Seasonal changes in hydrology and permafrost degradation control mineral element-bound DOC transport from permafrost soils to streams

Mineral elements bind to dissolved organic carbon (DOC) in permafrost soils, and this may contribute to the stabilisation or the degradation of organic carbon along the soil to river continuum. Permafrost thaw enlarges the pool of soil constituents available for soil to river transfer. The unknown is how changes in hydrology upon permafrost degradation affect the connection between soil-derived mineral element-bound DOC and headwater streams. Here, we study Al, Fe, Ca and DOC concentrations in water from a headwater stream at Eight Mile Lake, Alaska, USA (colloidal (0.22 µm–1 kDa) and truly dissolved (< 1 kDa) fractions) and in soil pore waters sampled across a gradient of permafrost degradation at the same location. We target the peak flow to base flow transition to show that there is a narrow window of mineral element-bound DOC colloid transport from soils to streams. We show that during spring thaw and maximum thaw there is an enhanced lateral transfer of mineral element-bound DOC colloids in extensively degraded sites compared to minimally degraded sites. This is explained by a more rapid response of hydrology at peak flow to base flow transition at degraded sites. Our results suggest that ongoing permafrost degradation and the associated response of soils to changing hydrology can be detected by targeting the composition and size of mineral element-DOC associations in soil waters and headwater streams during peak flow-baseflow transitions

Detecting Hydrological Connectivity in Polar Environments Using Silicon Isotopes

Climate change is having a direct impact on hydrological connectivity in permafrost environments1. In this work, we combine soil physics and silicon isotope geochemistry to locate pathways of hydrological connectivity in permafrost soils at Eight Mile Lake, Alaska. Silicon in soil pore waters (< 0.2 µm) can be a colloidal fraction (~ 0.2 µm to ~ 1 nm) and a truly dissolved fraction of silicic acid (~ < 1 nm), with an isotope fractionation associated with colloidal amorphous Si formation. Here we propose that soil pore waters contain different proportions of these Si pools during freezing and thawing, and apply this conceptual framework to detect the freezing and thawing conditions in permafrost soils during winter months. We propose that this approach could be applied in other cold, extreme environments to detect changes in water and nutrient flow paths. 1 Walvoord, M.A. and Kurylyk, B.L., 2016. Hydrologic impacts of thawing permafrost—A review. Vadose Zone Journal, 15(6)

Evidence for late winter biogeochemical connectivity in permafrost soils

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

Influence of thawing permafrost on soil-plant mineral element transfer: Case study in Interior Alaska

Current climate warming strongly affects the Arctic region by increasing thaw depths and modifying water table depths in soils. Exposing newly thawed permafrost at depth unlocks mineral nutrients that can boost plant growth, thereby contributing to modify the balance between carbon input and output from permafrost regions. In this changing Arctic environment, how will active layer deepening affect plant mineral nutrient availability, plant mineral nutrient uptake and soil-plant nutrient cycling via litter degradation? Studies have highlighted that upon thawing, deep-rooted plants benefit from a newly exposed pool of essential nutrients (e.g., N). It is hypothesized that during plant growth, additional mineral elements (e.g., Ca, K, Mg) are also bio-lifted and recycled in surface soil horizons through litter production, providing a potential source of nutrients for shallower rooted plants. To address this hypothesis and investigate mineral element distribution in plants and soils upon permafrost thaw, we determine the mineral element content (Ca, K, Mg, Na, P, Si, Mn) (i) in plant species from three different plant functional types (sedges, deciduous and evergreen shrubs) and (ii) in the corresponding soil profiles. The plant selection includes species with shallower (e.g., Vaccinium spp.) and deeper (e.g., Carex spp.) rooting depth, across a permafrost thaw gradient at Eight Mile Lake, Central Alaska. In soils, we also determine the availability of Ca, Mg, K, Na for plants from the soil exchange complex. Plant leaves and soil samples were collected in August – September 2019, corresponding to the maximal thaw depth. The sampling transect (vegetation and related soil profiles) encompasses a wide range of thaw depth (from -48 cm to –96 cm) and water table depth (from 0 cm to –40 cm). Results are discussed considering mineral element distribution in soils, plant rooting depth, and active layer depth to evaluate whether vegetation may access mineral elements from deeper soil horizons, related to the active layer deepening

Influence of permafrost degradation on foliar mineral element cycling upon changing subarctic tundra vegetation

Climate warming strongly affects the Arctic region by creating soil subsidence, increasing thaw depth and modifying water table depth. Thawing permafrost unlocks deeper soil mineral nutrients that may boost plant growth, and generates microtopography that may induce contrasted local soil moisture conditions. According to soil subsidence and drainage capacity, shift in vegetation through the Arctic and sub-Arctic region may vary, with sedges (as part of graminoids) expanding through wetter lowlands and shrubs expanding through drier uplands. Consequently, changes in the composition of Arctic tundra vegetation may influence local mineral element cycling through litter production, but this remains poorly constrained. In order to evaluate the influence of permafrost degradation on litter composition, we determined foliar mineral element stocks and annual litterfall fluxes from a typical Arctic tundra. We measured foliar elemental composition (Al, Ca, Fe, K, Mn, P, S, Si, and Zn) of leaf samples from 7 vascular species and 6 non-vascular species (mosses and lichens) from two contrasted Alaskan sites, i.e., under experimental (CiPEHR) and natural (Gradient) warming. We found that foliar composition is specific to the species and independent of the permafrost degradation. Therefore, the shift in the tundra vegetation related to climate change is expected to mostly influence the change in litter mineral element composition. Upon sedge expansion, foliar mineral element stocks largely increased for elements highly concentrated into sedge leaves, such as Si (i.e., Si foliar stock increased ~4 times over 8 years of warming experiment and related sedge expansion). Upon shrub expansion, foliar mineral element stocks increased for elements highly concentrated into shrub leaves, such as Ca and Mn (i.e., Ca and Mn foliar stocks were ~1.5 times higher upon shrub- than sedgeland). As a cascade reaction, changes in foliar mineral element stocks related to the shift in vegetation led to changes in their annual litterfall fluxes, with an increase in Si annual foliar fluxes upon sedge annual litterfall, and an increase in Ca and Mn annual foliar flux upon shrub annual litterfall (Figure 1). Consequently, sedge and shrub expansion led to contrasted litter elemental composition, and thereby contrasted nutrient cycling, with implications for further vegetation succession across the Arctic tundra

Mineral element cycling through the soil-plant system upon permafrost thaw: case study in Interior Alaska

Mineral element cycling through the soil-plant system upon permafrost thaw: case study in Interior Alaska Climate warming affects the Arctic region by exposing previously frozen permafrost to thaw, unlocking mineral nutrients, boosting plant growth, and modifying the carbon balance from permafrost regions. In changing Arctic environments, how will permafrost degradation affect plant mineral nutrient availability and soil-plant nutrient cycling via litter degradation? Studies have highlighted that upon thawing, deep-rooted plants benefit from new pools of essential nutrients such as N or P. Therefore, we hypothesized that other mineral elements such as Ca, K, or Mg may also be bio-lifted and recycled in surface soil horizons through deep plant uptake and litter production, providing a source of nutrients for shallower rooted plants. To test this hypothesis, plant leaves and soil samples were collected across a permafrost thaw gradient at Eight Mile Lake, Alaska in September 2019, corresponding to the season of maximal permafrost thaw depth. The sampling transect (vegetation and soil profiles) encompasses a range of active layer depth (−48 to –96 cm) and water table depth (0 to –40 cm). We investigate the influence of permafrost thaw on mineral element distribution in plants and soils by measuring the total content in Ca, K, Mg, Na, P, Si, and Mn: (i) in plant species from three different plant functional types (sedges, deciduous and evergreen shrubs); and (ii) in the corresponding soil profiles. The plant selection includes species with shallower (Vaccinium spp.) and deeper (Carex spp.) rooting depth. In soils, we also determined the content in exchangeable Ca, Mg, K, and Na. The large contrasts between the element distribution in organic and mineral horizons confirm the central role of the vegetation in the mineral elements cycling in these permafrost-affected ecosystems, and highlight the importance to consider jointly active layer depth, water table depth and plant rooting depth to assess the nutrient availability for the vegetation

Changing sub-Arctic tundra vegetation upon permafrost degradation: impact on foliar mineral element cycling

Arctic warming and permafrost degradation are modifying northern ecosystems through changes in microtopography, soil water dynamics, nutrient availability, and vegetation succession. Upon permafrost degradation, the release of deep stores of nutrients such as nitrogen and phosphorus from newly thawed permafrost stimulates Arctic vegetation production. More specifically, wetter lowlands show an increase in sedges (as part of graminoids), whereas drier uplands favor shrub expansion. In turn, shifts in the composition of vegetation may influence local mineral element cycling through litter production. In this study, we evaluate the influence of permafrost degradation on mineral element foliar stocks and potential annual fluxes upon litterfall. We measured the foliar elemental composition (Al, Ca, Fe, K, Mn, P, S, Si, and Zn) on ~500 samples of typical tundra vegetation species from two contrasting Alaskan sites, i.e., under experimental (CiPEHR) and ambient (Gradient) warming. The foliar concentration of these mineral elements was species specific, with sedge leaves having relatively high Si concentration, and shrub leaves having relatively high Ca and Mn concentrations. Therefore, changes in the species biomass composition of the Arctic tundra in response to permafrost thaw are expected to be the main factors that dictate changes in elemental composition of foliar stocks and maximum potential foliar fluxes upon litterfall. We observed an increase in the mineral element foliar stocks and potential annual litterfall fluxes, with Si increasing with sedge expansion in wetter sites (CiPEHR), and Ca and Mn increasing with shrub expansion in drier sites (Gradient). Consequently, we expect that sedge and shrub expansion upon permafrost thaw will lead to changes in litter elemental composition, and affect nutrient cycling across the sub-Arctic tundra, with potential implications for further vegetation succession

Changing sub-Arctic tundra vegetation upon permafrost degradation: impact on foliar mineral element cycling

Arctic warming and permafrost degradation are modifying northern ecosystems through changes in microtopography, soil water dynamics, nutrient availability, and vegetation succession. Upon permafrost degradation, the release of deep stores of nutrients, such as nitrogen and phosphorus, from newly thawed permafrost stimulates Arctic vegetation production. More specifically, wetter lowlands show an increase in sedges (as part of graminoids), whereas drier uplands favor shrub expansion. These shifts in the composition of vegetation may influence local mineral element cycling through litter production. In this study, we evaluate the influence of permafrost degradation on mineral element foliar stocks and potential annual fluxes upon litterfall. We measured the foliar elemental composition (Al, Ca, Fe, K, Mn, P, S, Si, and Zn) of ∼ 500 samples of typical tundra plant species from two contrasting Alaskan tundra sites, i.e., an experimental 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 and shrub leaves having relatively high Ca and Mn concentrations. Therefore, changes in the species biomass composition of the Arctic tundra in response to permafrost thaw are expected to be the main factors that dictate changes in elemental composition of foliar stocks and maximum potential foliar fluxes upon litterfall. We observed an increase in the mineral element foliar stocks and potential annual litterfall fluxes, with Si increasing with sedge expansion in wetter sites (CiPEHR), and Ca and Mn increasing with shrub expansion in drier sites (Gradient). Consequently, we expect that sedge and shrub expansion upon permafrost thaw will lead to changes in litter elemental composition and therefore affect nutrient cycling across the sub-Arctic tundra with potential implications for further vegetation succession