The total organic carbon pool of soils (SOC) of the circum-arctic permafrost zone has been esti-mated to contain ~400 Gt C in the upper 1m [1,2] and as much as 747 Gt C in the upper 3m, excluding peatlands and carbon in deep loess sediment . This is more than today’s global atmospheric C pool (730 Gt C). This globally significant C reservoir is vulnerable to climate warming through permafrost thawing and may thus become a future source of a large amount of CO2 and CH4 to the atmosphere .
Despite its undisputed importance for the global carbon cycle, the amount of SOC stored in arctic soils remains ambiguous and poorly constrained, which is especially true for arctic Russia, in comparison to other arctic regions . There is increasing evidence that a significant propor-tion of this SOC is stored in the subducted organic matter of Cryosols, suggesting that cryoturbation (mixing of soil layers due to freezing and thawing) may be one of the most important mechanisms of arctic carbon storage .
Aims, objectives and vision
The overarching goal of CryoCARB is to advance organic carbon estimates for cryoturbated soils, focusing on the Eurasian Arctic and to understand the vulnerability of these carbon stocks in a future climate. Our vision is that one can build on this knowledge to improve existing models to better predict the responses of cryoturbated soils to future climate conditions. The constraints to our understanding of carbon dynamics in cryogenic soils are currently manifold. First, due to cryoturbation, organic matter is unevenly distributed within the soil, making SOC estimation very difficult. There is evidence that the North American arctic carbon stock is larger than previously thought, also because of underestimation of carbon stored in distorted, broken and warped horizons . Second, most studies dealing with SOC in arctic soils fail to account for carbon stored in the upper permafrost, even though it is directly under threat in a rapidly warming Arctic . Thawing of the upper permafrost will also mobilize old C, deposited under prior climates [8,9], which is rarely addressed. Third, the mechanisms of carbon stabilization are largely unknown thus hampering the prediction of climate-CO2 feedbacks [3,5]. Knowledge of the chemical composition of organic matter and the processes on how carbon is stabilized is necessary to predict the magnitude and the time-scale at which SOC will be remobilized from thawing permafrost under climate change .
These constraints lead to the major objectives of CryoCARB, which are to:
- Quantify the SOC storage in cryoturbated arctic soils, including the upper permafrost.
Cryoturbation processes are caused by frost heave movements due to hydrothermal gradients and are prone to climatic changes. We will systematically quantify SOC storage along three transects in the Eurasian Arctic and in Greenland and Svalbard, including the distorted/warped horizons down to the upper permafrost.
- Identify the major SOC stabilization mechanisms in cryoturbated soils.
Cryoturbation processes lead to subduction of organic matter from the surface into deeper soil layers. Several studies have shown that this subduction retards carbon mineralization by centuries to millennia [3,5] thus effectively removing it from the fast carbon cycle. We propose to investigate the dominant stabilization mechanisms, focusing on soil organic matter (SOM) quality, microbial community composition, and abiotic factors (temperature, moisture, O2).
- Assess the vulnerability of SOC of cryoturbated soils in a future climate.
Due to cryoturbation, deeper SOM is not necessarily more recalcitrant than SOM in surface horizons. We will address the vulnerability of arctic SOM by two decomposition experiments, one in the laboratory and one in the field and by detailed ecosystem-scale greenhouse gas flux analyses. Finally, we will refine an existing global biosphere and carbon cycle model to address fundamental questions on stability of arctic SOM under global warming.
CryoCARB is structured in 5 work packages (WP) with distinct functions. These WPs are linked and integrated by (i) a set of joint sampling campaigns at three arctic transects in Eurasia, (ii) by two joint experiments, and (iii) by the development of a sound theoretical and conceptual framework and mathematical modelling (Figure 1). For contribution of individual projects (IPs) and associated projects (APs) to the workpackages, please refer to Table 2.
Joint Sampling Campaigns
The study has a regional focus and will cover the tundra regions (treeless arctic area) of Eurasia (Europe and Siberia), Greenland and Svalbard (Spitsbergen). Three transects are proposed in the Russian Arctic, covering bioclimatic subzones E to C (CAVM ), also called southern, typical and arctic tundra subzones in the Russian literature. In each transect, a core sampling site will be established in subzone E.
Laboratory Incubation Experiment (E1)
The laboratory incubation experiment aims at elucidating the effect of different abiotic conditions on microbial decomposition of SOM. Soil cores of different horizons (different SOM qualities) will be incubated at different abiotic conditions to address the relationship between abiotic factors, microbial community composition, SOM stabilization and decomposition processes.
Cryoturbation Experiment (E2)
The cryoturbation experiment aims at (i) assessing the (initial) vulnerability of subducted and deep organic matter to decomposition at surface conditions and (ii) at quantifying the effect of subduction on decomposition of top-soil organic material. This experiment will be conducted in-situ by transplanting top-soil SOM and plant litter to deeper soil layers and exposing buried soil layers (from past cryoturbation events) and regular deep soil at the surface, mimicking active cryoturbation processes. Measurements include gas exchange, microbial decomposition processes, microbial community composition, changes in SOM and DOM quality and nutrient analyses. The cryoturbation experiment will complement the laboratory incubation experiment by adding valuable information on the actual (in-situ) effect of cryoturbation on SOC losses.
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