Purpose Iron’s fluctuation between the II (ferrous) and III
(ferric) oxidation states has been coined as the “FeIII–FeII
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 FeIII–FeII redox wheel drives the biogeochemical cycles interactively in a fluctuating redox microenvironment. The interactions of the FeIII–FeII redox wheel with the biogeochemical cycles of C, N, S, and P in the fluctuating redox environments were reviewed in this paper.

Discussion 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 FeIII (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, electrontransfer from organic matter to FeIII (hydr)oxides via organic matter oxidation, oxidation of NH4
+ to NO2−, formation and oxidation of Fe sulfide minerals in the S cycle, and P transformation were discussed to couple with the FeIII–FeIIredox wheel.
Conclusions 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
FeIII–FeII redox wheel.