Influence of permafrost degradation and shift in vegetation on litter and soil properties: case study in Central Alaska

Global warming affects the Arctic and sub-Arctic regions by exposing previously frozen permafrost to thaw, unlocking mineral nutrients, stimulating microbial activity, and boosting plant growth. Initially composed of graminoids, forbs, deciduous and evergreen shrubs, mosses and lichens, Arctic tundra is subject to a shrub, forb and moss expansion, at the expense of graminoid species. For now, little is known about the intrinsic foliar properties of these plant species and how they may influence properties of the underlying litter. Therefore, we investigated vegetation foliar properties and litter properties (organic carbon, C/N ratio, cation exchange capacity (CEC), pH, and concentrations of major and trace elements), from a typical Arctic tundra across a natural gradient of permafrost degradation. Results show that vegetation foliar properties are intrinsic to plant species and do not depend on the permafrost degradation stage. Furthermore, the natural gradient of permafrost degradation showed contrasts in litter mineral element concentrations, related to the occurring shift in vegetation. For example, we found a decrease in silicon concentrations in the litter between the least and the most degraded permafrost site, which is concurrent with the decrease in graminoids (Si-rich plant species) and the increase in shrubs biomass (Si-poor plant species), upon permafrost degradation. Therefore, changes in vegetation composition may influence litter properties, such as C/N ratio, CEC and mineral element concentrations (Figure 1), which could in turn influence C dynamics with the change of nutrient availability for plant cover

Silicon Isotopes: A Tool to Capture Winter Biogeochemical Connectivity in Permafrost Soils (Stordalen, Sweden)

Silicon isotope fractionation upon amorphous silica precipitation is susceptible to freeze-thaw cycles. Here we make use of this characteristic to distinguish between a closed system in freezing soils: where silicic acid concentration is increasing, leading to amorphous silica precipitation that induces Si isotope fractionation where 28Si is preferentially incorporated into the colloidal amorphous silica; and an open system in freezing soils: where silicic acid is mixing with surrounding soil pore waters and amorphous silica precipitation is not induced. We analyzed a temporal series of soil pore water collected from September to November 2021 on a natural gradient of permafrost degradation from a palsa (closed system) to a fen (open system) in Stordalen, Sweden. We compare the evolution of the δ30Si values in soil pore waters where freeze-up has occurred (closed system) or where freeze-up is delayed or absent (open system). We couple our δ30Si data with variations in redox-sensitive element (e.g., Fe) concentrations, sensitive to limited biogeochemical connectivity in a closed system. The dual-approach of silicon isotope geochemistry with redox sensitive element analysis has important implications for capturing the lateral transfer of water and nutrients from permafrost soils during winter months

Identification of winter biogeochemical connectivity in permafrost soils with silicon isotopes and redox-sensitive elements (Stordalen, Sweden)

Climate change affects Arctic regions by exposing previously frozen permafrost to thaw and changing hydrological processes. As a result, permafrost soils in Arctic have recently developed unfrozen soil portions in winter. These unfrozen soil portions may increase the soil biogeochemical connectivity by creating lateral subsurface water flow, thereby contributing to the lateral transfer of nutrients including dissolved organic carbon. This winter connectivity is mainly expected if unfrozen soil portions are connected (open system). However, the proportion of connected (open system) relative to unconnected (closed system) unfrozen soil portions remains poorly quantified. Here, we investigate the silicon isotope composition (δ30Si) and the redox-sensitive element (e.g., Fe) concentrations in soil pore water collected from September to November 2021 on a natural gradient of permafrost degradation from a palsa (closed system) to a fen (open system) in Stordalen, Sweden. We use δ30Si measurements to distinguish between: a closed system in freezing soils where silicic acid concentration in soil pore water is increasing upon freezing, leading to amorphous silica precipitation that induces Si isotope fractionation due to the preferential incorporation of 28Si in colloidal amorphous silica; and an open system in freezing soils where silicic acid concentration in soil pore water is mixed with lateral contributions and amorphous silica precipitation is not induced. We then compare the evolution of the δ30Si values in soil pore waters where freeze-up has occurred (closed system) or where freeze-up is delayed or absent (open system). We couple our δ30Si data with variations in redox-sensitive element concentrations (e.g., Fe) to better constrain the biogeochemical connectivity with the atmosphere. The dual-approach of silicon isotope geochemistry with redox sensitive element analysis contributes to better understand the processes controlling the lateral transfer of water and nutrients from permafrost soils during winter months

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)

Changing conditions for mineral-organic carbon interactions across the permafrost landscape: hot moments more than hot spots?

The Earth’s high latitude regions are warming twice as fast as the global average which enhances the thawing of permafrost. According to geological archives, abrupt thaw and thermokarst formation in the past have induced changes in redox conditions in ice-rich permafrost deposits, affecting iron distribution and interactions between organic carbon and iron (Monhonval et al. 2021), modifying thereby the protective role of minerals for organic matter. This example illustrates that the evolution of mineral-organic carbon interactions with permafrost thaw is a potentially important player in the modulation of permafrost carbon emissions. The stability of mineral surfaces and the availability of metal ions for binding organic carbon are likely to vary upon changing water saturation in response to permafrost thaw and deepening of the active layer. Using the distribution of mineral elements with direct (Fe) or indirect (Si) interactions with organic carbon, we investigate the changing conditions for mineral-organic carbon interactions on a variety of scales across the modern permafrost landscape. Our results show that at the pedon scale localized freeze-thaw events occurring during the winter period create hot moments for soil biogeochemical reactions in microenvironments (~10 to 20 cm thick soil layers) and that these reactions affect the solute transfer from soils to rivers. Our data also support that hot spots for biogeochemical reactions are created in the novel active layer resulting from thermokarst slump deposits upon abrupt thaw. The temporal and spatial heterogeneity in the distribution of unfrozen soil microenvironments in the permafrost landscape is more complex than previously thought. The role played by ice and frost at structuring and compartmentalizing microenvironments hosting mineral-organic carbon interactions in permafrost soils should be seen as more variable seasonally and spatially. Monhonval et al. 2021. Iron redistribution upon thermokarst processes in the Yedoma domain. Front. Earth Sci. 9, doi.org/10.3389/feart.2021.703339

Changes in Nutrient Sources for Arctic Tundra Vegetation upon Permafrost Thaw

Permafrost thaw modifies resource acquisition for tundra vegetation in two major directions of vegetation shift across the Arctic: the expansion of deeply rooted sedges and the widespread increase in shallowly rooted woody shrubs. Assessing to what extent nutrient access is changing for the Arctic tundra vegetation development is crucial given the feedbacks of vegetation shifts on Arctic warming and permafrost stability by influencing the albedo, the snow accumulation and the litter decomposition rate. In this study, we evaluate the influence of permafrost degradation on the nutrient sources for plant uptake by using the radiogenic Sr isotope ratio as a tracer of source for plant nutrient, along a permafrost thaw gradient at Eight Mile Lake in Interior Alaska (USA). As plants take up Sr from the exchangeable soil fraction with no measurable fractionation, we determine the differences in 87Sr/86Sr ratio of the exchangeable Sr between shallow and deeper soil horizons, and we compare the 87Sr/86Sr ratio of foliar samples for three Arctic tundra species with contrasted rooting depths (B. nana, V. vitis-idaea, and E. vaginatum) upon different permafrost thaw conditions. The higher foliar 87Sr/86Sr ratios of shallow-rooted Arctic tundra shrubs (B. nana, V. vitis-idaea) reflect a shallow source of soil exchangeable Sr from surface soil horizons, whereas the lower foliar 87Sr/86Sr ratios of deep-rooted Arctic tundra sedges (E. vaginatum) reflect a source of Sr from deeper soil horizons. Importantly, our data highlight a shift in the range of foliar 87Sr/86Sr ratios towards lower values in plants grown on more deeply thawed permafrost soils, thereby supporting that the three Arctic tundra plant species access nutrients from deeper soil horizons upon permafrost thaw. The differences in plant strategy for nutrient acquisition is therefore expected to influence largely interactions between deeply and shallowly rooted plant species, and thereby the future shift in Arctic tundra vegetation

Mineral element recycling in topsoil following permafrost degradation and a vegetation shift in sub-Arctic tundra

Climate change affects the Arctic and sub-Arctic regions by exposing previously frozen permafrost to thaw, unlocking soil nutrients, changing hydrological processes, and boosting plant growth. As a result, sub-Arctic tundra is subject to a shrub expansion, called “shrubification”, at the expense of sedge species. Depending on the intrinsic foliar properties of these plant species, changes in foliar mineral element fluxes with shrubification in the context of permafrost degradation may influence topsoil mineral element composition. Despite the potential implications of changes in topsoil mineral element concentrations for the fate of organic carbon, this remains poorly quantified. Here, we investigate vegetation foliar and topsoil mineral element composition (Si, K, Ca, P, Mn, Zn, Cu, Mo, V) across a natural gradient of permafrost degradation at a typical sub-Arctic tundra at Eight Mile Lake (Alaska, USA). Results show that foliar mineral element concentrations are higher (up to 9 times; Si, K, Mo for all species, and for some species Zn) or lower (up to 2 times; Ca, P, Mn, Cu, V for all species, and for some species Zn) in sedge than in shrub species. As a result, a vegetation shift over ~40 years has resulted in lower topsoil concentrations in Si, K, Zn, and Mo (respectively of 52, 24, 20, and 51%) in highly degraded permafrost sites compared to poorly degraded permafrost sites due to lower foliar fluxes of these elements. For other elements (Ca, P, Mn, Cu, and V), the vegetation shift has not induced a marked change in topsoil concentrations at this current stage of permafrost degradation. A modeled amplified shrubification associated with a further permafrost degradation is expected to increase foliar Ca, P, Mn, Cu, and V fluxes, which will likely change these element concentrations in topsoil. These data can serve as a first estimate to assess the influence of other shifts in vegetation in Arctic and sub-Arctic tundra such as sedge expansion under wetter soil conditions

Tracing changes in base cation sources for Arctic tundra vegetation upon permafrost thaw

Upon permafrost thaw, the volume of soil accessible to plant roots increases which modifies the acquisition of plant-available resources. Tundra vegetation is actively responding to the changing environment with two major directions for vegetation shift across the Arctic: the expansion of deep-rooted sedges and the widespread increase in shallow-rooted shrubs. Changes in vegetation composition, density and distribution have large implications on the Arctic warming and permafrost stability by influencing the albedo, the snow accumulation and the litter decomposition rate. A better understanding of these cumulated effects of changing vegetation on warming and permafrost requires assessing the changes in plant nutrient sources upon permafrost thaw, nutrient access being a limiting factor for the Arctic tundra vegetation development. In this study, we determined the influence of permafrost degradation on the base cation sources for plant uptake by using the radiogenic Sr isotope ratio as a tracer of source, along a permafrost thaw gradient at Eight Mile Lake in Interior Alaska (USA). As plants take up Sr from the exchangeable soil fraction with no measurable fractionation, we determined the differences in 87Sr/86Sr ratio of the exchangeable Sr between shallow and deeper soil horizons, and we compared the 87Sr/86Sr ratio of foliar samples for three Arctic tundra species with contrasted rooting depths (Betula nana, Vaccinium vitisidaea, and Eriophorum vaginatum) upon different permafrost thaw conditions. The higher foliar 87Sr/86Sr ratios of shallow-rooted Arctic tundra shrubs (B. nana, V. vitis-idaea) was consistent with a shallow source of soil exchangeable Sr from surface soil horizons, whereas the lower foliar 87Sr/86Sr ratios of deep-rooted Arctic tundra sedges (E. vaginatum) reflected a source of Sr from deeper soil horizons. There is a shift between poorly and highly thawed soil profiles towards lower foliar 87Sr/86Sr ratios in both deep- and shallow-rooted plant species. This shift supports that micro-landscape variability in the exchangeable base cation reserve with soil depth represents a key source of readily available nutrients for both shallow- and deep-rooted plant species upon permafrost thaw. This study highlights a key change in plant nutrient source to consider upon thaw. This finding lies beyond the common view that nutrient release at the permafrost thaw front preferentially benefits deeprooted plant species

Does vegetation shift in Arctic tundra upon permafrost degradation influence mineral element recycling in the topsoil?

Climate change affects the Arctic and Subarctic regions by exposing previously frozen permafrost to thaw, unlocking nutrients, changing hydrological processes, and boosting plant growth. As a result, Arctic tundra is subject to a shrub expansion, called “shrubification” at the expense of sedge species. Depending on intrinsic foliar properties of these plant species, changes in foliar fluxes with shrubification in the context of permafrost degradation may influence topsoil mineral element composition. Despite the potential implications for the fate of organic carbon in the topsoil, this remains poorly quantified. Here, we investigate vegetation foliar and topsoil mineral element composition (mineral elements that influence organic carbon decomposition: Si, K, Ca, P, Mn, Zn, Cu, Mo and V) from a typical Arctic tundra at Eight Mile Lake (Alaska, USA) across a natural gradient of permafrost degradation. Results show that foliar element concentrations are higher (up to 9 times; Si, K, Mo, and for some species Zn) or lower (up to 2 times; Ca, P, Mn, Cu, V, and for some species Zn) in sedge than in shrub species. This induces different foliar flux with permafrost degradation and shrubification. As a result, a vegetation shift over ~40 years from sedges to shrubs has resulted in lower topsoil concentrations in Si, K, Zn and Mo (respectively of 52, 24, 20 and 51%) in poorly degraded permafrost sites compared to highly degraded permafrost sites. For other mineral elements (Ca, P, Mn, Cu and V), the vegetation shift has not induced a marked changed in topsoil concentrations at this stage of permafrost degradation. This observed change in topsoil composition involving beneficial or toxic elements for decomposers is likely to influence organic carbon decomposition. These data can serve as a first estimate to assess the influence of other shifts in vegetation in Arctic tundra such as sedge expansion with wildfires