Dissolved organic matter quality, hydrological connectivity and microbial activity shape phosphorus buffering in river-floodplain systems.

High loads of nutrients like phosphorus (P) persistently degrade water quality in floodplain waterbodies and cause eutrophication, i.e., elevated algae production. Despite continuous efforts to curb external nutrient loads entering freshwaters, internal P release from sediments frequently sustains eutrophication. Benthic microbial communities considerably impact nutrient turnover through the mineralisation of organic matter and may strongly affect P retention and release, which can be estimated via the P buffering potential. However, how these microbial processes shape the P buffering potential in hydrologically dynamic systems like floodplains is poorly understood. Here, we assessed the coupled effects of lateral hydrological connectivity, dissolved organic matter (DOM) quality and benthic microbial activity on the P buffering potential in a river-floodplain system. We examined seven sites in shallow floodplain waterbodies that were either connected or disconnected from the River Elbe (Magdeburg, Germany) after a summer contraction phase. Our findings show a significantly higher P buffering potential and thus, likely higher P release risk within connected sites, corroborated by distinct microbial community-level physiological profiles. The P buffering potential was positively correlated with more labile, low molecular weight DOM, increased autochthonous contribution, and a pronounced enzymatic degradation of hemicellulose (β-xylosidase activity). Our study underscores the pivotal role of hydrological connectivity, DOM quality and microbial enzyme activity in shaping sediment P buffering and potential P release in river-floodplain systems. Moreover, our results demonstrate the importance of critical yet often overlooked benthic sediment processes and microbe-organic matter interactions for P dynamics in floodplain waterbodies.