Exogenous dissolved organic matter redirects humic carbon fate at mineral interfaces through pre-contact molecular...

Mineral-associated organic carbon (MAOC) formation is commonly interpreted from the moment organic matter contacts mineral surfaces, emphasizing mineral reactivity, molecular functional groups, or adsorption strength. However, in soils receiving continuous organic inputs, humic precursors may first coexist with exogenous dissolved organic matter (DOM) in pore water and thin water films before mineral contact. Whether this upstream liquid-phase interaction reorganizes humic matter and redirects its subsequent mineral-interface fate remains poorly understood. Here, we introduce the concept of a pre-contact molecular conditioning and examine whether biochar-derived dissolved organic matter (BDOM) reorganizes the humic acid (HA) precursor state before contact with goethite (GOE), montmorillonite (MON), and a goethite-montmorillonite composite (MGA). FT-ICR MS spectrometry revealed that mineral-free co-incubation with BDOM generated a distinct HA-BDOM molecular pool rather than a simple addition of the two endmembers, producing 951 newly detected formulas, increasing O/C (17.97%) and nominal oxidation state of carbon (NOSC, 18.46%), and decreasing double-bond equivalent (DBE, 20.26%), modified aromaticity index (AI_mod, 15.76%), and (DBE-O)/C (55.41%). This pre-contact conditioning weakened the bulk affinity of HA for Fe-bearing minerals (53.45% on GOE, 63.12% on MGA). The lower first-contact retention did not imply a weaker contribution to MAOC formation. At the MGA interface, pristine HA underwent extensive molecular reorganization and released numerous transformation products, whereas HA-BDOM retained a larger proportion of its original formulas and preserved a broader molecular mass range. After remobilization, the HA-BDOM-derived pool showed a 44.2% greater capacity to associate with fresh MGA than the HA-derived pool. These results distinguish immediate mineral retention from the longer-term capacity of organic matter to undergo renewed association. The principal effect of BDOM was therefore not simply to alter adsorption strength, but to preserve molecular continuity across successive mineral-contact events. MAOC formation should thus be viewed as a sequential process whose outcome is partly determined by molecular conditioning in solution before mineral contact.