EVALUATION OF PHYTOTOXICITY OF COMPOSTS PRODUCED FROM SPENT MUSHROOM SUBSTRATE USING SEED GERMINATION BIOASSAY OF Raphunus sativus L.

Abstract

Spent mushroom substrate (SMS) has the potential to produce compost with different additives. However, some substances and by-products produced during the process can be toxic to seed germination and plant growth. Evaluation of the phytotoxicity is important for guaranteeing compost quality. Therefore, this study was conducted to evaluate the phytotoxicity of spent mushroom substrate compost (SMSC) using a seed germination bioassay. Compost aqueous extracts (CAEs) were prepared from SMSC produced by amending different additives (green leaves and urea) with and without an inoculum. Dilution series of 25%, 50%, and 100% were tested against seed germination of radish (Raphanus sativus L). Germination index (GI) in all 25% CEAs of this study were phytotoxic free and they indicated the availability of phytonutrients and/or phyto-stimulants except the non-inoculated SMSC without amendments. Similarly, all 50% CAEs were phytotoxic free except Tithonia amended inoculated SMSC with less phytotoxicity. Of 100% CAEs, non-inoculated Pueraria amended SMSC was highly phytotoxic, non-inoculated Tithonia and urea amended SMSC were phytotoxic free, and the rest were less phytotoxic. Seed germination of R. sativus was not inhibited by any of the CAEs of Tithonia amended SMSC indicating the higher usability as an amendment in SMS. The water extractable NH4 + concentration was significantly higher (p<0.05) in the inoculated SMSC amended with urea and it was significantly lower (p<0.05) in the non-inoculated SMSC amended with Tithonia and inoculated SMSC without amendments. Further, the inoculated SMSC extracts showed higher performance in GI than non-inoculated SMSC. GI showed a higher negative significant correlation (r=-0.841, p<0.01; r=-0.778, p<0.01) with EC in both inoculated and non inoculated compost samples, respectively. Moreover, the EC was significantly correlated with all extractable tested ions except Cu2+ and Ni2+ in inoculated treatments. pH values in all aqueous extracts were recorded in the range of 6.27 to 8.08. Extractable elements Cu2+ (r=0.645, p=0.01) and Zn2+ (r=0.577, p=0.05) had positive correlation with pH. SMS can be recycled as compost integrating nitrogenous additives and inoculum to use safely as a soil amendment