Phytotoxicity test

Immediately after the endpoint of the biodegradability test in thermophilic conditions, a germination test and a growth test were conducted to evaluate the potential terrestrial phytotoxicity of the residual test material and, if any, of their biodegradation products. Such tests are recommended in ASTM D6400. The phytotoxicity test procedure was adapted from OECD Guideline 208⁸⁴. Samples were mixed with wet sandy soil, then incubated for 19 days at 25 °C. Three parameters were monitored: the seedling emergence of 20 ryegrass seeds, the absence of visible phytotoxic effects (e.g., chlorosis, necrosis, leaf, and stem deformation), and the biomass production (wet weight of the resulting plants) after 19 days (“Phytotoxicity Test” in SI). ASTM D6400 suggests using two plant species for the phytotoxicity test: consequently, our test has to be considered only as a first indication of the terrestrial phytotoxicity of the materials. To reinforce the significance of the phytotoxicity test, the rate of respiration of the compost microbiota (as observed during the biodegradability test at 58 °C) was used as a second bioindicator of potentially toxic effects for the terrestrial trophic level⁸⁵.