Hydrochar as an emerging solution for soil health improvement: Insights from a pot trial

Anthropogenic soil degradation undermines essential ecosystem services such as food production, water purification, nutrient cycling and climate regulation. Unsustainable agricultural practices are among the main causes of soil degradation through pollution, soil loss and consequently lowering organic matter and nutrients. Adopting innovative solutions for agriculture soil management by adding low-decomposable organic amendments to the soil, such as hydrochar, can help reverse soil degradation. Hydrochar, derived from the hydrothermal carbonization of organic waste, may have the advantage of restoring the organic C stock in the soil, helping to mitigate climate change and improving soil health. Before using hydrochar at a large scale, a comprehensive assessment to exclude any potential adverse effects on the soil biotic community, playing a key role in the provisioning of ecosystem services, is needed.
This study, part of the interdisciplinary project ‘CHIMERA’ evaluating the impact of hydrochar on the soil-plant-atmosphere system, aims to investigate changes in the chemical and microbial properties of degraded agricultural soil following the application of hydrochar. Therefore, a controlled greenhouse experiment was conducted using pots (21 cm diameter, 16 cm height), each containing 1 kg of soil. Two types of hydrochar, produced by hydrothermal carbonization at 250 °C and 50 bar without oxygen, were tested: one derived from sewage sludge (HS) and the other from thistle (Cynara cardunculus L., HC) residues, respectively. Each hydrochar was applied at two doses (3 kg m^-² and 6 kg m^-²), and the resulting five treatments (four with hydrochar and one control) were assayed in five replicates. At different exposure times (from 18 to 517 days), the following soil properties were analysed: pH, total organic C (C_org) and its extractable fraction (C_ext), microbial biomass (C_mic), activity (as respiration), the quotient of mineralization (qM) and genetic bacterial diversity (richness).
The results showed no toxicity to the soil microbial community; moreover, a general improvement of microbial biomass, activity and richness was observed, compared to control, at each exposure time, together with a significant decrease in qM, suggesting that C added as hydrochar was at least in part retained in soil. This ability highlights the positive hydrochar's role in improving soil structure and promoting resilience against erosion, drought and other climate-related challenges.
Our findings suggest that hydrochar could be a tool for sustainable agricultural practices in restoring degraded soils. However, the application of hydrochar on soils requires further studies to confirm these positive effects and whether these effects can be observed using hydrochar derived from other raw materials and for other soil types.