Biochar-regulated transport of weakly hydrophobic antibiotics between macropore and matrix domains in structured soil

Abstract Biochar amendments show promise for mitigating accelerated antibiotic transport caused by macropore flow in soil, yet their effectiveness in such systems is poorly understood, lacking direct evidence differentiating biochar's role across macropore flow versus soil matrix infiltration. Using a novel macropore and matrix domains (dual-domain) separation apparatus, this study quantified the effect of biochar addition on the transport of weakly hydrophobic antibiotics, sulfadiazine (SDZ) and florfenicol (FFC), in soils under hydraulically isolated and connected domain conditions. Results suggested that biochar's efficacy is significantly amplified when hydraulic connectivity between the dual domains is present. In this state, it actively diverts antibiotics from macropore flow into the soil matrix infiltration, significantly reducing the total cumulative mass fluxes (CMFs) of SDZ from 0.72 ± 0.01 to 0.61 ± 0.00 and FFC from 0.81 ± 0.04 to 0.72 ± 0.02 ( p < 0.05). Partial least squares structural equation modeling (PLS-SEM) subsequently revealed that biochar rewires the system’s causal pathways. The model showed that it leverages mobile carriers (dissolved organic matter and colloids) to create a powerful immobilizing sink in the matrix, while counteracting the advective flux traced by Br – . These findings support the new hypothesis that biochar functions as a dynamic “biochar sorption pump” (BSP). This framework reframes biochar from a passive sink to an active flux regulator, providing a basis for designing precision remediation strategies based on soil hydraulic properties to protect vulnerable aquatic ecosystems. Graphical Abstract