Why care about warming effects on soil respiration under permafrost collapse?

Permafrost-affected regions stored large quantities of soil organic carbon (C), with an amount of 1014~1672 Pg C (1 Pg = 10¹⁵ g). These regions have been threaten by rapid climate warming which is reported to be 2-4 times faster than the global average, and the resultant permafrost thaw accelerates microbial breakdown of soil organic matter, potentially elevating atmospheric carbon dioxide (CO₂) or methane concentrations and triggering a positive permafrost C-climate feedback. Thermokarst landscapes are resulted from the melting of ice-rich permafrost which considerably impacts the trajectory of permafrost C cycle. Thermokarst landscapes account for 20% of the total area in the permafrost-affected region, but they exert a significant role in terrestrial C cycle across the entire permafrost region. Considering the fact that ongoing climate warming would affect many ecosystem processes in both thermokarst and non-thermokarst areas, an interesting question is whether the warming effects on soil CO₂ flux differ between the two distinct landforms. If warming effects are amplified upon thermokarst formation, the positive permafrost C-climate feedback will be intensified.

 How to conduct field monitoring and laboratory analyses?

Our research interest is to explore the interactive effects of permafrost thaw and warming on soil CO₂ release in permafrost regions. The principal investigator of our research team, Prof. Yuanhe Yang from Institute of Botany, Chinese Academy of Sciences, focuses on permafrost C cycle on the Tibetan Plateau. In 2017, we conceived an idea to figure out whether the warming response of soil CO₂ flux would change upon thermokarst formation. To achieve this goal, we identified three thermo-erosion gullies at a large thermokarst feature in Shaliuhe site on the northeastern Tibetan Plateau. The experimental design was determined with the help of Drs. Yuanhe Yang and Yunfeng Peng. During 2017-2018, Dr. Yunfeng Peng, Guanqin Wang, Jun Wang, Jianchun Yu established the field warming experiment. The warming experiments were set simultaneously inside (collapsed area) and outside the gully (non-collapsed area) using the open top chambers (OTCs). During the ice-free period from mid-May to mid-October of 2019 and 2020, soil CO₂ fluxes were monitored by Guanqin Wang, Bin Wei, Yang Liu and Qinlu Li. To verify the broader representativeness of single-site findings, we collected soils from the warming experimental site and five additional upland thermokarst-impacted sites in the year of 2020 and examined how temperature sensitivity of CO₂ release responded to permafrost collapse using a laboratory incubation experiment. Although being suffered extreme weather and heavy field work, we successfully collected the unique dataset, which made all worth it. This article builds on the efforts that have been given by our teammates and collaborators. When Dr. Leiyi Chen, Dianye Zhang, Shuqi Qin, Yutong Song and Luyao Kang joined, our research was strengthened in the microbiology and statistical analyses. We also appreciate the international collaborators, Drs. Benjamin W. Abbott, Philippe Ciais, Josep Peñuelas, Pete Smith and Jens Strauss for their constructive comments on an early version of the manuscript. This article would not have been possible without their attendance.

In-situ soil CO₂ flux measurement. Photograph is taken by Bin Wei.

What results were obtained?

Our research presents a novel understanding about the responses of soil respiration to warming upon permafrost collapse. The most significant finding of our research is that the response of soil respiration to warming was ~5.5 times stronger upon thermokarst formation. Laboratory incubation study also showed significant increases of temperature sensitivity of CO₂ release in collapsed relative to non-collapsed areas, generally supporting the results from the in-situ warming experiment. We further analyzed over 30 potential drivers of the warming response and found that the larger warming response was mainly associated with the lower substrate quality and higher abundance of microbial functional genes for C degradation in thermokarst-affected soils. Overall, our research helps to more accurately predict permafrost C-climate feedback on the thermokarst-impacted regions and make mitigation policies accordingly. More details can be found in our paper ‘Enhanced response of soil respiration to experimental warming upon thermokarst formation’ published in Nature Geoscience.

 Outlook

Our research has now revealed that warming-induced permafrost thaw can intensify the response of soil CO₂ emission to climate warming. However, whether the responses of other important greenhouse gas emissions, such as methane and nitrous oxide, will change upon thermokarst formation, are still unknown. To fill these knowledge gaps, we have conducted further monitoring of methane and nitrous oxide fluxes based on this warming experiment platform and the field work has been taken for a whole growing season, which provides great potentials to address the above scientific questions. In the future, we look forward to discovering more interesting phenomena and the underlying mechanisms based on this novel platform to better understand greenhouse gas emissions upon thermokarst formation.