Probing the toxicity of hydrothermal carbonised wastes on soil biota: Effect of reaction temperature and feedstock

Hydrothermal carbonised wastes (hydrochars) can have toxic effects on soil biota, but factors influencing toxin formation in hydrochar, and subsequent toxicity to soil organisms, have not been elucidated. This study investigated the toxicity of hydrochars on soil biota, with a focus on earthworm (Eisenia fetida) avoidance, microbial metabolic quotient (qCO₂) and microbial activities. Two reaction temperatures (200 °C and 260 °C) and different feedstocks (biosolids, chicken manure and rice straw) were used. Hydrochars produced at 260 °C were highly toxic to earthworms, causing earthworm avoidance of >84 %. Hydrochar from chicken manure and rice straw produced at 200 °C also caused significant avoidance (76-84 %), although with chicken manure, high salt (Na) concentration was the underlying factor rather than toxic organic compounds. In contrast, biosolids hydrochar produced at 200 °C showed no negative effect on earthworms. Further examination of biosolids hydrochar (260 °C) following extraction with water, methanol, acetone-hexane, or sequentially, indicated that toxins causing earthworm avoidance were both polar and non-polar organic compounds, as well as soluble salts. Despite increased qCO₂ suggesting microbial stress, hydrochars generally increased phospholipid fatty acids (bacteria and fungi), soil respiration, enzyme activities and N mineralisation. Findings reveal that while higher temperature hydrochars are highly toxic to earthworms, they do not adversely affect overall soil microbial health.