Significant contributions of biochar-derived dissolved matters to ecotoxicity to earthworms (Eisenia fetida) in soil with biochar amendment

Biochar (BC) as a soil amendment has been proved to pose potential risks to terrestrial invertebrates. However, the underlying toxic mechanism of BC to soil fauna remains to be elucidated, especially the effects of its different fractions. This work aimed to evaluate the contributions of biochar-derived dissolved matters (BC-DM) and particulate biochar (PBC) to ecotoxicity to earthworms in BC-amended soil, and then uncover the underlying toxic mechanisms. Results showed that the survival rates of earthworms in different treatments decreased by an order of BC (26.5% ± 0.05) < BC-DM (56.7% ± 0.03) < PBC (86.7% ± 0.09), indicating that BC was toxic to earthworms after addition to soil, and BC-DM appeared to be more toxic than PBC. Physiological response of earthworms further confirmed that BC-DM exposure induced more oxidative stress on earthworms than PBC. For example, the glutathione S transferase (GST) activity of earthworms in 4% BC-DM and PBC treatments decreased by 65.1% and 23.3%, respectively. BC and PBC caused slight damage to intestine of earthworms, while no significant change was observed in BC-DM treatments. Transmission electron microscopy (TEM) images revealed that BC, BC-DM and PBC resulted in obvious damage to epidermal tissue. Notably, the epidermis of earthworms suffered severe damage under BC exposure. This study was the first to comprehensively evaluate the ecotoxicity of biochar components to Eisenia fetida, and highlight the significant contribution of BC-DM to ecotoxicity to earthworms in BC-amended soil. The obtained results could contribute to scientifically assessing the ecological risk of biochar in agricultural soil from multiple perspectives.