Phosphorus removal from sediments by Potamogeton crispus:New high-resolution in-situ evidence for rhizosphere assimilation and oxidization-induced retention

Macrophytes are usually chosen for phytoremediation tools to remove P in eutrophic aquatic ecosystems, but the lack of test methods hinders the understanding of removal mechanism and application. In this study, we used the novel technologies combined of Diffusive gradients in thin films(DGT), Planar optode(PO), and Non-invasive micro-test technology(NMT) to explore P dynamics in water-sediment continuum and rhizosphere of Potamogeton crispus over time. Results of the high-resolution in situ measurement showed that labile P(LPDGT) fluxes at the surficial sediment significantly decreased from approximate 120, 140,and 200 pg/(cm~2·sec) via 30 days incubation period to 17, 40, and 56 pg/(cm~2·sec) via that of 15 days. Obvious synchronous increase of LPDGTwas not detected in overlying water, suggesting the intense assimilation of dissolve reactive P via root over time. PO measurement indicated that Oconcentration around the rhizosphere remarkably increased and radially diffused into deeper sediment until 100% saturation along with the root stretch downwards.NMT detection of roots showed the obvious Oinflow into root tissue with the uppermost flux of 30 pmol/(cm~2·sec) from surroundings via aerenchyma on different treatment conditions. Different from previous reports, gradually saturating Oconcentrations around the rhizosphere was principally driven by Openetration through interspace attributing to root stretch downward rather than root Oleakage. Increased Oconcentrations in deep sediment over time finally induced the oxidization of labile Fe(Ⅱ) into Fe(Ⅲ) bound P and local P immobilization.