A soil function indicator framework for evaluating biochar in agricultural soils

Agricultural soils underpin global food security but are increasingly constrained by declining organic matter, erosion, and loss of biological functioning under intensified production and climate stress. Biochar has been widely studied as a soil amendment capable of improving these functions while contributing to long-term carbon storage; however, its adoption remains limited by the absence of operational frameworks that translate soil responses into measurable and scalable outcomes. This work presents a soil-science-led conceptual framework that positions biochar application as a soil-function intervention and links biochar-induced processes to a minimal, field-feasible indicator set. The framework centres on a complementary triad comprising a soil enzyme composite index, microbial community composition metrics, and structural stability (aggregate–erosion resistance), representing biochemical, biological, and physical dimensions of soil function. Rather than exhaustive characterisation, the approach emphasises conservative interpretation, site-specific baselines, and repeatable measurements suited to heterogeneous farming contexts. Carbon storage and improvements in soil condition are treated as parallel but analytically distinct outcome pathways, avoiding conflation of carbon quantity with soil quality. By centring on soil functional change, this framework provides a transparent and operational basis for evaluating the impacts of biochar. It supports the integration of soil condition into emerging nature-positive frameworks without reliance on species-level biodiversity metrics.