Carbon and Nitrogen Fractionation in the Formation of Plant-Derived Iron Mineral-Associated Organic Matter

Molecular fractionation during iron mineral-associated organic matter (Fe-MAOM) formation determines the turnover of various organic components and plays a vital part in carbon (C) sequestration. Nitrogen (N) enrichment in Fe-MAOM, a key fractionation outcome, exacerbates the N limitation in soil C sequestration by restricting N bioavailability, particularly for plant-derived water-soluble organic matter (WSOM) with a high C/N ratio. Here, we investigated the C and N fractionation during plant-derived WSOM binding with ferrihydrite (Fh), a poorly crystalline Fe (oxy)hydroxide exhibiting high reactivity toward organic matter. The findings demonstrated that as the molar C/Fe ratio increased, the N enrichment degree of Fe-MAOM first increased and then plateaued. This trend, as observed in molecular-level fractionation and bound organic matter speciation, was attributed to the dominant WSOM binding mechanism shifting from Fh-organic matter (Fh-OM) interactions (high selectivity) to organic matter-organic matter (OM-OM) interactions (lower selectivity) as the C/Fe ratio increased. N enrichment originated from the direct binding of proteins via Fh-OM interactions and the indirect binding of amino acids via OM-OM interactions. While coprecipitation and adsorption processes exhibit similar N enrichment degrees, the former sequesters more N via Fe(III) complexation and precipitation and would further restrict N bioavailability. These findings enhance our understanding of the N limitation in soil C sequestration and contribute to refining the models of coupled C and N biogeochemical cycling.