Biochar is an effective amendment to remediate Cd-contaminated soils-a meta-analysis

Purpose Cd immobilization can be affected by many factors, among which the soil pH, soil organic matter (SOM), and amendment types are the most critical factors that have been considered in many previous studies. The objective of this study was to find the amendments with best performance for Cd remediation in soils with different range of soil pH and SOM. Materials and methods The effects of soil pH, SOM, and amendment types on soil Cd fractions and Cd uptake by rice were investigated by meta-analysis performed on MetaWin software, based on relevant data of 513 observations from 51 published papers that studied the soil fields in China from 2000 to 2020. Results and discussion Cd immobilization was more responsive to amendments when soil pH <= 6.0 as compared with those with pH higher than 6.0, which is probably because most amendments increased the soil pH and thus decreased the Cd activity. The Cd immobilization was most effective with the presence of 20-30 g/kg SOM; excessive SOM activated Cd in soil. Synthetic material appeared to be the most effective amendment on immobilization of the exchangeable fraction of Cd in soil by 42.5% inhibition rate. Biochar showed the strongest inhibition effect on the Cd uptake by rice (65.2% inhibition rate), while organic material exhibited the lowest immobilization effect. Conclusions We demonstrated that amendments showed best performance under conditions of soil pH <= 6.0 and SOM of 20-30 g/kg, and biochar is an effective amendment to remediate Cd-contaminated soils. The findings enable us to remediate Cd contamination on farmland more effectively and efficiently according to the local conditions.