Short-term phosphorus transformation in shallow-lake sediments mediated by nitrogen and iron under anoxic conditions

The release of internal phosphorus (P) is a key and complex process relative to a lake’s nutrient levels. The P response to nitrogen input should be clarified to obtain better detail, especially with respect to the mediation role of iron (Fe) connecting nitrogen and P. A simulation study was done in batch vials containing sediment and overlying water collected from Lake Moshui—a shallow lake. Because of nitrate (${\text{NO}}_3^{-}$) input, the abundances of Firmicutes and Proteobacteria at the phylum level increased, as did the abundances of Crenothrix, Sideroxydans, and Flavobacterium at the genus level. Moreover, nitrate input enhanced the proliferation of nitrate-reducing Fe(II) oxidization bacteria and the activity of denitrifying enzyme in sediment, but decreased the concentrations of total phosphorus (TP), soluble reactive P, and enzymatically hydrolyzable P in the water phase. The suppressive effect of nitrate on P release was mainly attributed to the increase in Fe(III) (hydr)oxides generated by Fe(II) oxidation. With sequential extraction, two types of dominant iron fractions—easily reducible oxides and reducible oxides (Fe_ox1 and Fe_ox2, respectively)—seemed to contribute to the fixation of P largely in the sediment. In addition, more P is converted to Fe_ox2, which is induced by the transformation of Fe_ox1 to Fe_ox2. Driven by nitrate, a higher content of P bound to Fe_ox1 and Fe_ox2 was observed after a 7-day incubation, indicating the potential function of the Fe(II) oxidation process. The current study revealed that the P transformation is influenced by nitrate input from the angle of iron as the connecting bridge to better understand the geochemical cycle of P in the anoxic environment of lakes.