Dual roles of crayfish shell biochar on the fate of arsenic in acid and alkaline aerobic soils: insights from dissolved organic matter and metabolism genes

Arsenic (As) contamination poses a significant global environmental and health concern. Crayfish shell biochar (CSBC) has shown considerable potential for remediating As-contaminated soil. However, the effect of CSBC on As mobility in acid and alkaline aerobic soils, and its underlying mechanism remains unclear. This study investigated the variation in As availability, dissolved organic matter (DOM), microbial community, and As metabolism genes in CSBC-amended soil. Results indicated that CSBC application decreased the available As content in alkaline soil (4.5–9.9%), but increased it in acid soil (19.6–26.8%). The increase in soil pH and DOM humification degree, along with the decrease in DOM molecular weight, contributed to enhanced As mobility in CSBC-amended acid soil. Additionally, CSBC application stimulated microbial membrane transport and signal transport function in acid soil. Furthermore, it increased the relative abundance of Bradyrhizobium, Sphingomonas, Streptomyces, and Ktedonobacter, which harbor As(V) reduction (arsC and arrB) and As(III) efflux (ACR3) genes, thereby promoting As mobility in acid soil. In alkaline soil, the CSBC provided additional bonding sites for As to mitigate its environmental risk. This study provides new insights into the critical role of biochar in influencing As fate in aerobic soil.