Ecotoxicological Assessment of Compost Amended with Compost’s Biochar: Balancing Compost Quality and Environmental Safety

Biochar can improve composting, yet the ecotoxicological consequences of returning compost’s biochar to composting are poorly understood. Mature but off-specification compost was pyrolysed at 550 °C, and the resulting biochar (CBC) was added at 0, 3, 6 and 9% of dry matter to pilot-scale piles (one ~15 Mg pile per rate) of green waste and sewage sludge (9:1). After 39 days, the composts were analysed for respiration activity (AT4), nutrients, heavy metals and water-extractable ions and tested with the Phytotoxkit (Triticum aestivum, Sinapis alba, Pisum sativum) and Ostracodtoxkit (Heterocypris incongruens). CBC tended to increase K, Mn and Cu and reduce Ca and Mg; total metals remained below regulatory limits. CBC9 had the lowest AT4 (<10 mg O2 g−1), interpreted as inhibited microbial activity, and the strongest biological response (72.5% shoot inhibition in wheat; 83.2% ostracod growth inhibition). In PCA, these responses aligned with K, Mn, pH and oxidised N forms, probably markers of biochar loading. In this exploratory study, 3% CBC showed the most favourable ecotoxicological profile, though not an established optimum for compost quality; composts meeting chemical and stability criteria may still be ecotoxic.