Stage-dependent counteracting effects of biochar on arsenic mobilization and immobilization in paddy soil

Biochar amendment has been widely reported to influence arsenic (As) mobility in flooded paddy soils. While previous studies have mainly attributed biochar-induced As behavior to feedstock, pyrolysis temperature and modification methods, how As dynamics evolve across different incubation stages remains poorly understood. In this study, four types of biochar were applied to flooded soil columns to investigate their effects on As risk. Results showed a potential stage-dependent counteracting effect: biochar may promote As mobilization during the first two weeks, while subsequently facilitated As immobilization (days 14-30), and subsequently re-mobilized As (days 30-90). In the early stage, biochar induced an alkaline, reducing, and DOC-enriched environment, which may have contributed to As mobilization in association with As(V) and Fe(III) reduction. Conversely, after two weeks, high-molecular-weight DOC (humic substances) was likely associated with As immobilization through complexation with Fe. Biochar addition enhanced nitrate- and sulfate-reducing microbial functions, which may have contributed to As mobilization through the dissolution of Fe (hydro)oxides. Although this enhancement was more pronounced during the early incubation stage, microbial processes may have remained important after day 30. Furthermore, biochar samples with higher ash content, pH, and H/C ratios showed stronger associations with both initial mobilization and subsequent immobilization. In contrast, biochar samples with higher BIX and FI indices showed stronger associations with microbial community variation, and correspondingly exhibited the greatest re-increase in aqueous As concentrations after day 30. This study highlights the importance of temporal dynamics in biochar application in flooded As-contaminated soil.