Fine-scale remobilization of phosphorus by rooted macrophytes (Phragmites australis) growth in lake sediments: evidence from a holistic growth period simulation study

Abstract

Aquatic vegetation serves an important structuring function in shallow freshwater ecosystems. Although increasing evidence indicates that sediment-associated phosphorus (P) is mobilized by aquatic macrophytes under P-deficient conditions, the influence of the holistic growth period of rooted macrophytes on transfer mechanism and bioavailability of sediment P around rhizosphere at millimeter scale remains unclear. In this present study, a 120-day batch intact sediment microcosm simulation was implemented to explore the effect of the whole Phragmites australis growth period on the stability and exchange of sediment P across critical micro-interfaces in lacustrine ecosystems. High-resolution dialysis peeper (HR-Peeper) was used to investigate the variations of pore water P in sediments around the P. australis rhizosphere and Zr-oxide diffusive gradients in thin-film (DGT) sampler was used to capture changes in the two-dimensional (2D) images of labile P over the whole growth period. Phosphorus fractionation showed a general decrease of total phosphorus (TP) and calcium-bound P (Ca-P), whereas an increase in iron-adsorbed P (Fe-P), loosely bound P (LS-P), and organic P (Org-P) was observed on day 120 compared to the values on day 0. Notably, the Ca-P content decreased by approximately 77%, while the Fe-P content increased by approximately 400%. Highly synchronous rises in P release flux and in the morphological characteristics of P. australis were the exponential function of incubation time. High-resolution data demonstrated that concentrations of soluble reactive P (SRP) and labile P concentrations in pore water were prominently enhanced by P. australis growth. Meanwhile, a top-down root-shaped patchy distribution pattern of labile P in the pore water was obviously stimulated over time. The reason for this phenomenon could be ascribed to remobilization of sediment mineral P by root organic exudates, as well as Fe-coupled accumulation of labile P due to oxygenation of Fe2+ followed by the formation of Fe plaques on the root surface within the more oxic rhizosphere. The annual growth period of P. australis could persistently enhance the mobility of sediment P, converting it from a more inert status to a redox-sensitive species. Our findings highlight that remobilization of sediment-associated P by P. australis accounts for a significant portion of the P cycle in eutrophic lake ecosystems.