Iron plays critical roles in nitrogen retention and removal in soils and sediments

Iron (Fe) and nitrogen (N), prevalent elements in the pedosphere, exhibit distinctly different properties, while a variety of complex interactions exist between Fe and N across ecosystems. On the one hand, some interactions, such as adsorption and coprecipitation, facilitate N retention in the presence of poorly soluble Fe(III) oxides or Fe(III)-bearing clay minerals. On the other hand, Fe participates in N cycling through biochemical processes such as ammonium oxidation coupled with Fe(III) reduction, and nitrate-reducing Fe(II) oxidation. As such, the effects of Fe on N transformations are variable; Fe can promote shifts between “N-rich” and “N-lean” conditions in soils and sediments. Consequently, modifying the availability of Fe can control these transformations. This regulatory mechanism is influenced by factors such as soil pH, structure, substrate availability (Fe, N, and carbon), vegetation type, microbial community composition, and the presence of electron shuttles between N compounds and Fe(III) oxides. Each of these factors can resulting in multiple synergistic effects on FeN coupling, making the prediction of changes in the soil N pool challenging. This review aimed to evaluate the driving mechanisms, significance, and effects of various factors on FeN coupling, thereby providing a basis for understanding geochemical element cycling and informing human intervention.