From Carbonaceous Residues to Engineered Fertilizers: A Performance-First Critical Review of Biochar and Hydrochar for Nutrient Recovery, Controlled Release, Agronomic Function, and Life-Cycle Climate Performance

Biochar and hydrochar are increasingly described as fertilizers, nutrient-recovery media, soil-carbon amendments, and climate-mitigation products, although these functions require different evidence. This structured critical narrative review reappraises a 122-source analytic corpus spanning 2019–2026 and applies the Dual-Performance Boundary Framework for Carbonaceous Fertilizers (DPB-CF), which independently tests agronomic fertilizer function and net life-cycle climate benefit. The corpus is strongly recent, with 67 sources (54.9%) published in 2024–2026 and 50 (41.0%) in 2025–2026. Evidence is synthesized from feedstock chemistry through pyrolysis, co-pyrolysis, and hydrothermal carbonization; nutrient partitioning and speciation; nutrient capture; controlled release; soil–plant response; product engineering; safety; techno-economics; life-cycle assessment; regulation; and scale-up. A three-generation author-developed framework is operationalized: amendment char, nutrient-enriched char, and engineered nutrient-recovery fertilizer. Phosphorus and potassium are often retained or concentrated in carbonaceous solids, whereas nitrogen is more vulnerable to volatilization during dry thermochemical conversion or partitioning into hydrothermal process water. Mg- and Ca-based engineering can increase phosphate capture and tailor release, but adsorption capacity or total nutrient content does not establish fertilizer value without evidence of release, plant uptake, nutrient-use efficiency, and crop response. Multi-year studies demonstrate both positive and neutral outcomes; soil dependence is therefore treated as part of product performance rather than experimental noise. For climate claims, carbon permanence must be integrated with feedstock counterfactuals, process energy, transport, co-products, avoided waste treatment, field greenhouse-gas emissions, and end-use assumptions. Evidence classification was performed as a single-reviewer interpretive synthesis and was not tested for inter-rater reliability. The central development priority is a mass-balanced pathway from nutrient recovery to crop uptake, coupled to safe product specifications, multi-season validation, and auditable techno-economic and life-cycle boundaries.