Long-term effects of Aloe vera-based hydrochar on earthworm life cycle and soil microbial functions in natural soil

Hydrochar application as a soil amendment has received increasing attention. However, limited research has addressed its environmental impacts on beneficial soil organisms, such as earthworms and microorganisms. This study investigated the toxicological effects of Aloe vera -based hydrochar, applied to soil at concentrations of 0.5%, 1%, and 3%, on Eisenia andrei (epigeic earthworm), examining both apical and biochemical effects over two generations. Additionally, microbial functional responses were assessed over 180 days, paying particular attention to interactions between the hydrochar amendments, soil microorganisms, and earthworms. Notably, earthworm reproduction was adversely affected in the parent generation (F0), with a reduction of up to 29%, but not in the offspring generation (F1). Antioxidant enzyme activities (catalase and glutathione S-transferase) were altered in both generations. Malondialdehyde levels, indicative of oxidative stress, increased in F1 juveniles, but not in F0 or F1 adults. In earthworm-free assays, enzyme activities associated with nitrogen and phosphorus cycling decreased after 28 days, except for urease activity. Over 180 days, however, ammonium oxidase, alkaline phosphatase and dehydrogenase activities increased by 141 ± 20%, 64 ± 5% and 150 ± 20%, respectively, relative to control. Earthworms mitigated the hydrochar-induced adverse effects on soil microbial functions. Overall, the negative effects of hydrochar on soil organisms diminished over time, likely due to the degradation of labile toxic compounds within the hydrochar. Hydrothermal carbonization shows promise as an alternative strategy for managing Aloe vera residues, since adverse effects of hydrochar were primarily observed at the highest concentrations (1–3%). Furthermore, hydrochar aging in soil may mitigate or eliminate the initial toxic effects. • Hydrochar reduced earthworm reproduction in parental but not in filial generation • Cellular damage, as evidenced by increased MDA levels, was observed in F1 juveniles • Oxidative stress biomarkers varied with the growth stages of earthworm life cycle • Hydrochar decreased phosphatase and ammonium oxidase soil activities after 28 days • Soil aging of hydrochar mitigated adverse effects on earthworms and microorganisms