Effect of soil application of biochar produced from Cd-enriched maize on the available Cd in a calcareous soil

This study aims to investigate the effect of pyrolysis (400 °C) on the stabilization of cadmium (Cd) during soil application of maize (Zea mays L.) biomass obtained during phytoremediation of soils contaminated with different levels of Cd. After pyrolysis, 1.95 g of biochar (Bs) was obtained from every 5 g of raw biomass (Rs). Both groups of Rs (R1–R6: 1–312 mgCd kg−1) and Bs (B1–B6: 2.5–780 mgCd kg−1) were incubated in a calcareous loamy soil for 90 days at 5% wt and 1.95% wt, respectively. Bs application slightly increased soil pH (0.20–0.30 units), whereas a reduction of soil pH was observed in Rs-treated soils (0.22–0.37 units). Both Rs and Bs increased cation exchange capacity (1.12- and 1.24-fold, respectively), total organic carbon (1.72–2.10- and 4.87–5.07-fold, respectively), and microbial biomass carbon (1.43–1.95- and 1.20–1.30-fold, respectively) compared to the control treatment. Dissolved organic carbon was only increased by the application of Rs (38–86%) compared to the control treatment, while Bs did not significantly affect this parameter. To better understand the potential risks of Cd in the environment when applying Rs and Bs, an investigation of its fractions in the soil was conducted. The DTPA-extractable Cd was 65–75% lower after Bs application when compared to Rs. Application of Bs also decreased exchangeable (5-fold), carbonate (1.3-fold), and oxidizable (1.6-fold) fractions of Cd but increased reducible (1.3-fold) and residual (1.7-fold) fractions of Cd when compared to Rs. The results showed that, by producing biochar from contaminated plant biomass and applying it to the soil, it is possible to prevent the redistribution of Cd in the soil so that it can be safe to cultivate plants in that area. In addition, its application improves soil physicochemical properties while increasing carbon sequestration. From environmental and agronomic risk perspective, stabilizing Cd in pyrolyzed maize biomass (biochar) is an improvement over either unmitigated presence of Cd in the soil or untreated maize biomass.