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 CO₂ and CH₄. 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.

Keywords: Abisko; Arctic; bioavailability; iron; microbial Fe(III) reduction and Fe(II) oxidation; permafrost collapse; seasonal fluctuations; soil organic carbon.

Figure 1

Seasonal fluctuations in weather and soil conditions for Abisko and Stordalen mire in the year 2019. (A) Air temperature [°C] and (B) precipitation [mm] were monitored by the Abisko Observatory. (C) Average soil temperature at Stordalen mire (average of the three thaw stages palsa, bog, and fen) at 2, 5, 10, 30, and 50 cm depth and (D) volumetric soil water content [%] in the upper 6 cm from the soil surface in palsa and fen were monitored by Integrated Carbon Observation System (ICOS) Sweden Abisko—Stordalen. For thaw stage-specific soil temperatures, see Figure S6. Early summer (yellow background) marks the time period when the short-term ferrihydrite (FH) bags were deployed for 2 weeks. “Late summer” bags (red background) were deployed at the same time as the early summer bags but remained in the soil for 2 months. The white arrow marks start of short- and long-term incubations. The light gray arrow marks the end of short-term deployment (only capturing early summer), and the dark gray arrow marks the end of the long-term deployment (deployed from early to late summer).

Figure 2

Gain and loss of solid-phase iron (Fe) along a thaw gradient in early (2 week incubation) and until late summer (2 month incubation). (A) Gain and loss in poorly crystalline Fe(III) (0.5 M HCl extractable) and more crystalline Fe(III) (6 M HCl extractable). Values are normalized to the reference material (unexposed ferrihydrite (FH)-coated sand with 2.19 ± 0.26 mg total Fe per g sand), which was transported to the field but then stored at room temperature until the end of the experiment. The reference material included a more crystalline Fe phase (1.01 ± 0.14 mg only 6 M HCl extractable per g sand), probably due to aging over time. Positive values indicate a net gain in Fe, and negative values indicate a net loss in Fe in comparison to the reference material. (B) Adsorbed/amorphous Fe(II) (1 M Na-acetate extractable). No Fe(III) was detected in the 1 M Na-acetate extracts. Reported values are the average of triplicate analysis, normalized to the reference material, of sand homogenized from all bags deployed at each thaw stage (palsa, bog, and fen). Error bars are the combined standard deviation of the triplicate analysis. Nine bags per thaw stage were combined from the early summer collection, and three bags per thaw stage were combined for the late summer collection.

Figure 3

Organic carbon (OC) associated with iron (Fe) mineral phases along the thaw gradient following 2 week (early summer collection) and 2 month incubation in soil (late summer collection). Reported values represent the total OC control-corrected by subtracting loosely bound OC (sodium pyrophosphate extractable OC). Error bars represent the combined standard deviation of total OC and sodium pyrophosphate extractable OC.

Figure 4

EDS-derived chemical distribution maps of iron (Fe)–organic carbon (OC) associations on ferrihydrite (FH)-coated sand grains incubated in the partially thawed bog (A) and in fully thawed fen (B) after 2 month incubation (late summer collection) in comparison to the reference material (unexposed FH-coated sand) (C). Results shown are representatives, and replicate analysis is reported in the Supporting Information (Figures S11 and S12). The orange color of the unexposed sand appears brighter than that of the samples retrieved from the bog and fen because of the co-occurrence with the purple color indicative of carbon.