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Abstract The permafrost active layer is a key supplier of soil organic carbon and mineral nutrients to Arctic rivers. In the active layer, sites of soil-water exchange are locations for organic carbon and nutrient mobilization. Previously these sites were considered as connected during summer months and isolated during winter months. Whether soil pore waters in active layer soils are connected during shoulder seasons is poorly understood. In this study, exceptionally heavy silicon isotope compositions in soil pore waters show that during late winter, there is no connection between isolated pockets of soil pore water in soils with a shallow active layer. However, lighter silicon isotope compositions in soil pore waters reveal that soils are biogeochemically connected for longer than previously considered in soils with a deeper active layer. We show that an additional 21% of the 0–1 m soil organic carbon stock is exposed to soil - water exchange. This marks a hot moment during a dormant season, and an engine for organic carbon transport from active layer soils. Our findings mark the starting point to locate earlier pathways for biogeochemical connectivity, which need to be urgently monitored to quantify the seasonal flux of organic carbon released from permafrost soils. 

Abstract. Permafrost ecosystems are limited in nutrients forvegetation development and constrain the biological activity to the activelayer. Upon Arctic warming, permafrost thaw exposes large amounts of soilorganic carbon (SOC) to decomposition and minerals to weathering but alsoreleases organic and mineral soil material that may directly influence thesoil exchange properties (cation exchange capacity, CEC, and base saturation,BS). The soil exchange properties are key for nutrient base cation supply(Ca2+, K+, Mg2+, and Na+) for vegetation growth anddevelopment. In this study, we investigate the distributions of soil exchangeproperties within Arctic tundra permafrost soils at Eight Mile Lake(Interior Alaska, USA) because they will dictate the potential reservoir ofnewly thawed nutrients and thereby influence soil biological activity andvegetation nutrient sources. Our results highlight much lower CEC density insurface horizons (∼9400 cmolc m−3) than in the mineralhorizons of the active layer (∼16 000 cmolc m−3)or in permafrost soil horizons (∼12 000 cmolc m−3). Together, with the overall increase in CEC density with depth andthe overall increase in BS (percentage of CEC occupied by exchangeable basecations Ca2+, K+, Mg2+, and Na+) with depth (from∼19 % in organic surface horizons to 62 % in permafrost soilhorizons), the total exchangeable base cation density (Ca2+, K+,Mg2+, and Na+ in g m−3) is up to 5 times higher in thepermafrost than in the active layer. More specifically, the exchangeablebase cation density in the 20 cm upper part of permafrost about to thaw is∼850 g m−3 for Caexch, 45 g m−3 forKexch, 200 g m−3 for Mgexch, and 150 g m−3 forNaexch. This estimate is needed for future ecosystem prediction modelsto provide constraints on the size of the reservoir in exchangeablenutrients (Ca, K, Mg, and Na) about to thaw. All data described in this paper are stored in Dataverse, the online repository of Université catholique de Louvain, and are accessible through the following DOI: https://doi.org/10.14428/DVN/FQVMEP (Mauclet et al., 2022b). 

Abstract. Arctic warming and permafrost degradation are modifying northernecosystems through changes in microtopography, soil water dynamics, nutrientavailability, and vegetation succession. Upon permafrost degradation, therelease of deep stores of nutrients, such as nitrogen and phosphorus, fromnewly thawed permafrost stimulates Arctic vegetation production. Morespecifically, wetter lowlands show an increase in sedges (as part ofgraminoids), whereas drier uplands favor shrub expansion. These shifts inthe composition of vegetation may influence local mineral element cyclingthrough litter production. In this study, we evaluate the influence ofpermafrost degradation on mineral element foliar stocks and potential annualfluxes upon litterfall. We measured the foliar elemental composition (Al,Ca, Fe, K, Mn, P, S, Si, and Zn) of ∼ 500 samples of typicaltundra plant species from two contrasting Alaskan tundra sites, i.e., anexperimental sedge-dominated site (Carbon in Permafrost Experimental Heating Research, CiPEHR) and natural shrub-dominated site(Gradient). The foliar concentration of these mineral elements was species specific, with sedge leaves having relatively high Si concentration andshrub leaves having relatively high Ca and Mn concentrations. Therefore,changes in the species biomass composition of the Arctic tundra in responseto permafrost thaw are expected to be the main factors that dictate changesin elemental composition of foliar stocks and maximum potential foliarfluxes upon litterfall. We observed an increase in the mineral elementfoliar stocks and potential annual litterfall fluxes, with Si increasingwith sedge expansion in wetter sites (CiPEHR), and Ca and Mn increasing withshrub expansion in drier sites (Gradient). Consequently, we expect thatsedge and shrub expansion upon permafrost thaw will lead to changes inlitter elemental composition and therefore affect nutrient cycling acrossthe sub-Arctic tundra with potential implications for further vegetationsuccession.