Hydrochar as a sustainable soil amendment

The increasing pressure on natural resources and the accumulation of organic waste highlight the urgent need for sustainable waste-to-resource strategies and effective soil restoration practices. Hydrothermal carbonization (HTC) has emerged as a promising technology for converting diverse waste streams into hydrochars—carbon-rich materials with the potential to improve soil quality, immobilize contaminants, and contribute to climate change mitigation. However, the scientific challenge lies in the considerable variability of hydrochar properties, which depend strongly on feedstock composition, HTC conditions, and subsequent modifications, thereby complicating the prediction of their environmental performance. This review comprehensively synthesizes advances published between 2017 and 2026 in the application of hydrochars as soil amendments. The assessment focused on three dimensions: (i) impacts on heavy metal immobilization, (ii) effects on soil microbiomes and respiration, and (iii) influences on crop productivity and greenhouse gas emissions. Reported outcomes were categorized as positive, neutral, or negative to ensure comparability. The synthesis of available evidence indicates that hydrochars enriched with oxygen-containing functional groups demonstrate high potential for contaminant immobilization. In contrast, manure- and nutrient-modified hydrochars frequently positively affect microbial activity and plant growth at low application doses. Conversely, high HTC severity (temperature and residence time) or excessive application rates often result in phytotoxicity or reduced microbial diversity. Life cycle assessment studies further reveal that the climate performance of hydrochar systems ranges from modest emission reductions to net-negative CO 2 balances, depending on system boundaries and utilization pathways. Hydrochars represent a versatile soil amendment with significant potential for integrating waste valorization and soil remediation.