Biochar-dissolved organic matter (DOM) retention is maximized under low-intensity rainfall, particularly in montmorillonite-rich soils

Biochar holds significant promises for carbon sequestration and environmental remediation. However, biochar application and aging releases dissolved organic matter (DOM), a highly reactive and mobile fraction whose migration through soil during rainfall remains poorly understood. We investigated vertical transport of DOM released from pristine and aged corn biochar with simulated soil columns (15cm inner diameter/60cm length) with hematite/quartz or montmorillonite/quartz mixtures (3:7=w:w) under low-intensity and high-intensity rainfall. Intensities were 177/880 ml/d with 30 ml/min flow rate for 6/30 min according to local mean annual precipitation. 100-mesh-nylon membranes and 5-cm-fine glass-bead layers at both column ends ensured uniform water flows. At every 4-days/rainfall event, biochar-derived DOM properties (stored 4^oC at dark) in column effluents were measured at 1st/3rd/5th/7th/10th/15th/20th events after 0.45mm nylon membrane filtration. Biochar migrations in collected soil solids were observed at 2/6/11/21/31cm column depths. Results revealed that montmorillonite powerfully inhibited DOM migration, reducing it by over 80% compared to pure quartz and by 50% compared to hematite/quartz. This clay mineral preferentially adsorbed humic-like DOM substances over polycyclic aromatic-like ones. A key finding involved rainfall pattern: under the same total rainfall, DOM concentration gradually increased with successive low-intensity events but dropped sharply under high-intensity rainfall. The higher concentrations from low-intensity rain promote DOM sorption onto minerals. Therefore, we conclude that low-intensity rainfall facilitates DOM retention in soil, particularly in montmorillonite-rich environments. This insight is crucial for optimizing biochar application strategies—suggesting that both local soil mineralogy and climate patterns should be considered to enhance long-term carbon stability and soil remediation effectiveness.