Modeling the effect of silicon and calcium availability on the future sustainability of Arctic permafrost carbon pools based on laboratory and field experiments

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Globally averaged temperature observations have increased since the onset of industrialization, and increase rates have been particularly high at the northern high latitudes. Warmer conditions threaten to degrade the vast pool of organic carbon currently stored in the in northern permafrost soils (estimated at 1330-1580 petagram), with potentially drastic consequences for global climate. The carbon cycle within Arctic permafrost ecosystems, as well as the sustainability of carbon pools, is not only controlled by temperature conditions. The amount of carbon emitted to the atmosphere in form of CO2 or CH4, depend strongly on other environmental boundary conditions such as moisture content, temperature, or pH of the soil. Also the quality of the organic matter (e.g. content of nutrients, nutrient stoichiometry and carbon compounds) plays a dominant role, with phosphorus (P) being of high importance. The potential influence of other, minor chemical elements have been largely neglected in this context. More specifically, two elements have been shown to be highly important for carbon fixation and turnover in marine systems, i.e. silicon (Si, important for C-fixation by diatoms) and calcium (Ca, important for C-fixation by coccolithophores). For terrestrial and semiaquatic systems, however, only few studies have analyzed links between Si and Ca content and carbon turnover rates, and even fewer results have yet been published for permafrost soils despite the potential importance for current and future carbon cycle processes. Silicon was shown to mobilize P from soil binding sites, therefore increasing P availability, whereas Ca is known to decrease P availability by forming insoluble Ca-P phases. The aim of this project is to constrain the net effect of competing processes linked to the availability of Si and Ca, and their effect on P availability, on the mineralization of organic matter in degrading permafrost soils, and quantify potential feedbacks with climate change. We will execute manipulation experiments both in-situ and in the laboratory, which will link enhanced Si and/or Ca contents (mimicking mobilization during permafrost thaw) to changes in decomposition/mineralization rates in the soil, and related shifts in CO2/CH4 emissions from permafrost ecosystems. These results will subsequently be used to test if the consideration of Si and Ca availability can improve the performance of a process-based model when compared against pan-Arctic carbon flux observations. Combining prognostic simulations of thaw depth with data on the vertical distribution of Si and Ca pools, we will furthermore estimate how Si and Ca availability may be affected through increases in thaw depth under future climate change. Taken together, these results will allow assessing the impact of Si and Ca availability on carbon fluxes in permafrost soils, and their contribution to the feedbacks between permafrost carbon cycle processes and future climate change.