It has been shown that reactive soil minerals, specifically iron(III) (oxyhydr)oxides, can trap organic carbon in soils overlying intact permafrost, and may limit carbon mobilization and degradation as it is observed in other environments. However, the use of iron(III)-bearing minerals as terminal electron acceptors in permafrost environments, and thus their stability and capacity to prevent carbon mobilization during permafrost thaw, is poorly understood. We have followed the dynamic interactions between iron and carbon using a space-for-time approach across a thaw gradient in Abisko (Sweden), where wetlands are expanding rapidly due to permafrost thaw. We show through bulk (selective extractions, EXAFS) and nanoscale analysis (correlative SEM and nanoSIMS) that organic carbon is bound to reactive Fe primarily in the transition between organic and mineral horizons in palsa underlain by intact permafrost (41.8 ± 10.8 mg carbon per g soil, 9.9 to 14.8% of total soil organic carbon). During permafrost thaw, water-logging and O2 limitation lead to reducing conditions and an increase in abundance of Fe(III)-reducing bacteria which favor mineral dissolution and drive mobilization of both iron and carbon along the thaw gradient. By providing a terminal electron acceptor, this rusty carbon sink is effectively destroyed along the thaw gradient and cannot prevent carbon release with thaw.
Reductive dissolution during permafrost thaw releases iron-bound organic carbon to porewaters, rendering previously stable carbon vulnerable to microbial decomposition and subsequent release to the atmosphere. How mineral iron stability and the microbial processes influencing mineral dissolution vary during transitional permafrost thaw are poorly understood, yet have important implications for carbon cycling and emissions. Here we determine the reactive mineral iron and associated organic carbon content of core extracts and porewaters along thaw gradients in a permafrost peatland in Abisko, Sweden. We find that iron mineral dissolution by fermentative and dissimilatory iron(III) reduction releases aqueous Fe2+ and aliphatic organic compounds along collapsing palsa hillslopes. Microbial community analysis and carbon emission measurements indicate that this release is accompanied by an increase in hydrogenotrophic methanogen abundance and methane emissions at the collapsing front. Our findings suggest that dissolution of reactive iron minerals contributes to carbon dioxide and methane production and emission, even before complete permafrost thaw.
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.
A. (2020). Role of in situ natural organic matter in mobilizing as during microbial reduction of FeIII-mineral-bearing aquifer sediments from Hanoi (Vietnam). Environmental Science and Technology.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.