The fate of exogenous organic matter on ferrihydrite: Critical controls of saturation on carbon and nitrogen sequestration.

Mineral-associated organic matter (MAOM) constitutes a substantial reservoir of soil organic carbon (SOC), yet the mechanisms governing the fixation of fresh exogenous organic matter by minerals with varying degrees of organic saturation remain poorly understood. In this study, we synthesized ferrihydrite (Fh)-fulvic acid (FA) complexes via coprecipitation and adsorption pathways at initial C/Fe molar ratios of 0, 1 and 5 to simulate distinct soil MAOM precursors. Water-soluble organic matter (WSOM) derived from corn residues was subsequently introduced to represent fresh organic inputs. Elemental analysis, excitation-emission matrix (EEM) fluorescence spectroscopy with parallel factor analysis (PARAFAC), and two-dimensional correlation spectroscopy (2D-FTIR-COS) were used to quantify carbon/nitrogen fixation behavior and elucidate molecular binding mechanisms, which demonstrated that the carbon sequestration efficiency and molecular selectivity of Fh were strictly regulated by its saturation state and formation pathway. Our results demonstrate that the sequestration capacity for fresh WSOM is strictly governed by the pre-existing mineral saturation and formation pathways, leading to divergent fates for carbon (C) and nitrogen (N). Specifically, coprecipitates with partially saturated ferrihydrite surface binding sites (initial C/Fe = 1) exhibited a strong capacity to sequester fresh WSOM, retaining up to 214.2 mg C/g Fe, whereas this capacity dropped to 110.4 mg C/g Fe in highly saturated coprecipitates (initial C/Fe = 5). Furthermore, highly saturated adsorption complexes exhibited almost no additional net C fixation from WSOM. In contrast to C, N retention remained robust across all saturation levels. Spectroscopic analyses revealed that the limitation on C sequestration from WSOM is driven by two underlying mechanisms: first, Fh already coated with FA exhibits a changed surface charge, thus electrostatically repelling fresh inputs; and second, in saturated systems, weaker "OM-OM" interactions allow high-affinity WSOM fractions (e.g., polysaccharides) to competitively displace pre-adsorbed FA. In contrast, the sustained N accumulation is attributed to the specific affinity of N-rich components in WSOM, which maintain strong binding interactions that are relatively independent of the mineral's saturation state. Our research has confirmed the critical requirements of insufficient mineral saturation and formation pathways for the soil carbon pool, providing a theoretical basis for a more comprehensive assessment of the potential of soil carbon sinks and management strategies.