Dissolved organic matter drives fraction-specific mobilization and transformation of redox-sensitive constituents in groundwater.

The anomalous mobilization of redox-sensitive elements (Fe, Mn, N, As, and P) in riparian groundwater is frequently associated with dissolved organic matter (DOM) influx; however, the contrasting roles of labile protein-like and recalcitrant humic-like DOM remain poorly resolved. Here, we employed tyrosine (Tyr) and humic acid (HA) as model compounds for labile, protein-like DOM and recalcitrant, humic-like DOM, respectively, and conducted gradient incubation experiments (0, 20, and 100 mg/L) under both sterile and non-sterile conditions. By integrating hydrochemical time-series, solid-phase speciation, fluorescence parallel-factor analysis, amplicon sequencing, and metagenomic functional profiling, we distinguished the biotic and abiotic drivers governing elemental coupling. Our results reveal that DOM substantially altered redox-driven elemental mobilization, with effects that were strongly dependent on DOM type, concentration, and incubation stage. In biotic treatments, Tyr profoundly stimulated microbial loops, enriching Fe/Mn-reducing and N-cycling taxa (relative abundances of 32.25% and 26.13% at 20 mg/L, versus 11.70% in controls) and upregulating genes involved in dissimilatory nitrate reduction and extracellular electron transfer. Notably, low-concentration Tyr (20 mg/L) favored Mn release, whereas high-concentration Tyr (100 mg/L) promoted late-stage Fe oxide dissolution and concomitant P liberation. In stark contrast, HA exerted a dualistic influence: although it weakly stimulated canonical metal reducers and even suppressed gene abundance at elevated loads, it facilitated Fe, As, and P migration via abiotic pathways-electron shuttling, ligand complexation, and competitive adsorption-as evidenced by sustained release in sterilized systems. Intriguingly, the decoupling of As from Fe after mid-term release suggests rapid re-adsorption or secondary mineral sequestration, complicating risk prognostics. Collectively, this work not only advances a conceptual framework for multi-element coupling at the DOM-mineral-microbe triple interface, but also provides critical insights for predicting groundwater quality trajectories under shifting DOM input regimes.