Biochar-derived dissolved organic matter induced changes in the bacterial communities structure and metabolic functions in As and Cd contaminated soil.

This study investigates the effects of pyrolysis temperature and feedstock type for biochar production on the release of biochar-derived dissolved organic matter (BDOM) and its impact on the soil bacterial community and the composition of soil dissolved organic matter (SDOM). The BDOM was extracted from biochars produced from sheep bones, rice husk, and rabbit manure, prepared at low (400 °C, LPT) or high (700 °C, HPT) pyrolysis temperatures. The BDOM was then applied at a concentration of 2.5 % (w/w). LPT-BDOM produced higher contents of BDOM (up to 1440 ± 43 mg kg-1), resulting in higher SDOM (up to 78 %, compared with control soil) after application and higher availability of nutrients (Ca, Mg, and P) and toxic metalloids (TMs; As and Cd) in soil. The addition of BDOM to soil altered the bacterial community composition, with increased bacterial richness and diversity observed in the soil with HPT-BDOM compared to the control soil. The community shift was linked with higher levels of volatile organic compounds and increased nutrient availability compared with HPT. The increase in fluorescence (up to 54 %), freshness (up to 29 %), biological (up to 112 %), and humification (up to 52 %) indices was associated with LPT-derived BDOM, particularly produced from sheep bones. Manure-LPT and sheep bone-HPT enhanced hydrocarbon degradation, while rice husk-LPT enriched bacterial taxa related to nitrogen fixation and nitrate reduction. LPT biochar addition favored cellulolysis and fermentation, whereas HPT biochars stimulated methylotrophy, aligning with their contrasting carbon lability. These findings highlight the dual role of biochar labile fraction in shaping carbon availability, influencing SDOM dynamics, nutrient and heavy metals bioavailability, underscoring the need for feedstock- and temperature-specific selection in environmental applications.