Soil Responses to Hydrothermal Liquefaction Hydrochar Application: Greenhouse Gas Emissions and Earthworm Avoidance

Hydrothermal liquefaction (HTL) converts wet biomass into biocrude and a solid by-product, hydrochar (HC), for which no established valorization pathway exists. Valorizing HC as a soil amendment is promising due to its nutrient and carbon-rich composition, but its effects on soil dynamics and biota remain not understood. This study evaluated the impact of HTL-derived HC and post-treated materials on soil greenhouse gas (GHG) emissions and earthworm behavior. HC was produced in a continuous HTL pilot plant using a cattle manure–straw mixture and subsequently treated by pyrolysis at 400 °C and 600 °C or by water washing (W-HC). The materials were incubated with a southern German soil for 90 days in microcosms to monitor GHG fluxes and calculate the associated global warming potential (GWP). In parallel, an earthworm avoidance test was conducted for 21 days using the same soil. The earthworm tests showed no avoidance behavior, with avoidance rates between 47% and 59%, indicating no ecotoxic effects under the tested conditions. Pyrolyzed materials, particularly PY600, reduced GWP compared with untreated HC and the control, while simultaneously increasing microbial carbon biomass. Therefore, high-temperature pyrolysis is a key step to enable HTL-derived HC use as a soil amendment with promising carbon capture potential.