The importance of iron redox reactions and mineral transformations for the fate of phosphorus in the environment

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Phosphorus (P) is an essential but limiting micronutrient for all living organisms and can significantly affect global biogeochemical cycles, ultimately regulating global primary productivity. Environmental P cycle is known to strongly couple with Fe redox cycling, since Fe minerals are abundant in nature, have high surface areas, and have strong P adsorption capacity. Fe(III) oxide-bound P is reported to account for about 9% of P sequestration from water columns of lakes and up to 70.4% in lake sediments and soils. Fe(III) oxides are readily subject to microbial or abiotical reductive dissolution under anoxic conditions, leading to P mobilization or the formation of the Fe(II)-phosphate mineral vivianite. Aqueous and solid Fe(II) species can undergo microbial and abiotic oxidation under both oxic and anoxic conditions in natural environments, resulting in P retention by newly formed Fe(III) oxide minerals. However, an in-depth understanding of how Fe redox cycling affects the transformation of Fe phases and the subsequent fate (mobilization vs retention) of phosphorous is still lacking. The key aim of this proposal is to understand the critical roles of Fe redox reactions and mineral (trans)formations for P cycling in complex environmental settings. Specifically, we will (1) follow the (trans)formation of abiogenic and biogenic Fe(III) oxide minerals during microbial reduction in the presence of P and determine its effect on the fate of P; (2) determine the (trans)formation of vivianite during microbial vs abiotic oxidation and its effect on the fate of P; (3) determine the (trans)formation of Fe(III) phosphate minerals in the co-presence of Fe(III)- and sulphate-reducing bacteria and its effect on the fate of P; (4) determine the (trans)formation of Fe minerals under cyclic redox conditions and its effect on the speciation and fate of P in redox transitions of natural environments. The expected results will deepen our current knowledge on the coupled Fe and P cycles in natural environments and will help to comprehensively understand global P cycling on Earth.