Biological Science: Research Developments and Innovations Vol. 2

Biochar and hydrochar are increasingly discussed as carbon-rich amendments that can connect biomass valorisation, soil rehabilitation and climate mitigation, yet the two materials are produced by fundamentally different thermochemical routes and cannot be treated as agronomically or climatologically interchangeable. This critical narrative review evaluates how production conditions and post-production engineering govern material properties, soil conditioning functions and the persistence of amendment-derived carbon. Literature published from 1 January 2006 to 4 July 2026 was searched using major open scholarly indexes and agricultural and biomedical databases, supplemented by citation chasing and DOI-level bibliographic verification. The evidence indicates that pyrolysis generally produces more condensed, aromatic and persistent carbon than hydrothermal carbonisation, whereas hydrothermal carbonisation offers a practical conversion route for wet feedstocks but commonly yields hydrochar with a larger labile carbon fraction and a greater risk of phytotoxic or nitrogen-immobilising effects when freshly applied. Engineering strategies including activation, mineral functionalisation, nutrient loading and co-composting can strengthen targeted functions, but their agronomic value depends on whether the introduced functionality remains effective in chemically complex soil and whether modification increases contaminant burdens, salt inputs or life-cycle emissions. Soil responses are strongly context dependent. The most reproducible benefits of biochar occur where the amendment corrects an identifiable constraint, particularly acidity, low nutrient retention, low water-holding capacity or poor soil structure; average effects are weaker and less predictable in fertile, fine-textured or weakly constrained soils. Carbon sequestration is controlled by intrinsic carbon structure together with ageing, organo-mineral association, aggregation, priming of native soil organic matter, non-carbon-dioxide greenhouse-gas responses and supply-chain emissions. Hydrochar can contribute to soil carbon management, but current evidence supports lower confidence in its long-term permanence than for well-carbonised biochar. The central implication is that agricultural char deployment should move from product-centred claims to function-matched design, long-term field validation and full carbon accounting.