A global hierarchical meta-analysis identifies multiple factors controlling the effect of biochar application on soil cadmium levels

Soil cadmium (Cd) contamination is a persistent global threat due to its high mobility and bioaccumulation. While biochar is a common remediation tool, empirical results remain inconsistent. This global hierarchical meta-analysis synthesized 5,437 observations from 239 studies to quantify biochar’s efficacy in reducing soil Cd and identify key regulatory drivers. The study confirms that biochar application significantly reduces soil Cd levels, though results vary by context. Feedstock type and pyrolysis temperature emerged as primary regulators. Specifically, biochars derived from wood, organic waste, and lignocellulosic materials were most effective. Interestingly, the response exhibited a non-linear pattern, with both low and high pyrolysis temperatures outperforming intermediate ranges. The model further identified 605.85 t ha⁻¹ as a statistical inflection point for soil Cd response, beyond which the marginal reduction effect weakened; however, this threshold far exceeds conventional field application rates and should therefore be interpreted only as an upper-bound statistical feature of the model fit. Random forest modeling identified the application rate, pyrolysis temperature, soil texture, and study type as the dominant factors controlling Cd reduction. Among soil properties, increases in cation exchange capacity (CEC) and soil organic carbon (SOC) were the most significant contributors to variability. Interaction analyses further revealed that while manure-derived biochar might increase soil Cd in controlled laboratory settings, it effectively inhibits Cd in fine-textured soils. These findings provide quantitative evidence for understanding the context-dependent performance of biochar in Cd-contaminated soils. By identifying drivers and nonlinear patterns, this study informs cautious site-specific biochar application strategies.