Reactive organic matter destabilizes Fe-mediated phosphorus retention through Fe-S-P decoupling in lake sediments

Despite reductions in external inputs, internal phosphorus (P) release from sediments remains a key barrier to lake recovery. Conventional mineral-centered frameworks inadequately account for the fundamental role of reactive organic matter in driving redox-mediated shifts in sediment P stability. We analyzed a 180-year sediment record from a subtropical plateau lake and quantified a critical threshold that controls phosphorus stability. When the ratio of reactive dissolved organic carbon (rDOC) to reactive Fe exceeds 8.04 (95% CI: 4.42∼10.53), abrupt destabilization of the Fe-mediated P sink is observed, indicating that electron donor supply has exceeded the Fe(III) buffering capacity of the sediment. Below this threshold, the geochemical and isotopic record is consistent with dissimilatory Fe reduction as the predominant anaerobic terminal electron-accepting process, sustaining a redox-sensitive Fe-bound P sink. Above this threshold, DGT profiles, stable isotope records (δ¹⁵N, δ¹³C), and SEM-EDS data collectively indicate progressive sulfide accumulation and geochemical conditions consistent with competition between sulfide and phosphate for dissolved Fe²⁺ at the sediment-water interface, suggesting weakening of Fe-mediated P retention. This transition coincides with a shift in organic matter sources from terrestrial to sewage- and algae-derived inputs, as recorded by rising δ¹⁵N and shifting δ¹³C values. Anthropogenic nitrogen enrichment (δ¹⁵N > 2.5‰) further amplifies this process. These findings identify the rDOC:reactive Fe ratio as a quantitative early-warning indicator for Fe-mediated P sink failure, and demonstrate that reactive organic matter loading history, rather than mineral composition alone, is a critical determinant of sediment P stability in anthropogenically stressed lakes.