Divergent stability of dissolved organic matter-iron-phosphorus (DOM-Fe-P) complexes in macrophyte-versus...

Endogenous phosphorus (P) release is a key driver of eutrophication, largely regulated by the stability of dissolved organic matter-iron-phosphorus (DOM-Fe-P) complexes. However, conventional citrate-bicarbonate-dithionite (CBD) extraction fails to discern DOM's role in stabilizing DOM-Fe-P under natural conditions, as it indiscriminately extracts both active and stable iron oxyhydroxide pools. Here, we employed bicarbonate-dithionite (BD) extraction to separate these pools in algae-dominated (ADL) and macrophyte-dominated lakes (MDL), integrating microbial community analysis to unravel DOM-driven stabilization mechanisms. DOM-Fe-P in ADL exhibited higher P/Fe molar ratios but lower organic-carbon/Fe (OC/Fe) molar ratios than MDL, aligning with the divergent DOM degradability, carbon-phosphorus stoichiometry, and microbial assemblages. In ADL, humic-like fulvic components in the active pool likely served as electron shuttles, increasing DOM-Fe-P redox susceptibility. Conversely, MDL showed enhanced stability due to sulfide-Fe competition and the stabilizing effect of polyphenols on Fe. Microbial data revealed that algal DOM fostered diverse communities promoting iron reduction and DOM degradation, while macrophyte DOM intensified microbial cooperation and sulfur cycling. These findings highlight organic matter sources as critical controllers of DOM-Fe-P stability, offering a mechanistic basis for managing endogenous P release in eutrophic lakes.