Interactions of biochar with metal mobility in soils and uptake by plants

Biochar can be an effective amendment that can immobilize heavy metals in soils, reduce metal uptake by plants, and increase crop yields. However, there are only limited studies about biochar application in the field. Uncertainties in the results remain as these studies have wide ranging biochar properties, environmental conditions, and study design parameters. To investigate the effectiveness of biochar in the field conditions, this study reviewed 34 biochar field trials published before 30 June 2020. Cd mobility was analysed in depth as most of the data available was on Cd pollution. Results showed that across all studies, biochar addition to soils resulted in average reductions of 33% and 28% in exchangeable fraction of soil Cd and Cd enrichment of plant tissues, respectively. Crop yield increased by 21% on average. The efficacy of biochar varied depending on soil properties, biochar characteristics such as feedstock, biochar dosage rate and climatic factors such as precipitation. It was found that rice straw or hardwood-derived biochar may be the best for Cd immobilization in soils. Increases in soil pH and OC due to biochar addition significantly reduced Cd mobility in soil. Greater effects of biochar on Cd immobilization were observed for biochar dosages less than 50 t/ha. Hence, this rate could be the maximum threshold for biochar application in Cd-contaminated paddy soils. It was noted that despite the remediating effects of biochar, metal concentrations in soils and plants may still be greater than legislative standards; hence, guidelines are needed for safe and effective use of biochar