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 pyrolytic temperature and feedstock type 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) pyrolytic 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), 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 altered the bacterial community composition, with increased bacterial richness and diversity observed in the HPT-BDOM compared to the control. 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 with sheep bone-BDOM. Manure-LPT and sheep bone-HPT enhanced hydrocarbon degradation, while rice husk-LPT enriched taxa related to nitrogen fixation and nitrate reduction. LPT treatments favored cellulolysis and fermentation, whereas HPT treatments promoted methylotrophy, aligning with their contrasting carbon lability. These findings highlight the dual role of biochar's labile fraction in shaping carbon availability, influencing SDOM dynamics, nutrient and total metals (TMs) bioavailability, and microbial ecology, underscoring the need for feedstock- and temperature-specific selection in environmental applications.