Effects of the increases in soil pH and pH buffering capacity induced by crop residue biochars on available Cd contents in acidic paddy soils

To explore the effects of the increases in pH and pH buffering capacity (pHBC) induced by crop residue biochars on the changes in soil available Cd content, six acidic paddy soils developed from different parents were amended with seeded sunflower plate biochar (SSPBC), peanut straw biochar (PSBC) and corn straw biochar (CSBC). The pH, pHBC, and available Cd of the soils were measured after laboratory incubation. The results showed that the incorporation of crop residue biochars led to the increases in soil pH and pHBC, but a decrease in soil available Cd content. The decreasing order of available Cd content was SSPBC > PSBC > CSBC and was consistent with the changes in soil pH induced by the biochars. During submerging and draining, soil pH increased first and then declined, however the content of available Cd decreased first and then increased significantly. Soil pH in the treatments with biochars showed little change during draining, which was different from the control without the biochars added. This was attributed to the enhancing effect of the biochars on soil pHBC. Also, there was a significant negative correlation between the change in available Cd content and soil pHBC during submerging/draining alternation and suggested that higher pHBC corresponded to smaller soil available Cd content. Consequently, the amount of Cd absorbed by rice was reduced, thereby reducing the potential risk of soil Cd to humans. These results can provide useful references for the remediation of Cd-contaminated paddy soils.