Transformation of redox-sensitive to redox-stable iron-bound phosphorus in anoxic lake sediments under laboratory conditions.

Phosphorus (P) can be retained in mineral association with ferrous iron (Fe) as vivianite, Fe(II)(PO) ∙ 8 HO, in lake sediments. The mineral is formed and remains stable under anoxic non-sulphidogenic conditions and, therefore, acts as a long-term P sink. In laboratory experiments under anoxic conditions, we investigated whether P adsorbed to amorphous Fe(III)-hydroxide functioned as a precursor phase of vivianite when added to different sediments as a treatment. The untreated sediments served as controls and were naturally Fe-rich (559 µmol/g DW) and Fe-poor (219 µmol/g DW), respectively. The solid P binding forms analysed by sequential extraction and X-ray diffraction were related to coinciding pore water analyses and the bacterial community compositions of the sediments by bacterial 16S rRNA gene amplicon sequencing. In the treatments, within a period of 40 d, 70 % of the redox-sensitive Fe(III)-P was transformed into redox-stable P, which contained vivianite. The mineral was supersaturated in the pore water, but the presence of Fe(III)-P functioning as a precursor was sufficient for measurable vivianite formation. The composition of the microbial community did not differ significantly (PERMANOVA, p = 0.09) between treatment and control of the naturally Fe-rich sediment. In the naturally Fe-poor sediment, the microbial community changed significantly (PERMANOVA, p = 0.001) in response to the addition of Fe(III)-P to the sediment. The freshly formed redox-stable P was not retransferred to a redox-sensitive compound by aeration for 24 h until 90 % O saturation was reached in the sediment slurry. We conclude that 1) Fe(III)-hydroxide bound P, resulting from oxic conditions at the sediment-water interface, is immobilised during anoxic conditions and stable even after re-oxygenation; 2) the process is feasible within the time scales of anoxic lake stratification periods; and 3) in relatively Fe-poor lakes, Fe dosing can provide excess Fe to form the precursor.