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Permafrost thaw – decadal responses to climate change

Periodic Reporting for period 1 - PERMTHAW (Permafrost thaw – decadal responses to climate change)

Periodo di rendicontazione: 2016-05-01 al 2018-04-30

Permafrost soils contain approximately 1672 Petagram carbon (C), twice the amount of the current atmosphere, and constitute 50% of the world’s belowground C pool. Along with the current change in climate these high latitudinal soils experience increased temperatures, more than any other region, with permafrost degradation and change of ecosystem functions as a result. The thaw of permafrost releases ancient organic matter that has been stored in the frozen soils for centuries. Following microbial degradation, this organic matter can be released to the atmosphere as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), further influencing the climate systems. Changes in ecosystem type and function can further influence these fluxes. Thus, a changing climate leads to server alterations of the carbon (C) and nitrogen (N) balance in the Arctic and high altitude ecosystems. However, research up to today has mostly focused on the impact of permafrost thaw and the time horizon immediately following this degradation.
This project aims to understand the future that lies ahead, following thaw and establishment of new non-permafrost ecosystems, and how the predicted climate variability will influence these soils on a decadal timescale. By using a natural occurring permafrost degradation transects, this project investigates how the C and N cycling changes following thaw. Moreover, by using laboratory incubation the project provided unique insights in how these cycles will respond to the changing climate long after the formation of ‘new’ ecosystems, giving a decadal perspective on permafrost thaw.
The investigated natural permafrost thaw gradient used in this study provided contrasting result compared to the heavily investigated carbon-rich emission hotspots usually highlighted in climate change discussion. Our result indicated that with increased vegetation and change towards a shrub-dominated landscape these mineral-based soils significantly reduces the methane emission due to a reduction in the anaerobic environment in the surface soil and an increase in methane consumption in this part of the soil. Furthermore, the long-term decomposition of carbon in the deeper former permafrost soils remains at a low emission rate, attributing most of the C and N cycling to the surface layer where even the carbon sequestration balance the decomposition rates from the deeper soils.
This project aimed at understanding the decadal time response of how ‘newly’ thawed permafrost ecosystems will respond to the future predicted climate variability. Therefore, this project includes a novel outline of field and laboratory studies in a cross-disciplinary approach. To generate a solid estimate of the ecosystem response to the decadal changes following final thaw up an extensive filed sampling approach was adopted with bi-weekly samplings during the growing seasons. The harsh and challenging winter season enabled only a few sites visits due to the remote locations of the sites. The overall response in carbon turnover showed a positive increase in the carbon uptake by the plant community (Gross Primary Production) with increasing time after permafrost decay. The ecosystems total respiration was largest at the intermediate that stage, confirming a similar pattern earlier described for partly thawed permafrost system.
The in situ observations also revealed striking evidence of a change in methane dynamics with a reduction in methane emission and even a pronounced consumption of atmospheric methane following the transition to post permafrost shrub-dominated ecosystems. These changes in fluxes were confirmed by a microbial scanning of the soils where an increase of the methane-consuming methanotrophs was evident in surface soils following also with time after the final thaw. Similar a decrease in the abundance of methane-producing methanogens was evidenced in deeper soil layers. Furthermore, a pronounced seasonality of was observed among the methanotrophs indicating that several strands are active with different optima in environmental conditions as pH, soil moisture and temperature.
The decomposition rates and temperature sensitivity of the deeper, former permafrost, soils indicate a rapid release of carbon at the initial incubation decreasing to a slower decomposition. This is in line with several earlier studies, but the release of carbon from these soils is profoundly lower than the previous report that highlights the carbon-rich systems of Siberia and Northern Canada/Alaska. Our results are more in line with the up to 87% of the Arctic that can be characterized as mineral soils providing a new view of the permafrost dynamics in the Arctic region.
The results from the PERMTHAW project is currently under preparation for two major publications to be submitted during 2019, with additional one already submitted manuscript and two upcoming once based on additional data generated from the project.
With the PERMTHAW project, we have expanded the knowledge of how an ecosystem adapts to the new environmental conditions following after the final decay of permafrost. This project thereby expands the state-of-the-art knowledge to also include permafrost boundary regions, a region more pronounced to experience changes due to the south location and with the annual temperature right on the tipping point between permafrost prevention or loss. Even though the project did not target socioeconomic aspects the result and final outcome will improve our understanding of how the landscape of the Arctic and Sub-Arctic will change in a future climate, enabling a better adaptation of northern societies to the changes that will come. The direct impact of the observed changes in vegetation and nutrient dynamics is of special interest for the local Sami reindeer herding population, where the project has interacted with two of the Sami-villages (Talma and Gabna).
Conceptual model