Chemical fractions, mobility and release kinetics of Cadmium in a light-textured calcareous soil as affected by crop residue biochars and Cd-contamination levels

The aim of this study was to examine the effect of maize straw biochar (MSB), cotton residue biochar (CRB) and wheat straw biochar (WSB) application (1.5% wt.) on Cd immobilisation in a light-textured calcareous soil with varying levels of Cd-contamination. Application of all three crop residue biochars were found to significantly enhance the immobilisation of Cd in the sandy loam calcareous soil. Cotton residue biochar was by far the most effective biochar for reducing the more labile soluble and exchangeable (WsEx-Cd) (−44.4%) and carbonate (Car-Cd) (−44.7%) Cd fractions and increasing the less available residual (Res-Cd) fraction (+82.9%) compared to the maize and wheat straw biochars (WsEx-Cd: −18.8–20.3%; Car-Cd: −27.1–29.0%; Res-Cd: +43.8–44.8%). Application of CRB also significantly reduced EDTA-extractable Cd by 45–52%, and specifically reduced the two-first-order kinetic model Q1 parameter (−55.3%) and increased the Q3 parameter (26.1%). The greater effectiveness of the CRB was associated with its substantially higher C and lower ash content compared to the other crop residue biochars. Results of this study demonstrate the potential use of crop residue biochars, especially cotton residue biochar, in remediation of Cd contaminated calcareous soils.