Phosphorus-mediated decoupling of Fe-Si cycling under variable oxygenation: implications for dissolved silicon release from natural sediments

Abstract

The availability of dissolved silicon (DSi) relative to dissolved phosphorus (DP) regulates the dominance of siliceous phytoplankton in eutrophic waters, yet the redox controls on DSi internal loading from natural sediments remain uncertain. We incubated surficial sediments in ten parallel flow-through columns to disentangle Fe-P-Si interactions under alternating oxic and anoxic overlying waters. Columns containing 10 cm of sediment received anoxic influents with different combinations of Fe(II), DSi, and DP delivered through the bottom port using a peristaltic pump, while the effluent were monitored; post-experiment sediments were characterized using buffered ascorbate-citrate (BAC) and 1 mol/L NaOH extractions. Under oxic conditions, more than 90% of inflow Fe(II) was retained in the upper 1 cm of sediment, enriching BAC-extractable Fe and P by up to 40% relative to initial values. Transition to anoxia caused reductive dissolution of these phases, leading to Fe release of up to 193 ± 26 µmol/L and DP peaks of 160 ± 30 µmol/L in the overlying water. In contrast, DSi exhibited neither measurable retention during oxia nor systematic increases under anoxia, with effluent concentrations consistently exceeding influent levels by 20%–50%, indicating steady internal production. Mass-balance calculations show that DSi fluxes (20–40 µmol/d) were dominated by continuous dissolution of amorphous silica, while elevated DP suppressed Fe-Si co-precipitation via competitive sorption on Fe(III) solids. We conclude that the interplay among porewater DP and DSi production rates, along with the pool of reactive iron, governs the extent to which internal Si release responds to redox variations.

Graphical abstract

Keywords

Internal silicon loading / Iron-phosphorus-silicon coupling / Oxygen conditions / Flow-through column experiment / Eutrophication control

Highlight

	● Flow-through columns revealed redox effects on Fe-P-Si cycling in sediments.
	● Oxic Fe(II) oxidation retained phosphorus but not dissolved silicon.
	● Elevated P suppressed Fe-Si interactions by competitive sorption and precipitation.
	● Amorphous silica dissolution sustains steady DSi efflux independent of redox shifts.
	● Redox-driven DSi:DP shifts regulate phytoplankton composition in eutrophic systems.

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