Experimental assessment of tundra fire impact on element export and storage in permafrost peatlands

Extensive studies have been performed on wildfire impact on terrestrial and aquatic ecosystems in the taiga biome, however consequences of wildfires in the tundra biome remain poorly understood. In such a biome, permafrost peatlands occupy a sizable territory in the Northern Hemisphere and present an extensive and highly vulnerable storage of organic carbon. Here we used an experimental approach to model the impact of ash produced from burning of main tundra organic constituents (i.e., moss, lichen and peat) on surrounding aquatic ecosystems. We studied the chemical composition of aqueous leachates produced during short-term (1 week) interaction of ash with distilled water and organic-rich lake water at 5 gsolid L−1 and 20 °C. The addition of ash enriched the fluid phase in major cations (i.e., Na, Ca, Mg), macro- (i.e., P, K, Si) and micronutrients (i.e., Mn, Fe, Co, Ni, Zn, Mo). This enrichment occurred over <2 days of experiment. Among 3 studied substrates, moss ash released the largest amount of macro- and microcomponents into the aqueous solution. To place the obtained results in the environmental context of a peatbog watershed, we assume a fire return interval of 56 years and that the entire 0–10 cm of upper peat is subjected to fire impact. These mass balance calculations demonstrated that maximal possible delivery of elements from ash after soil burning to the hydrological network is negligibly small (<1–2 %) compared to the annual riverine export flux and element storage in thermokarst lakes. As such, even a 5–10 fold increase in tundra wildfire frequency may not sizably modify nutrient and metal fluxes and pools in the surrounding aquatic ecosystems. This result requires revisiting the current paradigm on the importance of wildfire impact on permafrost peatlands and calls a need for experimental work on other ecosystem compartments (litter, shrubs, frozen peat) which are subjected to fire events

Dispersed ice of permafrost peatlands represents an important source of labile carboxylic acids, nutrients and metals

Thawing of frozen organic and mineral soils and liberation of organic carbon (OC), macro- and micro-nutrients and trace elements from pore ice in high latitude regions represent a potentially important but poorly quantified retroactive linkage to climate warming. This is especially true for permafrost peatlands, occupying a sizable proportion of all permafrost territories and presenting a large and highly vulnerable stock of soil OC which can be subjected to fast thawing at currently circum-zero temperatures. The conventional method of assessing the labile water-soluble fraction of permafrost soils is aqueous extraction from dried soil. However, this technique does not allow collecting native ice present in soil pores and is therefore likely to underestimate or overestimate the pool of labile soil C and nutrients. Here, we present results of direct pore ice analyses performed on native peat cores from the western Siberia Lowland in comparison to the water extraction (10 and 100 gdry peat L-1) of soluble components from the same peat subjected to freeze drying. Aqueous leachates of permafrost peat from both thawed (0–45 cm) and frozen (45–130 cm) layers yielded high concentrations of DOC, nutrients, carboxylic acids and trace metals, comparable or higher to those in peat porewater and dispersed peat ice. We found strong (a factor of 3 to 30) enrichment in the frozen part of the core (below 45 cm, which is active layer depth) in dissolved OC, many carboxylates (acetate, formate, lactate, butyrate, propionate, pyruvate), inorganic nutrients (Si, P, N) and trace elements (Fe, Al, Mn, Zn, Sr and Ba). The dispersed ice which is present in peat below active layer represents highly labile reservoir of organic and inorganic nutrients which should be considered in permafrost thaw scenario

Bacterial number and genetic diversity in a permafrost peatland (Western Siberia): Testing a link with organic matter quality and elementary composition of a peat soil profile

Permafrost peatlands, containing a sizable amount of soil organic carbon (OC), play a pivotal role in soil (peat) OC transformation into soluble and volatile forms and greatly contribute to overall natural CO2 and CH4 emissions to the atmosphere under ongoing permafrost thaw and soil OC degradation. Peat microorganisms are largely responsible for the processing of this OC, yet coupled studies of chemical and bacterial parameters in permafrost peatlands are rather limited and geographically biased. Towards testing the possible impact of peat and peat pore water chemical composition on microbial population and diversity, here we present results of a preliminary study of the western Siberia permafrost peatland discontinuous permafrost zone. The quantitative evaluation of microorganisms and determination of microbial diversity along a 100 cm thick peat soil column, which included thawed and frozen peat and bottom mineral horizon, was performed by RT-PCR and 16S rRNA gene-based metagenomic analysis, respectively. Bacteria (mainly Proteobac-teria, Acidobacteria, Actinobacteria) strongly dominated the microbial diversity (99% sequences), with a negligible proportion of archaea (0.3–0.5%). There was a systematic evolution of main taxa according to depth, with a maximum of 65% (Acidobacteria) encountered in the active layer, or permafrost boundary (50–60 cm). We also measured C, N, nutrients and ~50 major and trace elements in peat (19 samples) as well as its pore water and dispersed ice (10 samples), sampled over the same core, and we analyzed organic matter quality in six organic and one mineral horizon of this core. Using multiparametric statistics (PCA), we tested the links between the total microbial number and 16S rRNA diversity and chemical composition of both the solid and fluid phase harboring the microor-ganisms. Under climate warming and permafrost thaw, one can expect a downward movement of the layer of maximal genetic diversity following the active layer thickening. Given a one to two orders of magnitude higher microbial number in the upper (thawed) layers compared to bottom (frozen) layers, an additional 50 cm of peat thawing in western Siberia may sizably increase the total microbial population and biodiversity of active cells

Soils and vegetation of the riverside floodplain in the hydrological continuum of the southern tundra within the Pur–Taz interfluve (Western Siberia)

Climate warming has significantly impacted the ecosystems of the Subarctic and Arctic. It has most strongly affected highly productive ecosystems, including those formed in river floodplains. Due to the initially high (background) values of NDVI, remote monitoring methods are not suitable for detecting changes in the biological productivity of floodplain vegetation. Research for both individual regions and landscapes is needed. However, for the floodplains of many rivers in Western Siberia, there are no primary descriptions of soils and vegetation. We have studied the soils and vegetation of the riverside floodplains in the lower reaches of the Taz River within the Pur–Taz interfluve. The studies were carried out within the hydrological continuum from the stream to the main Taz River. A regular change in soils and vegetation along the hydrological continuum was established, with fluvial processes intensifying. Ecosystems with the greatest diversity of plants, with thick layered soils such as Pantofluvic Fluvisol (Polyarenic, Polysiltic, Humic), are formed in the valleys of the tributaries of the Taz River on the natural riverside levee. The floodplain of the Taz River is distinguished by small differences in the heights of topographic elements, loamy soil texture, waterlogging and permafrost. The soils of the studied hydrological continuum were assigned to two Reference Soil Groups (Gleysol and Fluvisol). To describe the diversity of basic soil properties, six principal qualifiers and nine supplementary qualifiers were used. An assumption was made about the replacement of willow bushes by alder bushes during the warming period with the growth of some species of forbs (Parasenecio hastatus). The study made it possible to outline ways of further studying the floodplains of the Subarctic of Western Siberia

Evaluating the potential of capillary rise for the migration of Pt nanoparticles in Luvisols and Phaeozems (Western Siberia)

Numerous experiments with nanoparticles have recently led to a better understanding of the migration of colloids and larger particles in soils. However, it remains unclear how colloidal particles migrate in soil horizons without macropores, and whether they can move with the fl ow of capillary water. In this article, we tested the hypothesis that colloidal particles can be transported by water flow in capillary-sized soil pores. To test our hypothesis, column experiments with platinum nanoparticles were carried out. The columns contained undisturbed monoliths from the Luvisols and Phaeozems soil horizons in the southeast of Western Siberia. The lower part of the soil columns was immersed in a colloidal solution with platinum nanoparticles. Thus, we checked whether the nanoparticles would rise to the top of the columns. Platinum nanoparticles are a usable tracer of colloidal particle migration pathways. Due to the minimal background concentrations, platinum can be detected by inductively coupled plasma mass spectrometry (ICP-MS) in experimental samples. Due to their low zeta potential, nanoparticles are well transported over long distances through the pores. Our experiments made it possible to establish that the process of the transfer of nanoparticles with a fl ow of capillary water is possible in almost all the studied horizons. However, the transfer distances are limited to the fi rst tens of centimeters. The number of migrating nanoparticles and the distance of their transfer increase with an increase in the minimum moisture-holding capacity and decrease with an increase in the bulk density of soil horizons and an increase in the number of direct macropores. The migration of nanoparticles in capillary pores is limited in carbonate soil horizons. The transfer of colloidal particles through soil capillaries can occur in all directions, relative to the gravity gradient. Capillary transport plays an important role in the formation of the ice composition of permafrost soils, as well as in plant nutrition

Colloidal organic carbon and trace elements in peat porewaters across a permafrost gradient in Western Siberia

The majority of organic carbon (OC), nutrients, and dissolved trace elements in soil porewaters are present in the form of colloids which determine element transport, bioavailability, and overall impact on ecosystems. Climate warming and permafrost thaw in high latitudes will primarily affect the soil liquid phase thereby modifying delivery of colloids to the hydrological network and their role in C transport and emission. Here we studied colloids in peat porewaters across a natural gradient of sporadic, isolated, discontinuous and continuous permafrost zone in the Western Siberian Lowland (WSL), the largest peatland in the world. The depth of sampling and the microrelief (mounds and hollows) had a generally weak impact on the proportion of colloidal forms (3kDa 0.45 μm) of OC, major (Fe, Al, P, alkali and alkaline-earth metals) and trace elements (TE) including micronutrients (Zn, Mn, Ni, Co, Cu), toxicants (Sb, As, Cd, Pb) and geochemical tracers (trivalent and tetravalent cations). Considering all micro-landscapes together, there was no sizable change in the proportion of colloidal fraction of OC, Fe, Al, P, micronutrients and toxicants across the permafrost zones. The majority of colloidal forms of all elements of these groups were represented by a size fraction between 3 and 30 kDa and were essentially Fe-Al-organic compounds with an average Fe:Al:OC molar ratio of 1.9:1:308. Overall, the degree of impact from environmental factors on OC and metal distribution among various colloidal fractions can be classified as depth ≤ permafrost type < microlandscape. Applying a substitution “space for time scenario” for the climate warming and permafrost thaw in Western Siberia, we do not expect sizable changes in C and element colloidal status during active layer thickness increase and permafrost boundary shift northward. Future studies of colloids in peat ice (below the active layer) are needed to assess possible changes in delivery of C and metals from soil to rivers and onward into the Arctic Ocean under massive permafrost thawing in the WSL

Organic carbon, and major and trace elements reside in labile low-molecular form in the ground ice of permafrost peatlands: a case study of colloids in peat ice of Western Siberia

The fate of organic carbon (OC), nutrients and metals accumulated in thawing permafrost ice is at the forefront of environmental studies in the Arctic. In contrast to a fairly good understanding of the chemical nature of dissolved OC (DOC) and metals in surface Arctic waters, the speciation and colloidal status of solutes accommodated in the dispersed ground ice remain virtually unknown. Here we used a size fractionation procedure (centrifugal ultrafiltration) to quantify the proportion of colloidal (3 kDa to 0.45 μm) and conventionally dissolved low molecular weight (LMW<3 kDa) fractions of DOC, and major and trace elements in the porewater and ice of 5 peat cores sampled along a 400 km permafrost and climate gradient in the largest peatland in the world, the Western Siberian Lowland (WSL). We discovered that the strong (a factor of 2 to 10) increase in the total dissolved (<0.45 μm) concentration of DOC and most major and trace elements in the peat ice relative to the peat porewater from the thawed layer was essentially linked to an increase in the LMW<3 kDa fraction. This increase in the potentially bioavailable fraction in the peat ice relative to the porewater was especially pronounced for DOC, P and many trace elements including metal micronutrients, and was observed throughout all permafrost zones. This contrasted with element distribution in the upper (thaw) layer, where the majority of these elements were present in the colloidal pool. Following previous experiments on permafrost peatland surface waters, we hypothesized that the freeze-thaw cycles of peat porewater were responsible for generation of the LMW fraction in the bottom part of the peat core. Results of this study demonstrate that carbon, and macro- and micro-nutrients as well as trace metals in ground ice of permafrost peatlands are essentially present in a low molecular weight (<3 kDa) and potentially bioavailable form that can strongly impact the riverine export fluxes of solutes during permafrost thaw

Charcoals in the middle taiga podzols of Western Siberia as an indicator of geosystem history

A middle-taiga iron-illuvial podzol (Glossic Endogleyic Albic Podzol) was studied on an ancient eolian dune in the Bol’shoi Yugan River basin (the Ob’ River tributary, Surgut region), near the large-scale archaeological research site. The radiocarbon age of 31 charcoals was determined, and 8 variants of the location of charcoal-containing soil zones relative to other morphological patterns were identified. It was proved that charcoal can help in dating the time of mosaic pattern formation, and the development of podzolic horizon coincided with intense wildfires in the second half of the Holocene. It was found that charcoal-containing soil zones first appeared in soils about 5 ka ago. Charcoals older than 5 ka cal. BP were not found. Pyrogenic events became twice more frequent at the beginning of the third millennium BP, with their maximum in the middle of the third millennium BP. The frequency of pyrogenic events decreased noticeably at the very end of the second millennium BP. Many peaks of pyrogenic events during the last five millennia coincided with the periods of archaeological cultures. An assumption that the continuous existence of forest environment leads to the permanent burial of charcoal due to fall of a tree accompanied by its uprooting was partially confirmed

Fractionation of organic C, nutrients, metals and bacteria in peat porewater and ice after freezing and thawing

To better understand freezing - thawing cycles operating in peat soils of permafrost landscapes, we experimentally modelled bi-directional freezing and thawing of peat collected from a discontinuous permafrost zone in western Siberia. We measured translocation of microorganisms and changes in porewater chemistry (pH, UV absorbance, dissolved organic carbon (DOC), and major and trace element concentrations) after thawing and two-way freezing of the three sections of 90-cm-long peat core. We demonstrate that bi-directional freezing and thawing of a peat core is capable of strongly modifying the vertical pattern of bacteria, DOC, nutrients, and trace element concentrations. Sizeable enrichment (a factor of 2 to 5) of DOC, macro- (P, K, Ca) and micro-nutrients (Ni, Mn, Co, Rb, B), and some low-mobile trace elements in several horizons of ice and peat porewater after freeze/thaw experiment may stem from physical disintegration of peat particles, leaching of peat constituents, and opening of isolated (non-connected) pores during freezing front migration. However, due to the appearance of multiple maxima of element concentration after a freeze-thaw event, the use of peat ice chemical composition as environmental archive for paleo-reconstructions is unwarrante

Fractionation of organic C, nutrients, metals and bacteria in peat porewater and ice after freezing and thawing

To better understand freezing-thawing cycles operating in peat soils of permafrost landscapes, we experimentally modelled bi-directional freezing and thawing of the three sections of 90-cm long peat core collected from a discontinuous permafrost zone in western Siberia. We measured translocation of microorganisms and changes in porewater chemistry (pH, UV absorbance, dissolved organic carbon (DOC), and major and trace element concentrations) after thawing and two-way freezing of peat cores. We demonstrate that bi-directional freezing and thawing of a peat core is capable of strongly modifying the vertical pattern of bacteria, DOC, nutrients, and trace element concentrations. Sizeable enrichment (a factor of 2 to 5) of DOC, macro-(P, K, Ca) and micro-nutrients (Ni, Mn, Co, Rb, B) and some low-mobile trace elements in several horizons of ice and peat porewater after freeze/thaw experiment may stem from physical disintegration of peat particles, leaching of peat constituents and opening of isolated (nonconnected) pores during freezing front migration. However, due to the appearance of multiple maxima of element concentration after a freeze-thaw event, the use of peat ice chemical composition as environmental archive for paleo-reconstructions is unwarranted