We synthesized 20 years of research to explain the interrelated processes that determine soil and plant responses to biochar. The properties of biochar and its effects within agricultural ecosystems largely depend on feedstock and pyrolysis conditions. We describe three stages of reactions of biochar in soil: dissolution (1–3 weeks); reactive surface development (1–6 months); and aging (beyond 6 months). As biochar ages, it is incorporated into soil aggregates, protecting the biochar carbon and promoting the stabilization of rhizodeposits and microbial products. Biochar carbon persists in soil for hundreds to thousands of years. By increasing pH, porosity, and water availability, biochars can create favorable conditions for root development and microbial functions. Biochars can catalyze biotic and abiotic reactions, particularly in the rhizosphere, that increase nutrient supply and uptake by plants, reduce phytotoxins, stimulate plant development, and increase resilience to disease and environmental stressors. Meta‐analyses found that, on average, biochars increase P availability by a factor of 4.6; decrease plant tissue concentration of heavy metals by 17%–39%; build soil organic carbon through negative priming by 3.8% (range −21% to +20%); and reduce non‐CO₂ greenhouse gas emissions from soil by 12%–50%. Meta‐analyses show average crop yield increases of 10%–42% with biochar addition, with greatest increases in low‐nutrient P‐sorbing acidic soils (common in the tropics), and in sandy soils in drylands due to increase in nutrient retention and water holding capacity. Studies report a wide range of plant responses to biochars due to the diversity of biochars and contexts in which biochars have been applied. Crop yields increase strongly if site‐specific soil constraints and nutrient and water limitations are mitigated by appropriate biochar formulations. Biochars can be tailored to address site constraints through feedstock selection, by modifying pyrolysis conditions, through pre‐ or post‐production treatments, or co‐application with organic or mineral fertilizers. We demonstrate how, when used wisely, biochar mitigates climate change and supports food security and the circular economy.

Interactions between biochar, soil, microbes, and plant roots may occur within a short period of time after application to the soil. The extent, rates, and implications of these interactions, however, are far from understood. This review describes the properties of biochars and suggests possible reactions that may occur after the addition of biochars to soil. These include dissolution–precipitation, adsorption–desorption, acid–base, and redox reactions. Attention is given to reactions occurring within pores, and to interactions with roots, microorganisms, and soil fauna. Examination of biochars (from chicken litter, greenwaste, and paper mill sludges) weathered for 1 and 2 years in an Australian Ferrosol provides evidence for some of the mechanisms described in this review and offers an insight to reactions at a molecular scale. These interactions are biochar- and site-specific. Therefore, suitable experimental trials—combining biochar types and different pedoclimatic conditions—are needed to determine the extent to which these reactions influence the potential of biochar as a soil amendment and tool for carbon sequestration.

Biochar properties can be significantly influenced by feedstock source and pyrolysis conditions; this warrants detailed characterisation of biochars for their application to improve soil fertility and sequester carbon. We characterised 11 biochars, made from 5 feedstocks [Eucalyptus saligna wood (at 400°C and 550°C both with and without steam activation); E. saligna leaves (at 400°C and 550°C with activation); papermill sludge (at 550°C with activation); poultry litter and cow manure (each at 400°C without activation and at 550°C with activation)] using standard or modified soil chemical procedures. Biochar pH values varied from near neutral to highly alkaline. In general, wood biochars had higher total C, lower ash content, lower total N, P, K, S, Ca, Mg, Al, Na, and Cu contents, and lower potential cation exchange capacity (CEC) and exchangeable cations than the manure-based biochars, and the leaf biochars were generally in-between. Papermill sludge biochar had the highest total and exchangeable Ca, CaCO3 equivalence, total Cu, and potential CEC, and the lowest total and exchangeable K. Water-soluble salts were higher in the manure-based biochars, followed by leaf, papermill sludge, and wood biochars. Total As, Cd, Pb, and polycyclic aromatic hydrocarbons in the biochars were either very low or below detection limits. In general, increase in pyrolysis temperature increased the ash content, pH, and surface basicity and decreased surface acidity. The activation treatment had a little effect on most of the biochar properties. X-ray diffraction analysis showed the presence of whewellite in E. saligna biochars produced at 400°C, and the whewellite was converted to calcite in biochars formed at 550°C. Papermill sludge biochar contained the largest amount of calcite. Water-soluble salts and calcite interfered with surface charge measurements and should be removed before the surface charge measurements of biochar. The biochars used in the study ranged from C-rich to nutrient-rich to lime-rich soil amendment, and these properties could be optimised through feedstock formulation and pyrolysis temperature for tailored soil application.