Are the biogeochemical cycles of carbon, nitrogen, sulfur, and phosphorus driven by the FeIII-FeII redox wheel in dynamic redox environments?

Iron's fluctuation between the II (ferrous) and III (ferric) oxidation states has been coined as the Fe-III-Fe-II redox wheel. Numerous studies have coupled the iron redox wheel with the biogeochemical cycle of carbon (C), nitrogen (N), sulfur (S), or phosphorus (P) individually in soils or sediments, but evidence suggests that the Fe-III-Fe-II redox wheel drives the biogeochemical cycles interactively in a fluctuating redox microenvironment. The interactions of the Fe-III-Fe-II redox wheel with the biogeochemical cycles of C, N, S, and P in the fluctuating redox environments were reviewed in this paper. In this review, we discuss the importance of iron with regard to each of the biogeochemical cycles individually as well as interactively. The importance of crystalline and non-crystalline Fe-III (hydr)oxides is highlighted as they serve as terminal electron acceptors for organic matter mineralization and N and S transformation and also act as sorbents for dissolved P compounds. Mechanically, electron transfer from organic matter to Fe-III (hydr)oxides via organic matter oxidation, oxidation of NH (4) (+) to NO (2) (-) , formation and oxidation of Fe sulfide minerals in the S cycle, and P transformation were discussed to couple with the Fe-III-Fe-II redox wheel. The knowledge gaps are identified at the end of the review. The natural environmental relationships still require further studies that link the iron redox wheel as a driver of the biogeochemical cycles of C, N, S, and P. Anthropogenically altered environments (nutrient and metal elevation, global warming, and acidification) require intensive studies to allow for improved integrated modeling of global C, N, S, and P biogeochemical cycles driven by the Fe-III-Fe-II redox wheel.