Optimizing biochar application strategies to improve soil properties and crop productivity in salt-affected croplands: Evidence from a global meta-analysis and machine learning approach

CONTEXT Salt-affected soils have become a major constraint on global food security and sustainable agricultural development by degrading soil structure, limiting nutrient availability, and suppressing crop growth. Biochar, characterized by its porous architecture, high carbon stability, and diverse physicochemical properties, is increasingly regarded as a promising amendment for the reclamation of salt-affected soils. However, the effects of biochar on soil salinity, pH, carbon accumulation, and crop yield vary substantially across climatic conditions, soil backgrounds, management practices, and biochar characteristics. OBJECTIVE This study aimed to systematically quantify the effects of biochar application on soil properties and crop yield in salt-affected croplands, identify the key factors governing these responses, and provide a scientific basis for the rational deployment of biochar under contrasting environmental and management conditions. METHODS We conducted a global meta-analysis based on 831 paired observations extracted from 78 published studies to evaluate the effects of biochar application on soil bulk density (BD), electrical conductivity (EC), pH, salinity, soil organic carbon (SOC), and crop yield (Yield). Subgroup analyses were further performed according to crop type, climatic zone, climatic conditions, agricultural management practices, soil properties, and biochar properties. Random forest models were then used to identify the relative contributions of climatic factors, soil properties, management practices, and biochar characteristics to variations in soil and yield responses. RESULTS AND CONCLUSIONS Biochar application significantly reduced BD, EC, pH, and salinity by 8.2%, 30.7%, 0.5%, and 13.7%, respectively, while increasing SOC and crop yield by 38.4% and 28.9%, respectively. Subgroup analyses showed that biochar-induced reductions in BD and increases in SOC and yield were relatively consistent across most environmental and management contexts, whereas the responses of EC, pH, and salinity were strongly context dependent. These effects were jointly regulated by hydrothermal conditions, soil salinization degree, soil texture, nutrient status, biochar application rate, experimental duration, and biochar physicochemical properties. Random forest models exhibited strong predictive performance for BD, EC, pH, salinity, SOC, and Yield. Key predictors included biochar application rate, mean annual temperature, soil total nitrogen, soil available phosphorus, biochar cation exchange capacity, biochar pH, biochar electrical conductivity, and biochar bulk density. SIGNIFICANCE This study provides a global synthesis of the role of biochar in improving salt-affected soils and enhancing crop productivity, offering evidence-based guidance for soil reclamation under diverse environmental and management constraints. Our findings have important implications for the sustainable restoration of salt-affected croplands and the safeguarding of global food security.