Impacts of exogenous Fe species on C dynamics in peat soils

Carbon dynamics in peatlands are regulated by biogeochemical processes, including heterotrophic respiration, where microorganisms utilize organic carbon (OC) as an electron donor and respire CO₂. In the absence of oxygen, ferric iron (Fe^III) is an important electron acceptor. However, the presence of Fe^III minerals can also modify carbon dynamics by adsorbing OC or occluding OC in microaggregates, thus limiting the mineralization of OC. Along with its speciation and heterogeneity across soils, the overall impact of iron (Fe) on OC mineralization in mineral-rich peatlands remains unclear. To investigate this complex role of Fe and its impact on OC mobilization, we designed model anoxic soil incubations, where multiple Fe species were added to mimic soil heterogeneity. To this end, reactive Fe species (ferrihydrite; Fh, a ferrihydrite-silicate coprecipitate; FhSi in which Si:Fe = 0.05 mol/mol, Fe^III-peat complex; FePeat) and more stable Fe species (goethite; Gt) either in pure forms (Fh, FhSi, and Gt) or mixtures of the two (95/5% Gt/Fh; GtFh, 95/5% Gt/FePeat; GtFePeat) were added to an ombrotrophic peat, increasing the Fe content of the soil from 0.1% to 6% (w/w). The incubations were prepared anoxically in crimp-sealed vials and lasted for 70 days. Two incubation series were established to allow for (1) measurements of the headspace CO₂ concentrations (over 60 days) and (2) sampling of the soil slurry after 4, 17, 35, and 70 days. The latter was used to follow trends in pH, Eh, dissolved organic carbon (DOC) and Fe speciation in the aqueous-phase, and amounts of OC and Fe mobilized from the solid-phase in sequential chemical extractions: 0.5 M HCl (sorbed Fe), hydroxylamine-HCl (short-ranged-ordered Fe oxyhydroxides), and 6 M HCl (crystalline Fe hydroxides).

The results show that the addition of Fe species changes the carbon dynamics. The addition of reactive Fe species (Fh, FhSi) promoted CO₂ production and resulted in higher concentrations of aqueous Fe, suggesting reductive dissolution of the minerals as they served as extra electron acceptors for microbial respiration. In contrast, in the Gt treatment, the goethite addition alone did not affect CO₂ production until the 20^th day, after which CO₂ production was first inhibited and then promoted (after 42 days) compared to a control treatment which received no Fe additions. However, when small fractions (5%) of reactive Fe species were added alongside goethite (GtFh and GtFePeat), CO₂ production was up to 1.5-2.2 times higher than in the Gt treatment. Yet, the lowest DOC concentrations were measured in Fh and FhSi, suggesting that the ferrihydrites re-adsorbed the released OC. Furthermore, while fractions of extractable Fe in Fh and FhSi did not change significantly over the incubation, a strong increase in 6 M HCl extractable Fe in all goethite-containing treatments suggests that, although less reductive dissolution occurred, mineral recrystallization may have occurred.

These results highlight the complex impacts of exogenous Fe species on carbon dynamics and shed light on the vulnerability of peatlands as carbon sinks in the context of climate change, where changes in groundwater geochemistry, including Fe content and watertable fluctuation, may be expected.