk) indicated the best composting mix, as shown by the highest temperature profile and OM loss, and lowest CO 2 evolution rate and OUR.

Purpose Composting of harvested Salvinia natans weed of Loktak lake of Manipur, India can protect the precious lake from the unwanted growth of the weeds but there is a need to assess the total amount and bioavailable forms of heavy metals and minerals of the final compost before application as a soil conditioner. Methods Studies were conducted on the transformations of the physico-chemical parameters and the total amount, bioavailable and leachable forms of heavy metals and minerals during agitated pile composting of S. natans with rice husk and cattle manure in five different combinations [trial 1 (5 S. natans: 4 cattle manure: 1 rice husk), trial 2 (6 S. natans: 3 cattle manure: 1 rice husk), trial 3 (7 S. natans: 2 cattle manure: 1 rice husk), trial 4 (8 S. natans: 1 cattle manure: 1 rice husk) and trial 5 (10 S. natans: 0 cattle manure: 0 rice husk)]. Results The highest temperature (52.2°C) was monitored in trial 2 having 30 % cattle manure during the process. Highest reduction of moisture content and volatile solid in trial 2 corroborated the temperature profile. Total concentrations of heavy metals (Zn, Cu, Mn, Fe, Ni, Pb, Cd and Cr), total concentration of nutrients (Na, K, Ca, and Mg) and the water-soluble forms of the nutrients were enhanced significantly. The water-soluble, plant-available and leachable metals decreased favorably with maximum reduction observed mostly in trial 2. The study also revealed that the total concentrations of Zn (211-254 mg/ kg), Ni (310-345 mg/kg) and Pb (805-891 mg/kg) of the final composts were higher than that of Cr (140-207 mg/ kg) but the water-soluble forms of Zn (3.6-4.7 % of total Zn), Ni (NIL) and Pb (0.7-1.0 % of total Pb) were lower than that of .3 % of total Cr) indicating that Cr in the composts had higher toxicity potential. The leachable heavy metals were within the threshold limits prescribed for agricultural application. Conclusions Composting of S. natans biomass with appropriate proportion of cattle manure reduced the bioavailable and leachable forms of the heavy metals in the biomass.

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