Biochar effectively remediates Cd contamination in acidic or coarse- and medium-textured soils: A global meta-analysis

Soil cadmium (Cd) contamination is one of the most serious environmental problems on a global scale. Biochar has a great potential to reduce Cd bioavailability in contaminated soils even though biochar effects on soil Cd bioavailability has been inconsistent among studies. In this meta-analysis, we used 802 paired observations from 84 peer-reviewed articles to evaluate the effect of biochar application on Cd bioavailability among different soil types and to elucidate the factors governing that effect. Our key findings are: 1) biochar application reduced Cd bioavailability across various biochar and soil types, with the greatest reduction (70%) in urban/anthropogenically contaminated soils, and reductions of 41.1, 42.3 and 30.2% were found in acidic soils (pH < 6.5), and in coarse- and medium-textured soils, respectively. However, biochar increased Cd bioavailability in fine-textured soils by 16.2%; 2) in acidic soils, biochars produced from rice straw, pyrolyzed at 450–550 °C, with a heating rate of 1–5 °C min−1 and a residence time of <60 min were most effective; whereas in alkaline soils, biochars produced from sewage sludge, pyrolyzed at <350 °C, with a residence time >60 min were more effective in reducing Cd bioavailability; and 3) the effect of biochar on soil Cd bioavailability was mainly governed by the induced changes in soil pH and dissolved organic C, and by the surface area, ash content, H/C and abundance of O-containing functional groups of biochars. We conclude that biochar application to acidic or coarse- and medium-textured soils is effective for remediating Cd contamination, but application to fine-textured soils should be avoided.