Seasonal CO2 source partitioning in thawing permafrost ecosystems: Contribution of winter processes to annual carbon cycling

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Winters in the Arctic are long and cold but cold-adapted plants and soil microorganisms, covered by insulating snow layers, continue to respire carbon dioxide (CO2), which can account for 20–50% to annual CO2 emissions from these ecosystems. And even though winter represents a major part of the year, the mechanisms of ecosystem respiration in winter are poorly understood.High-latitude ecosystems are governed by strong seasonal gradients in temperature, light and snow cover. Permafrost, i.e. ground that is at or below 0 °C for at least two consecutive years, is very sensitive to climatic change. Climate warming over the last decades resulted in wide-spread permafrost thaw with accompanying shifts in vegetation from primarily shallow-rooting plant communities to deep-rooting species. However, the effects of root-soil interactions on ecosystem respiration during winter and at depths where permafrost thaws remain unclear. Furthermore, the rate of climatic change in high-latitude regions has been twice the rate of the global average and most of the changes have been observed in winter, with an overall decrease in the duration of snow cover but an increase in snow depth. Climatic change, permafrost thaw, and deeper rooting systems are expected to result in higher ecosystem respiration and thus higher CO2 emissions but partitioning ecosystem respiration into its two sources, relatively fast cycling plant-derived CO2 and older, slow cycling soil organic carbon, remains a major research challenge. However, only the separation of these two sources will enable us to identify the origin of the additionally respired CO2 and thus help us to better understand the future role of thawing permafrost regions in the global carbon cycle.I will combine in situ 13C-isotopic labelling studies with a common garden experiment to (1) identify the drivers and quantify the contribution of winter ecosystem respiration to annual carbon cycling along a permafrost thaw gradient in northern Sweden as well as the relative contributions of plant- and soil microbial respiration to ecosystem respiration during different seasons and (2) examine how permafrost thaw affects soil-root dynamics in winter. This project will thus give new insights into the drivers of winter processes in degrading permafrost-affected ecosystems and the partitioning of CO2 sources will help to determine the effect of climatic change on the annual sink strength of permafrost-affected systems for atmospheric CO2 in a warming Arctic. Neither winter nor permafrost processes are currently represented in climate models with high statistical confidence. A better understanding of seasonal interactions between plants, soil microorganism, and carbon cycling in these unique ecosystems is therefore urgently needed.