Biochar Application Improves Chemical Properties and Nutrient Availability in Saline–Alkali Soils: A Meta-Analysis

Saline–alkali soils are a major global challenge because they degrade soil chemistry, reduce nutrient availability, and limit crop productivity. Despite increasing interest in biochar as a sustainable soil amendment, its overall effects on soil chemical properties under saline–alkali conditions remain inconsistent. Therefore, we conducted a meta-analysis of 113 peer-reviewed publications and 714 paired comparisons published between 2014 and 2025 to evaluate the effects of biochar on soil chemical properties in saline–alkali soils. Biochar significantly (p < 0.05) increased soil organic carbon (SOC) (90.4%), available nitrogen (AN) (47.9%), available phosphorus (AP) (51.1%), available potassium (AK) (101.8%) and cation exchange capacity (34.1%) compared to the control. Biochar also reduced soil pH (1.5%), electrical conductivity (7.7%), and exchangeable sodium percentage (32.7%). In addition, it decreased soluble sodium (3.2%) and chloride (41.7%), while increasing soluble calcium (41.8%) and magnesium (24.0%). Furthermore, stalk-derived biochar applied at high rates (≥ 3%) showed the best performance, increasing SOC by 123%. Stalk- and wood-derived biochar feedstocks produced the greatest improvements in AK (165.9%), AP (66.1%), and AN (54.2%). Correlation analysis revealed that SOC was positively correlated with calcium (Ca²⁺) (r² = 0.038), magnesium (Mg²⁺) (r² = 0.298), and AP (r² = 0.061), while negatively correlated with AN (r² = −0.034). In summary, medium-temperature pyrolyzed biochar derived from stalk and wood feedstocks, when applied at higher rates, represents an effective and sustainable strategy for improving chemical properties and nutrient status in saline–alkali soils. These findings provide valuable insights for developing evidence-based biochar application protocols and formulating ecological restoration strategies for large-scale saline–alkali soil remediation.