Exposure of earthworm (Eisenia fetida) to rice straw biochar: Ecotoxicity assessments for soil-amended programmes

Increasing attentions of agricultural straw-derived biochar on biological interactions in soil environment have been gained following the benefits obtained from its role of soil conditioner. However, scientific probing on its unintended effects on soil organisms remain largely understudied. In this study, an avoidance test and a 14-day of incubation experiment in earthworm Eisenia fetida were conducted in field soil amended with rice straw-derived biochar (RSB) to evaluate its eco-toxic effect. Results showed earthworms strongly avoided soil amended with RSB from 7.5% to 15% at statistically significant levels. Subsequent 14-day incubation experiment with RSB rates up to 5% without avoidance responses showed these doses did not cause statistically significant difference in relative growth rate and survival rate regardless of incubation time of 7 or 14 d. Data obtained from 5% RSB application not only led to decrease in activities of digestion-related enzymes including Na⁺-K⁺-ATPase and cellulose, but facilitated some abnormities of intestinal epithelial tissue. Meanwhile, skin structure deterioration mainly related to upper stratum corneum appeared both in 2.5% and 5% amendments application. Additionally, RSB usage rates up to 2.5% and 5% also led to significant decrease in ROS and MDA but accompanied with unaffected antioxidant enzymes of SOD and CAT. Even importantly, only 5% amendment induced coelomocyte DNA damage supported by increased values of tail DNA% and OTM. Our research identified statistical effect thresholds related to RSB application rates and suggests that lower amendment with RSB no more than 1% have no eco-toxic effect on earthworms Eisenia fetida, whereas some adverse impact mainly found in higher addition up to 5%, which provide safe supporting for usual loading of soil with lower rates of RSB as soil conditioner to soil organisms in practice production.