Does biochar amendment of e-waste contaminated soil improve earthworm growth and reproduction?

Inadequate management of e-waste at informal recycling sites contaminates soil. Biochar offers agricultural and environmental benefits, but its effects on soil fauna remain poorly understood. We evaluated biochar's effects on the growth and reproduction of the earthworm, Alma nilotica (Grube, 1855), to determine an application level that mitigates harmful effects of e-waste contaminants without harming soil organisms. Biochar was prepared from dried maize cobs using a closed drum technique at 500 to 530°C. Effects on earthworm growth and reproduction were assessed using Organisation for Economic Co-operation and Development (OECD) test guidelines and test methods. Results indicated that growth and reproduction were inhibited in a dose-dependent manner, by up to 50 ± 6.4% and 68.2 ± 6.6%, respectively, at a 5% application rate without e-waste contaminants. Earthworm length was more sensitive to the e-waste contaminants than weight, showing disproportionate growth. A dose-dependent growth and reproduction inhibition was also observed in biochar-amended e-waste contaminated soil, giving growth and median reproduction inhibitory concentration (95% confidence interval) of 3.75% (3.25-4.30%) and 2.1% (1.9-2.3%) biochar, respectively. Biochar amendment of e-waste soil at 1% significantly improved earthworm growth (2.2 times longer) and reproduction (5 times more offspring) compared to the unamended soil, unlike at 5% biochar amendment, where growth and reproduction were not different from the unamended soil. These findings underscore the importance of considering optimal biochar dosage in soil amendments to enhance earthworm growth and reproduction.