Seasonal cryogenic processes control supra-permafrost pore water chemistry in two contrasting Cryosols
Introduction
Circumpolar permafrost soils (i.e., Cryosols) represent 16% of the global soil area and store 1035 ± 150 Pg of organic carbon (C) within the first three meters (Hugelius et al., 2014). Cryosols are developed in both organic and mineral materials in which cryogenic processes such as freeze-thawing cycles, cryoturbation, frost heave, cryogenic sorting, cracking and ice segregation dominate pedogenetic processes (Jones et al., 2010, WRB, 2007). With permafrost aggradation and due to low temperatures and poor drainage, the soil weathering and microbial decomposition of organic matter has been limited since the last deglaciation (Davidson and Janssens, 2006, Lindgren et al., 2018, Ping et al., 2014). Consequently, a large pool of organic C has built up in weakly developed soils (Hugelius et al., 2014, Tarnocai and Bockheim, 2011).
Global permafrost warming (Biskaborn et al., 2019) currently leads to permafrost degradation through the gradual thickening of the active layer (seasonally thawed surface layer) (Romanovsky et al., 2015) and the development of thermokarst landforms (land surface subsidence due to the thawing of ice-rich permafrost and the melting of ground ice) (Kokelj and Jorgenson, 2013, Lafrenière and Lamoureux, 2013). The expected increase in precipitation will likely augment the stream discharge, the supra-permafrost contribution to river discharge amplifying the whole hydrological cycle of Arctic landscapes (Bintanja and Selten, 2014, McClelland et al., 2006, Peterson et al., 2002, Zhang et al., 2013). With permafrost degradation, the large pool of previously frozen soil organic matter (SOM) is exposed to decomposition and mobilization, thereby having the potential to generate greenhouse gas emissions that could fuel further global warming (Keuper et al., 2020, Turetsky et al., 2020, Turetsky et al., 2019). Warming and degradation of permafrost have been shown to increase solute concentrations in surface water bodies via surface and groundwater flow (Beermann et al., 2017, Fouché et al., 2017b, Frey and McClelland, 2009, Harms et al., 2014, Mann et al., 2015, Wickland et al., 2018) and release sediments into rivers (Jolivel and Allard, 2013, Kokelj et al., 2013, Lewis et al., 2012) because of enhanced weathering and element leaching (Drake et al., 2018, Kendrick et al., 2018, Kokelj et al., 2013, Vonk et al., 2019, Zolkos et al., 2018). Climate-driven changes in dissolution and solute mobilization from soils to streams will interplay with the carbon cycling in northern watersheds (Hobbie et al., 2002, Mack et al., 2004, Tank et al., 2012). The mobilization of organic and inorganic elements will evolve in uncertain pathways depending on permafrost soil formation histories, ages, geology, and climate conditions. Therefore, accounting for the contrasting pedogenetic processes in Cryosols and collecting data from soils to streams are crucial to better anticipate the evolution of biogeochemical cycles of northern ecosystems undergoing rapid changes (Ping et al., 2014, Zolkos et al., 2018, Bouchard et al., 2020, Ulanowski and Branfireun, 2013).
Within the Cryosol diversity, permafrost affected peatlands (i.e., Organic Cryosols) represent 14% of Cryosols. Turbic Cryosols (i.e., cryoturbated soils) dominate Cryosols developed in mineral parental materials representing 61% of northern Cryosols (ACECSS, 1998, Hugelius et al., 2014). Peatlands can be distinguished depending on their chemistry, their hydrology and the hosted plant communities between bogs (i.e. ombrotrophic peatlands) and fens (i.e. minerotrophic peatlands) (Payette et al., 2001, Vitt, 2006). In bogs, only atmospheric depositions provide water and nutrients. Consequently, bogs are strongly limited in nutrients and display low pH. In fens, the soil water is nutrient-rich, slightly acidic to neutral, originating from rainfall and lateral groundwater inputs, and the SOM is moderately to well decomposed (Vasander and Kettunen, 2006). Turbic Cryosols are developed in various mineral substrates such as glacial, loess or alluvial or marine deposits (Jones et al., 2010, Tarnocai and Bockheim, 2011).
The water availability in the active layer controls the nutrient availability for vegetation and microorganisms (Chu and Grogan, 2010, Schmidt et al., 2002). The composition of pore water (i.e., both capillary and gravitational) integrates atmospheric deposition, supra-permafrost groundwater chemistry, dissolution equilibrium with mineral phases in the soil porosity, root uptake and exudation as well as the dissolution and decomposition of the plant litter and SOM (Nieminen et al., 2013, Ulanowski and Branfireun, 2013). In Cryosols, water only occurs in the liquid state in the active layer during summer and in taliks (i.e., unfrozen ground surrounded by permafrost) strongly limiting pedogenetic processes (EDWARDS et al., 2006, French, 2007, Ping et al., 2014, Stutter and Billett, 2003). Successive summer leaching has depleted the active layer in major ions while freezing of solute-rich pore water has enriched the deep horizons that thaw at the decadal timescale (i.e., transient layer) (Lamhonwah et al., 2017b, Shur et al., 2005). The major solute concentrations could be ten to one hundred times higher in ice lenses just beneath the permafrost table than in the pore water of the active layer (Lamhonwah et al., 2017) thus creating a pool of solutes awaiting release with active layer thickening under warmer climates (Kokelj et al., 2002, Kokelj and Burn, 2005).
The biogeochemical functioning of Arctic watersheds has been mainly investigated through dissolved major solute concentrations and CO2, CH4 emissions in ponds and streams (Bagard et al., 2011, Frey and McClelland, 2009, Guo et al., 2007, Humborg et al., 2002, Petrone et al., 2006, Pokrovsky et al., 2012), limiting our ability to infer the intra-catchment processes responsible for the observed changes. Very few studies focused on pore water composition in Arctic Cryosols (Hobbie et al., 2002, Kokelj et al., 2002, Kokelj and Burn, 2005, Neff and Hooper, 2002, Prokushkin et al., 2007, Prokushkin et al., 2006, Prokushkin et al., 2005, Shibata et al., 2003), and even less used field lysimeters (Carey, 2003, Elberling and Jakobsen, 2000, MacLean et al., 1999, Raudina et al., 2017, Stutter and Billett, 2003), all consistently showing a major control of permafrost conditions and the nature of soils in water biogeochemistry. While some studies have focused on dissolved organic matter cycling (Prokushkin et al., 2006, Prokushkin et al., 2005, Wickland et al., 2018, Wickland et al., 2007) and inorganic elements (Carey, 2003, Kokelj et al., 2002, Raudina et al., 2017, Reyes and Lougheed, 2015, Shibata et al., 2003) in permafrost soils, none performed a high frequency monitoring of the pore water chemistry along summer, investigating the thaw front deepening control on pore water chemistry. As many studies showed that terrestrial ecosystems and surface waters in the Arctic undergo large temporal variations along the year (Beel et al., 2021, Csank et al., 2019, Fouché et al., 2017b, Hughes-Allen et al., 2021, Treat et al., 2018), investigation of seasonal variations of soil functioning is important to better understand the processes at the intra-catchment scale and at soil–water interfaces, which play an crucial role in biogeochemical cycles (Bouchard et al., 2020, Drake et al., 2015, Kokelj et al., 2020, Mann et al., 2015, Wologo et al., 2021). We assume that seasonal variations in permafrost physical states strongly affect soil processes while this control remains unclear and is current knowledge gap (Bouchard et al., 2020). Both chemical (e.g., dissolution of the mineral phase or release of previously frozen solutes) and biological processes (e.g., roots exudates or microbial activity) evolve from the snow melt and the start of deepening of the thaw front to the relatively warm period when the maximum thaw depth is reached and the following late summer experiencing some rainfall events. A better understanding of seasonal processes in Cryosols will provide insights on intra-catchment processes occurring in permafrost landscapes that affect global cycles.
In the present study, we aim at investigating the seasonal thaw effects on biogeochemical cycles at the soil profile scale, for two contrasted soils, in order to provide local processes that may explain the catchment-integrated effects previously observed. To do so, we aim to 1) capture differences in pore water chemistry in Cryosols displaying contrasting parental materials and formation histories, and 2) assess the controls of pore water chemistry evolution along summer. Consequently, we collected water samples daily using tension-free and tension lysimeters in the surroundings of Salluit, in Nunavik (Northern Québec, Canada) along a complete growing season. We monitored for the first time the intra-seasonal variations of concentrations of organic and inorganic solutes in soil pore water in relation with meteorological data, and temperatures, soil moisture and thickness of the active layer in Histic and Turbic Cryosols. We postulate that although the main differences in pore water chemistry are explained by soil parental materials, the seasonal climate (temperature and precipitation) and seasonal cryogenic processes (i.e., thaw front deepening) affect the soil solution chemistry along the growing season in both sites. Although the present study is restricted to a full summer, we believe it will provide a significant assessment of the local interplay between climate-induced active layer thickening and biogeochemical dynamics. We postulate that our results will help evaluating the catchment vulnerability to biogeochemical changes based on the soil extents.
Section snippets
Study sites and experimental design
The present study focused on two sites situated in Salluit (62°12′N; 75°38′W), lying within the deep continuous permafrost zone on the shore south of Hudson Strait. The permafrost was estimated to be thicker than 150 m under the village near the shore and the active layer thickness locally vary among permafrost landforms (Fouché et al., 2014, Gagnon and Allard, 2020). Measured in many boreholes for more than 30 years, the mean annual ground temperature is −5.6 °C at 23 m depth in gneissic
Results
Soil thermal regimes, variations in volumetric water contents, water table and thaw front depths during summer 2011 in both Histic and Turbic Cryosols are presented in Fouché et al. 2017a. Ground thaw occurred in three phases: 1- slow deepening of the thaw front from the beginning of June to mid-July, followed by 2- a quick thawing phase up to early August; 3- since mid-August, the maximum thaw depth was reached being respectively ~60 cm in the Histic Cryosol and ~100 cm in the Turbic Cryosol.
Water sources and chemistry from rainfall to soils and streams
Upon permafrost degradation, a portion of the permafrost organic and mineral pools is mobilized and processed from soils to surface waters. A better characterization the water chemistry in permafrost landscapes will allow for understanding intra-catchment processes that affect global cycles. The geochemistry of sampled rainfall water was dominated by Cl−, SO42− and Na+ illustrating the contribution of sea sprays from coastal wet and dry deposition. Rainfall pH was slightly higher than reported
Conclusion
The parental material composition and various water sources controlled the chemistry of both capillary and gravitational waters. Both soils displayed low concentrations in nitrates. Gravitational and capillary water chemistry only differed in the Turbic Cryosol, the latter being enriched in DOC and cations while being less concentrated in anions than the former. We assumed that Cl− was mainly provided by precipitation and sea sprays. On the other hand, cations were more concentrated in
Data availability
The authors declare that all data supporting the findings of this study are available within the paper and its supplementary information. All data associated with this study are available from the corresponding author upon request and will shortly be posted on the Polar Data Catalogue metadata website (https://www.polardata.ca).
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research is part of a PhD project that was funded by INSU, CNRS (grant n°19-2009-2012). Travels were helped by the PhD mobility grant from the Programme Frontenac funded by the “Fonds Québécois de Recherche sur la Nature et les Technologies” (FQRNT, grant n°163828). Financial support was provided by CNRS INSU Chantier Arctique, the Arctic Net Network of Centers of Excellence, and the Arctic Development and Adaptation on Permafrost in Transition (ADAPT) program funded by the Natural Science
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