Changes in nutrient and heavy metal content after vermicomposting of water hyacinth-based spent mushroom substrate

The spent biomass of water hyacinth (Eichhornia crassipes) after oyster mushroom cultivation is not much suitable for application as soil amendment. As a means to avoid environmental pollution and to decrease waste to landfill, the spent biomass was primarily composted with cow dung and then feed to red wiggler earthworm (Eisenia fetida) to transform it into vermicompost. The produced vermicompost showed significant reduction in salt concentration, electrical conductivity, available K concentration, C/N ratio and increase in pH, available N and P. The representative heavy metals (Fe, Cu, Zn, Pb and Cd) detected in the spent biomass of water hyacinth were derived from the initial plant biomass. Among these metals, concentration of Fe increased and that of Zn decreased significantly in the vermicompost, while the other metals did not show significant change. When applied as amendment to control soil for growth of radish plants, the vermicompost treatment resulted in significantly higher shoot and root biomass. Also, the amendment decreased the bioavailability of metals (especially Cd) and their accumulation in radish plants. The metal concentrations in the vermicompost and in the radish plants were found to fall within the permissible limits for compost and the normal range present in plants, respectively, indicating the suitability of the vermicompost for use as organic fertilizer. Utilization of the water hyacinth as substrate for oyster mushroom cultivation and recycling the spent mushroom substrate through vermicomposting could therefore provide food and fertilizer with no waste to landfill, changing the status of the invasive aquatic weed from a prolific pest to a potentially usable product.