Seasonal variation in release characteristics and mechanisms of sediment phosphorus to the overlying water in a free water surface wetland, southwest China

Geochemical cycling of iron (Fe) mediated by sediment microbes drives the remobilization of phosphorus (P). Understanding the underlying mechanism is essential for the evaluation of P retention by wetlands. The diffusive gradients in thin film (DGT) and 16S rDNA sequencing techniques were combined to explore seasonal variations in the remobilization mechanism of sediment P in a free water surface wetland in southwest China. A significantly positive correlation between labile P and Fe concentrations was found from the sediment profiles, indicating coupled remobilization of Fe and P in the sediment. Fe-reducing bacterial genera, particularly Sphingomonas and Geothermobacter, were responsible for the reductive dissolution of Fe oxides and subsequent P release in sediment. The efflux of sediment P was higher in the rainy season (95 +/- 87 ng cm-2 d-1) than in the dry season (39 +/- 29 ng cm-2 d-1). Based on the significantly positive relationship between the efflux and total concentration of sediment P, we propose a promising regression equation for quantifying the release risk of sediment P. The Luoshijiang Wetland exhibited a higher release potential as indicated by a greater regression slope (0.558) compared to the other water bodies (0.055), which was mainly attributed to the lower labile Fe:P molar ratio in the sediment. Based on estimations of the diffusive flux of P at the sediment-water interface, sediment contributed more than 172 and 413 g of P per day to the water column in the dry and rainy seasons, respectively, accounting for 14.0% and 1.9% of the P mass in the surface water of the wetland.

Automated summary

This study investigates the remobilization mechanism of sediment phosphorus (P) in a wetland in southwest China. The results show a coupled remobilization of iron (Fe) and P in the sediment, with Fe-reducing bacteria responsible for the dissolution of Fe oxides and subsequent P release. The study also highlights the high release potential of the wetland, with sediment contributing a significant amount of P to the water column.