Interactive effects of biochar type and pH on the bioavailability of As and Cd and microbial activities in co-contaminated soils

The differing chemical properties of arsenic (As) and cadmium (Cd) make co-remediation in soil systems a major challenge. This study aimed to gain a mechanistic understanding of how two contrasting biochars (poultry litter and sugar-gum wood) impacted the potential bioavailability of As and Cd in soil matrix varying in initial pH (4.6, 5.7, 6.4, 7.1 and 7.4). The application of poultry-litter biochar significantly increased potential bioavailability of As from 11 to 58 mg kg−1 across all soil pH while the potential bioavailability of Cd was decreased from 15 to 2 mg kg−1 at pH ≤ 6.4. Biochar derived from sugar-gum wood decreased bioavailable As from 11 to 8 mg kg−1 and 14 to 12 mg kg−1 in soils with pH 4.6 and 5.7, respectively, while increased it from 14 to 31 mg kg−1 at pH ≥ 6.4. Biochars transformed the residual fraction of As into the exchangeable fraction but the reverse was true for Cd. Biochar application changed the microbial respiration with contrasting dynamic patterns throughout the 30-d incubation. Soil respiration and microbial biomass C responded more quickly in low-pH than high-pH soils due to low As bioavailability at low pH. These new findings provide strong evidence regarding how contrasting biochars control the mobility of As and Cd over a wide range of soil pH within the same soil matrix.