Biochar reduces soil thermal conductivity, diffusivity and volumetric heat storage: A global meta-analysis

Biochar amendments are increasingly applied to improve soil physical functioning and support carbon dioxide removal, but their effects on intrinsic soil thermal properties remain poorly characterised. We conducted the first global systematic meta-analysis of 19 independent studies, 231 control-biochar comparisons, and 529 property-specific effect sizes to test how biochar changes soil heat transfer and storage. Biochar reduced thermal conductivity by 17.6% (95% CI, -22.7 to -12.2), thermal diffusivity by 11.0% (-14.5 to -7.3), and volumetric heat capacity by 8.3% (-12.3 to -4.1). Gravimetric heat capacity showed no significant overall response (+3.3%; -7.6 to 15.4) but was supported by fewer studies. Negative responses were directionally consistent for thermal conductivity, diffusivity, and volumetric heat capacity. Moderator analyses showed that responses were most consistently associated with post-application bulk density and changes in bulk density, while application rate modulated response magnitude and soil texture constrained context dependence. Co-variation among thermal conductivity, thermal diffusivity, and volumetric heat capacity matched expected physical dependencies, indicating coordinated structural reorganisation rather than independent shifts in isolated parameters. These estimates describe intrinsic conductive and storage properties; field-scale soil temperature responses may also be modified by albedo, evaporation, vegetation, and surface energy balance. Improved integration of soil thermal measurements with moisture dynamics, structural changes, and carbon cycling is essential to accurately represent biochar effects in soil and land-surface models. Significance StatementBiochar is increasingly used to store carbon while improving soils, but its effects on how soils conduct, transmit, and store heat remain poorly understood. By synthesising 19 studies and 529 property-specific effect sizes, we show that biochar consistently reduces soil thermal conductivity, diffusivity, and volumetric heat capacity, primarily through structural changes in bulk density and pore architecture. These findings identify soil thermal behaviour as an overlooked component of biochar-soil interactions, with implications for soil temperature buffering, water-energy coupling, plant and microbial processes, and land-surface and carbon-cycle modelling.