Seasonal Fluctuations in Iron Cycling in Thawing Permafrost Peatlands

Abstract

In permafrost peatlands, up to 20% of total organic carbon (OC) is bound to reactive iron (Fe) minerals in the active layer overlying intact permafrost, potentially protecting OC from microbial degradation and transformation into greenhouse gases (GHG) such as CO 2 and CH 4 . During the summer, shifts in runoff and soil moisture influence redox conditions and therefore the balance of Fe oxidation and reduction. Whether reactive iron minerals could act as a stable sink for carbon or whether they are continuously dissolved and reprecipitated during redox shifts remains unknown. We deployed bags of synthetic ferrihydrite (FH)-coated sand in the active layer along a permafrost thaw gradient in Stordalen mire (Abisko, Sweden) over the summer (June to September) to capture changes in redox conditions and quantify the formation and dissolution of reactive Fe(III) (oxyhydr)oxides. We found that the bags accumulated Fe(III) under constant oxic conditions in areas overlying intact permafrost over the full summer season. In contrast, in fully thawed areas, conditions were continuously anoxic, and by late summer, 50.4 ± 12.8% of the original Fe(III) (oxyhydr)oxides were lost via dissolution. Periodic redox shifts (from 0 to +300 mV) were observed over the summer season in the partially thawed areas. This resulted in the dissolution and loss of 47.2 ± 20.3% of initial Fe(III) (oxyhydr)oxides when conditions are wetter and more reduced, and new formation of Fe(III) minerals (33.7 ± 8.6% gain in comparison to initial Fe) in the late summer under more dry and oxic conditions, which also led to the sequestration of Fe-bound organic carbon. Our data suggest that there is seasonal turnover of iron minerals in partially thawed permafrost peatlands, but that a fraction of the Fe pool remains stable even under continuously anoxic conditions.

Iron (hydr)oxide formation in Andosols under extreme climate conditions

“…Furthermore, an iron redox cycling of this type requires a highly dynamic reciprocal action between dissolved species, microbial biomass, OL and RAMPs, resulting in a mechanism with a frequent exchange of electrons, which was previously called a biogeobattery 23 . Drastic changes in the redox conditions (i.e., different oxygen penetration depths) due to waterlevel uctuations should strongly reinforce this biogeochemical cycling 13,34 .…”

“…Here, the proportion of decomposed OM, which is also controlled by water-table uctuations 34,35 , plays an important role due to its various functional groups (e.g., hydroxylic, carboxylic and phenolic acids 11 ). Considering the relationship between changing Al o /TOC ratios with lower δ 13 C and higher δ 15 N values (Fig. 1b), we assume that Al-humus complexation is a function of (i) Al 3+ -supersaturation and (ii) the presence of a large pool of labile OM in soil solutions 20 .…”

“…1, 5-6), which predominantly precipitate during periods of higher rainfall (RAMP production in Fig. 6a) when oxidization to phases with higher crystallinity is inhibited 9,13,45,46 .…”

“…Recent studies highlight that redox (trans)formations of iron in soils are controlled by a complex cascade of abiotic and biotic processes 2,6,11 . The reduction and oxidation of iron by microorganisms should be inseparably linked to the amount of organic ligands (OL) and DOC released from intense OM turnover 12,13,35,40 as well as to the cycling of speci c nutrients (especially N, P and S) 10,43 . This suggests an e cient interaction of the iron redox cycling with the simultaneously formed Al-humus complexes and the dominant oxyanions adsorbed, e.g., NO x − , PO 4 3− , SO 4 2− .…”

“…The micronutrient iron is critical to all living organisms and colloidal secondary Fe-minerals mutually interact with OM as either important electron acceptors or donors in microbially-mediated redox reactions 6 . Hence, Fe-(hydr)oxides play an important role in carbon sequestration and in biogeochemical cycles by the xation or mobilization of iron and other essential bio-available elements for, e.g., the nutrient status of terrestrial ecosystems 2,12,13 or the regulation of marine primary productivity 14 . Apart from glacial meltwater plumes, the peatland-sourced riverine element transport by Fe-and OM-rich colloids represents a key mechanism for the terrigenous nutrient supply in coastal regions and fjords of mid-and high-latitudes 15,16,17 .…”