Elemental composition of grass phytoliths: Environmental control and effect on dissolution.

Plant phytoliths, which represent the main pool of silica (Si) in the form of hydrous Si oxide, are capable of providing valuable information on different aspect of environmental issues including paleo-environmental reconstruction and agricultural sustainability. Phytoliths may have different chemical composition, which, in turn, affects their preservation in soils ad impacts terrestrial cycle of the occluded elements including micro-nutrients and environmental toxicants. Yet, in contrast to sizable work devoted to phytoliths formation, dissolution and physico-chemical properties, the mechanisms that control total (major and trace) elemental composition and the impact that various elements exert on phytolith reactivity and preservation in soils remains poorly known. In order to fil this gap in knowledge, here we combined two different approaches - analytical trace element geochemistry and experimental physical chemistry. First, we assessed full elemental composition of phytoliths from different plants via measuring major and trace elements in 9 samples of grasses collected in northern Eurasia during different seasons, 18 grasses from Siberian regions, and 4 typical Si-concentrating plants (horsetail, larch, elm and tree fern). We further assessed the dissolution rates of phytoliths exhibiting drastically different concentrations of trace metals. In the European grasses, the variations of phytolith chemical composition among species were highly superior to the variations across vegetative season. Compared to European samples, Siberian grass phytoliths were impoverished in Ca and Sr, exhibited similar concentrations of Li, B, Na, Mg, K, V, Zn, Ni, Mo, As, Ba, and U, and were strongly enriched (x 100-1000) in lithogenic elements (trivalent and tetravalent hydrolysates), P, Mn, Fe and divalent metals. Overall, the variations in elemental composition between different species of the same region were lower compared to variations of the same species from distant regions. The main factors controlling phytoliths elemental composition are the far-range atmospheric (dust) transfer, climatic conditions (humidity), and, in a lesser degree, local lithology and anthropogenic pollution. Despite significant, up to 3 orders of magnitude, difference in TE composition of grass and other plant phytoliths, the dissolution rates of grass phytoliths measured in this study were similar, within the experimental uncertainty, to those of other plants studied in former works. Therefore, elemental composition of phytoliths has relatively minor impact on their preservation in soils.

Assessment of soil development during rapid urbanization using the carbon and nitrogen stable isotope composition.

The proposed paper is devoted to the analysis of the urbanization effects on soil cover using physico-chemical parameters and stable isotopic signatures on the example of Tyumen. The study methods included analysis of C and N elemental and isotope (δ13C and δ15N) composition, analysis of soil physico-chemical properties and major oxides content. The results of the survey have shown that soil properties within the city limits vary significantly depending on both anthropogenic activities and geomorphological context. The urban soils of Tyumen differ from very strongly acidic with pH values down to 4.8 to strongly alkaline with pH values reaching 8.9, and from sandy loams to silty loams by texture. The study results have shown that δ13C values varied from - 33.86 to - 25.14‰ and δ15N values varied in range, especially, from - 1.66 to 13.38‰. The range of these signatures was smaller than those reported for urbanized areas in Europe and USA. The δ13C values in our case were more related to the geological and landscape parameters of the study area, rather than with urban disturbances and development of urban ecosystems. At the same time, the δ15N values probably indicate zones with increased atmospheric N deposition in the case of Tyumen. The application δ13C and δ15N is a promising tool for analysis of urban soil disturbances and functions, though the regional context should be taken into account.

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 unwarranted.

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 micro-components 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.

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.

Pollution status and human health risk assessment of potentially toxic elements and polycyclic aromatic hydrocarbons in urban street dust of Tyumen city, Russia.

This study investigated levels and sources of pollution and potential health risks associated with potentially toxic elements (PTEs) and polycyclic aromatic hydrocarbons (PAHs) in urban street dust collected from Tyumen city, a large transport centre with one of the highest motorization rates in Russia. Twenty street dust samples were collected from four grades of roads in five different land use areas. Research methods included measurements of physical and chemical properties of street dust, concentrations of 18 PTEs using inductively coupled plasma mass spectrometry, 12 PAHs using high-performance liquid chromatography, and statistical analysis of the data. Concentrations of Ni, Cr, Sb, and Mo, as well as medium and high molecular weight PAHs in urban street dust, were notably higher than in soils within the city, which indicates that transport is the main source of these elements. Concentrations of Cu, Cd, Pb, Zn, Mn, and As in street dust of Tyumen were lower compared to many large cities, while Cr, Ni, and Co were higher. Concentrations of PAH were comparable to other large nonindustrial cities. Total contamination of street dust by both PTEs and PAHs showed more robust relationships with the number of roadway lanes rather than land use. The estimated carcinogenic risks were low in 70% of samples and medium in 30% of samples. Noncarcinogenic risks were attributed to exposure to Co, Ni, V, and As. The total noncarcinogenic risk for adults was found to be negligible, while the risk was found to be moderate for children.

Vulnerability of the Ancient Peat Plateaus in Western Siberia.

Based on the data of the plant macrofossil and palynological composition of the peat deposits, the evolution and current state of polygonal peatlands were analyzed at the southern limit of continuous permafrost in the Pur-Taz interfluve. Paleoreconstruction shows that peat accumulation began in the Early Holocene, about 9814 cal. year BP, in the Late Pre-Boreal (PB-2), at a rate of 1 to 1.5 mm year-1. Intensive peat accumulation continued in the Boreal and early Atlantic. The geocryological complex of polygonal peatlands has remained a stable bog system despite the predicted warming and increasing humidity. However, a rather rapid upper permafrost degradation and irreversible changes in the bog systems of polygonal peatlands occur with anthropogenic disturbances, in particular, a change in the natural hydrological regime under construction of linear objects.

Polycyclic aromatic hydrocarbons in permafrost peatlands.

The concentrations of 15 individual PAHs in 93 peat cores have been determined by using high-performance liquid chromatography methods. In the profile the qualitative and quantitative composition of PAHs was non-uniform estimated in a wide range: from 112 to 3673 ng/g with mean 1214 ± 794 ng/g. Among 15 identified individual PAHs, the main contribution to their total amount was made by heavy highly condensed PAHs in the Eastern European peat plateaus, in particular, 6-nuclear benzo[ghi]perylene (1021 ± 707 ng/g), whereas in West Siberian permafrost peatlands, light PAHs were dominating, mostly naphthalene and phenanthrene (211 ± 87 and 64 ± 25 ng/g, respectively). The grass-equisetum peat contained the maximum of heavy PAHs and the dwarf shrub-grass-the minimum. In grass-dwarf shrub, grass-moss and moss peat, the share of 2-nuclear PAHs was most significant: naphthalene and fluorene, as well as 6-nuclear benzo[ghi]perylene. The presence of benzo[ghi]perylene in the entire peat strata, including its permafrost layer, was a marker of the anaerobic conditions that persisted throughout the Holocene and they were necessary for the synthesis of this compound.

Evaluating the effect of historical development on urban soils using microartifacts and geochemical indices.

The proposed research is devoted to the effects of the urbanization on the soil cover through the assessment of common organic and inorganic pollutants as well as anthropogenic microartifacts (MAs) over 20 sites characterizing different land use areas within the city of Tyumen, Russia. The analytical methods included measurements of physical-chemical properties of soils (total organic carbon content, pH, and texture), the total concentrations of potentially toxic elements (V, Cr, Co, Ni, Cu, Zn, As, Sr, and Pb) using X-ray fluorescence analysis, and the concentrations of 12 priority polycyclic aromatic hydrocarbons using high-performance liquid chromatography, and the evaluation of the composition of anthropogenic MAs assemblies by optical microscopy. The results of the study showed that the soils of all studied sites within the industrial areas were characterized by a high level of combined pollution with elements and compounds. For other sites, the duration of urbanization was the strongest factor that influenced the levels of pollution. The sites with the highest pollution levels were also characterized by the highest proportion and diversity of MAs. In contrast to the urban soils of the cities in North America and Western Europe, anthropogenic MAs associated with the use of coal (ash, slag, and silicon spheres) were present, but to a far lesser extent. Apparently, this is due to the fairly late development and intensive growth of Tyumen in the second half of the twentieth century, as well as the use of natural gas and fuel oil. At the same time, MAs associated with construction and domestic debris prevailed both in the soils of demolition sites and residential areas, which indirectly indicates the rapid and chaotic nature of urbanization, characteristic of Tyumen.

Lichen, moss and peat control of C, nutrient and trace metal regime in lakes of permafrost peatlands.

Permafrost thaw in continental lowlands produces large number of thermokarst (thaw) lakes, which act as a major regulator of carbon (C) storage in sediments and C emission in the atmosphere. Here we studied thaw lakes of the NE European permafrost peatlands - shallow water bodies located within frozen peat bogs and receiving the majority of their water input from lateral (surface) runoff. We also conducted mesocosm experiments via interacting lake waters with frozen peat and dominant ground vegetation - lichen and moss. There was a systematic decrease in concentrations of dissolved C, CO2, nutrients and metals with an increase in lake size, corresponding to temporal evolution of the water body and thermokarst development. We hypothesized that ground vegetation and frozen peat provide the majority of C, nutrients and inorganic solutes in the water column of these lakes, and that microbial processing of terrestrial organic matter controls the pattern of CO2 and nutrient concentrations in thermokarst lakes. Substrate mass-normalized C, nutrient (N, P, K), major and trace metal release was maximal in moss mesocosms. After first 16 h of reaction, the pCO2 increased ten-fold in mesocosms with moss and lichen; this increase was much less pronounced in experiments with permafrost peat. Overall, moss and lichen were the dominant factors controlling the enrichment of the lake water in organic C, nutrients, and trace metals and rising the CO2 concentration. The global significance of obtained results is that the changes in ground vegetation, rather than mere frozen peat thawing, may exert the primary control on C, major and trace element balance in aquatic ecosystems of tundra peatlands under climate warming scenario.

Dispersed ground ice of permafrost peatlands: Potential unaccounted carbon, nutrient and metal sources.

The physical and chemical consequences of massive ground ice (wedges) melt upon permafrost thaw is one of the central issues of environmental research linked to climate warming in the Arctic. Little is known about the chemical properties of dispersed ground ice abundant throughout permafrost peatlands that can easily melt with increasing active layer thickness (ALT). This is especially pertinent in continental lowlands, that account for sizeable areas of the Arctic, and contain high amount of organic carbon in both solid (peat) and liquid (porewater) phases. Here we studied 8 peat cores (0-130 cm depth)-comprised of porewater from the active layer (0-45 cm) as well as ice dispersed in frozen peat (40-130 cm)-across a latitudinal profile of Western Siberia Lowland (WSL) extending from discontinuous into continuous permafrost zones. Dissolved Organic Carbon (DOC), alkali and alkaline-earth metals (Ca, Mg, Sr, Ba, Li, Rb, Cs), sulfate, phosphorus, some trace elements (Al, Fe, Mn, Zn, Ni, Co, V, As, Y, REE, Zr, Hf, U) were sizably [more than 3 times] enriched in peat ice compared to peat porewaters from the active layer. In most sampled cores, there was a local maximum of strong enrichment (up to factors between 14 and 58) in DOC, P, Ca, Mg, Mn, Fe, Sr, As located 30-50 cm below the active layer. This maximum likely occurred due to solute concentration during full freezing of the soil column during winter. There was a sizable correlation between DOC, Al, Fe and other major and trace element concentrations that suggests strong control of organic complexes and organo-mineral (Al, Fe) colloids on element migration throughout the peat profile. The pool of C, major cations and trace metals in peat ice (40-130 cm) was approximately 3-55 times higher than the pool of these elements in porewaters from the active layer (0-40 cm). A 1-m increase of the ALT over the next 100 years is capable of mobilizing 58 ± 38 Tg of DOC from soil ice into the rivers and lakes of the WSL latitudinal belt (63-67 °N). This fast lateral export of C (3.7 ± 2.7 t C km-2 y-1) may double current C yields in WSL rivers (3.4 ± 1.3 t C km-2 y-1). A strong increase (150-200%) in riverine export of Zn, P and Cs may also occur while other micronutrients (Fe, Ni, Co, Ba, Mo, Rb) and toxicants (Cd, As, Al) may be affected to a lesser degree (20-30% increase). We propose a global peat ice inventory in permafrost regions is essential for assessing the consequences of permafrost thaw on surface aquatic systems.

Lake Drainage in Permafrost Regions Produces Variable Plant Communities of High Biomass and Productivity.

Climate warming, increased precipitation, and permafrost thaw in the Arctic are accompanied by an increase in the frequency of full or partial drainage of thermokarst lakes. After lake drainage, highly productive plant communities on nutrient-rich sediments may develop, thus increasing the influencing greening trends of Arctic tundra. However, the magnitude and extent of this process remain poorly understood. Here we characterized plant succession and productivity along a chronosequence of eight drained thermokarst lakes (khasyreys), located in the low-Arctic tundra of the Western Siberian Lowland (WSL), the largest permafrost peatland in the world. Based on a combination of satellite imagery, archive mapping, and radiocarbon dating, we distinguished early (<50 years), mid (50-200 years), and late (200-2000 years) ecosystem stages depending on the age of drainage. In 48 sites within the different aged khasyreys, we measured plant phytomass and productivity, satellite-derived NDVImax, species composition, soil chemistry including nutrients, and plant elementary composition. The annual aboveground net primary productivity of the early and mid khasyrey ranged from 1134 and 660 g·m-2·y-1, which is two to nine times higher than that of the surrounding tundra. Late stages exhibited three to five times lower plant productivity and these ecosystems were distinctly different from early and mid-stages in terms of peat thickness and pools of soil nitrogen and potassium. We conclude that the main driving factor of the vegetation succession in the khasyreys is the accumulation of peat and the permafrost aggradation. The soil nutrient depletion occurs simultaneously with a decrease in the thickness of the active layer and an increase in the thickness of the peat. The early and mid khasyreys may provide a substantial contribution to the observed greening of the WSL low-Arctic tundra.

Enhanced particulate Hg export at the permafrost boundary, western Siberia.

Arctic permafrost soils contain large amounts of organic carbon and the pollutant mercury (Hg). Arctic warming and associated changes in hydrology, biogeochemistry and ecology risk mobilizing soil Hg to rivers and to the Arctic Ocean, yet little is known about the quantity, timing and mechanisms involved. Here we investigate seasonal particulate Hg (PHg) and organic carbon (POC) export in 32 small and medium rivers across a 1700 km latitudinal permafrost transect of the western Siberian Lowland. The PHg concentrations in suspended matter increased with decreasing watershed size. This underlines the significance of POC-rich small streams and wetlands in PHg export from watersheds. Maximum PHg concentrations and export fluxes were located in rivers at the beginning of permafrost zone (sporadic permafrost). We suggest this reflects enhanced Hg mobilization at the permafrost boundary, due to maximal depth of the thawed peat layer. Both the thickness of the active (unfrozen) peat layer and PHg run-off progressively move to the north during the summer and fall seasons, thus leading to maximal PHg export at the sporadic to discontinuous permafrost zone. The discharge-weighed PHg:POC ratio in western Siberian rivers (2.7 ±â€¯0.5 µg Hg: g C) extrapolated to the whole Ob River basin yields a PHg flux of 1.5 ±â€¯0.3 Mg y-1, consistent with previous estimates. For current climate warming and permafrost thaw scenarios in western Siberia, we predict that a northward shift of permafrost boundaries and increase of active layer depth may enhance the PHg export by small rivers to the Arctic Ocean by a factor of two over the next 10-50 years.