Enhanced vermicomposting of Sesbania bispinosa through carbonaceous biochar amendment for nutrient-rich vermicompost production

Sesbania bispinosa (Jacq.) W. Wight is a weedy green manure plant of the family Fabaceae. The legume plants play a major role in nitrogen fixation and soil fertility while biochar plays a significant role in environmental remediation. The present study has aimed to convert S. bispinosa in combination with cow dung (1:1, v/v) into enriched vermicompost through the amendment of different concentrations of activated Prosopis wood biochar. Totally 10 treatments were maintained: VSB1-VSB5 and SB1-SB5 with and without earthworms, respectively. The treatments were maintained under controlled environmental conditions in triplicate till 28 days. The initial and final samples of the treatments with and without earthworms were analyzed for physicochemical characteristics, FTIR analysis and phytotoxicity assay. The pH, total organic carbon, C/N ratio and C/P ratio showed a declining trend while the nutrients, total nitrogen, phosphorus, potassium, calcium and sodium contents displayed increment towards the progression of vermicomposting. A total nitrogen content of 2.78% was recorded in vermicompost amended with 3% biochar followed by 2.61% in 2% biochar amendment. The functional group changes from initial substrates to final vermicompost inferred through FTIR analysis denote the decomposition of complex organic materials into simple forms. The lowest C/N ratio (19.06) with a higher germination index (102.85%) of Vigna mungo (black gram) seeds were observed in the vermicompost of S. bispinosa + cow dung substrates amended with 3% biochar within a very short period of time (28 days). Hence, the use of biochar at a 3% level is recommended for the vermiconversion of green manure biomass to obtain nutrient-amplified vermicompost. It is also beneficial to use 2% biochar in vermicomposting, however, 3% biochar amendment is highly beneficial and a better choice from a vermicompost quality perspective.