How much biochar should be used in global ecological restoration?

Abstract Biochar applied in ecological restoration shows a significant dose-dependent effect. Therefore, determining an appropriate rate for specific environments is essential in practical applications. To this end, we integrated soil profiles and biogeochemical and microbiome datasets from six major ecosystems (agricultural, grassland, forest, coastal wetlands, desert, and polar tundra), constructed a global biochar remediation threshold map, and investigated the driving mechanisms of threshold formation. Simultaneously, differentiated application frequency strategies tailored to each ecosystem are proposed. The results showed that the appropriate thresholds for different ecosystems were 5–30 t ha −1 (agricultural), 5–40 t ha −1 (grassland), 5–40 t ha −1 (forest), 10–50 t ha −1 (coastal wetlands), 10–40 t ha −1 (desert), and 20–60 t ha −1 (polar tundra). Within the threshold range, in combination with customized application frequency, biochar enhances ecological functions by increasing soil water-holding capacity (by approximately 10–14.3%), reducing greenhouse gas emissions (by approximately 16.4–31.5%), lowering soil heavy metal content, and increasing soil organic matter. Exceeding the threshold can cause sharp fluctuations in soil pH, increases in bioavailable polycyclic aromatic hydrocarbons, and decreases in microbial diversity, thereby inhibiting remediation. Inadequate application frequency also weakens the ecological restoration efficacy of biochar. The thresholds are environmentally dependent. The threshold window can be expanded or narrowed by the joint regulation of preparation parameters, soil characteristics, climate, and human management. Based on this, we propose a framework of “threshold identification–mechanism analysis–targeted intervention” and customized application paths for ecosystems (e.g., on-farm biochar + fertilizer single/strip application, low-dose staged application in grassland, forest low-dose dispersal single intervention, combination of wetland surface and spot combined with seasonal application, desert inter-root precision single application, and zoned management of medium and low doses in polar regions). The framework provides a quantitative and mechanistic basis for formulating standards for the production and application of biochar and for promoting precise remediation. Graphical Abstract