The frozen ground in the northern hemisphere is estimated to contain about twice as much carbon as Earth’s atmosphere. But now, these permafrost soils are collapsing under warmer conditions, and releasing unknown amounts of carbon in the process.
An international team of researchers led by CSU Professor Thomas Borch investigated how the thawing process may be affecting microorganisms in the soil.
The experts tested the theory that thawing increases the availability of organic carbon for microorganisms to process, subsequently releasing vast amounts of carbon dioxide and methane. Both of these heat-trapping gases accelerate climate warming and cause more permafrost to thaw in a vicious cycle.
“The organic material naturally present in the samples accumulated as peat over thousands of years. With permafrost thaw, microbes become active and are able to decompose the peat,” said study co-lead author Andreas Kappler. “We also know that iron minerals preserve organic carbon from biodegradation in various environments – and thus they could be a carbon sink even after the permafrost has thawed.”
The reactive iron is present as a kind of rust that could potentially trap the organic material in what is referred to as a “rusty carbon sink.” The team analyzed the storage potential of the rusty carbon sink at a permafrost peatland in Abisko, Sweden. The researchers collected samples of the soil porewater and drill cores of the active layer along a permafrost thaw gradient.
The permafrost soil samples were used to evaluate how much organic material was bound to reactive iron minerals, how stable these iron-carbon associations are with permafrost thaw, and whether the microorganisms present could use the material as a source of energy.
The study revealed that microorganisms are, in fact, able to use the iron as a food source, thereby releasing the bound organic carbon.
“That means the rusty carbon sink cannot prevent the organic carbon from escaping from the thawing permafrost,” said Kappler. “Based on data available from elsewhere in the northern hemisphere, we expect that our findings are applicable for permafrost environments worldwide,” added study co-lead author Casey Bryce.
The researchers explained that the rusty carbon sink is only found in intact permafrost soils and is lost during permafrost thaw.
“It appears that the previously iron-bound carbon is highly bioavailable and, therefore, bacteria could immediately metabolize it into greenhouse gas emissions,” said study co-author Monique Patzner. “This is a process which is currently absent from climate-change prediction models and must be factored in.”
The researchers will now investigate how this newly discovered process affects greenhouse gas emissions, and will study the chemical nature and fate of the organic matter released from permafrost soils. According to Professor Borch, these further inquiries should help to improve our understanding of carbon cycling in these sensitive ecosystems.
The study is published in the journal Nature Communications.