Reframing biochar soil application from mass-based to function-based through the principle of functional delivery

Abstract Biochar is widely applied in soils to improve environmental quality. However, reported biochar effects remain highly variable and often contradictory, even under similar experimental conditions. This inconsistency is commonly attributed to differences in feedstock, pyrolysis conditions, and soil properties. A fundamental limitation persists as biochar is usually applied on a mass basis, despite large variability in the functional properties that govern its environmental performance. Equal masses of different biochar do not deliver equivalent functional capacity. This perspective introduces the Principle of Functional Delivery (PoFuD), which reframes biochar application from a mass-based to a function-based. PoFuD proposes that biochar effects are determined by the type and quantity of functional capacity delivered rather than by the applied mass. This article outlines a functional standardisation approach in which biochar application is normalised to a target property (or process-relevant function), while all co-delivered properties are measured transparently and treated as contextual background. Adopting PoFuD would clarify dose-response relationships, improve comparability across studies, and strengthen mechanistic interpretation. This conceptual shift has broad implications for soil remediation, biogeochemistry, greenhouse gas mitigation, and policy-relevant biochar research, and provides a foundation for more robust experimental design and standardisation in environmental applications.