Yakovleva E., Gabov D. Polycyclic aromatic hydrocarbons in plants of natural frost mound bogs // Principy èkologii. 2019. № 2. P. 119‒128. DOI: 10.15393/j1.art.2019.8822

Issue № 2

Original research

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Polycyclic aromatic hydrocarbons in plants of natural frost mound bogs

Introduction

Polycyclic aromatic hydrocarbons (PAHs) are organic compounds of the benzene series, which, on the one hand, are carcinogenic and mutagenic compounds (Hamid et al., 2017; Liu et al., 2017), on the other hand – they can be synthesized and used as nutrient materials (Аниськина, Яковлева, 2016). Plants of natural habitats, according to a number of authors, contain small amounts, mainly light polyarenes (Migaszewski et al., 2009; Dijk et al., 2015; Яковлева и др., 2016).

At high latitudes, bumpy swamps occupy a significant part of the land surface and are the main carbon reservoirs (Пастухов и др., 2018). At the same time, little attention is paid to the study of the chemical composition of swamp vegetation. Most of the works are devoted to the elemental composition of macrophytes (Kufel et al., 2004; Манасыпов и др., 2012).There are data on the composition of PAHs of plants, growing on bogs, but they are devoted to the study of the impact of  burnout processes on changes in the composition of PAHs and do not deal with the natural phytocenoses (Vane et al., 2013). The study of the mass share of polyarenes in plants of bumpy swamps of the background territories will allow estimating the natural levels of these organic pollutants in forest-tundra ecosystems. The results can be further used when conducting environmental monitoring of the impact of industrial facilities on the plant communities of the bumpy swamps. The results of the studies will make it possible to trace the change in the composition of PAHs in different organs of plants in the conditions of anthropogenic impact and thus to investigate the patterns of absorption of polyarenes by different species.

In swamps, plants are in an undecomposed state for a long time, to a greater extent decompose grassy plants and leafy litter, to a lesser extent mosses (Головацкая, Никонова, 2013). The study of the composition of plant polyarenes of bumpy swamps will allow estimation the different plant species contribution into the formation of the composition of soil PAH. The study of the mechanisms of peat soils formation of as a depot of organic carbon is the most important task of modern research.

The aim of this work is to assess the background content of polyarenes in the dominant species of plants in bumpy swamps.

Materials

The research was carried out in the forest-tundra zone of the Komi Republic in the Seida river basin. The species composition of the studied swamp is extremely poor, according to preliminary data, it has about 60 species, including higher vascular plants, mosses and lichens. According to the complex hydrolog-morphological approach, the studied swamp belongs to the type of flat-bumpy swamps. In accordance with the Botanical and geographical principles (Юрковская, 1992), the swamp belongs to the type of grass-lichen swamps of the North-East European region: grass-shrub – moss-lichen on mounds, cotton-grass – sedge – sphagnum and sedge in depressions. The swamp is characterized by a well-defined micro-relief, largely due to permafrost processes. About 60% of the massif is occupied by elevations or mounds, the rest of the space is occupied by pits, swamps flooded with runoff hollows and secondary lakes. The complexity of the vegetation cover is also caused by microrelief. Flat mounds with a height of 0.8–1 m prevail. 

Due to the small-banded surface, the horizontal structure of the communities has a mosaic character, the curtains of low shrubs on lichen pads alternate with tall (up to 0.5 m) individuals of Betula nana or moss groups of micro-plants, including with the participation of fluff (Eriophotum vaginatum and Eriophotum russeolum), sedges (Carex limosa, Carex rotundata) and sphagnum mosses.

Lowering the microrelief of the flooded and formed predominantly by cotton grass-sphagnum and sedge-sphagnum phytocenoses. The herb-shrub layer is dominated by one of the following species: Eriophorum russeolum, Carex limosa, or C. rotundata. In addition to these, Carex cinerea, C. rostrata, and Comarum palustre are abundant in some cases, such as along the edge of a reservoir. Participation of other species is insignificant. The ground cover is formed by sphagnum mosses (mainly Sphagnum riparium, S. lindbergii, S. balticum).

Methods

Dominant plant species were selected from the surface of the swamp in three-fold repetition. The selection was carried out in early August 2018. The following species were selected on the knolls: Polytrichum strictum Brid., Eriophorum vaginatum L., Betula nana L. hollows – Sphagnum riparium Angstr., Carex limosa L., Eriophorum russeolum Fries, branches and leaves of Salix lapponum L. Near the swamp – Comarum palustre L., Carex aquatilis L., Equisetum fluviative L. along the edges of the swamp – branches and leaves of Betula pubescens Ehrh. and shoots of Picea abies L. age 1-7 years. The choice of plant species for the study was based on the Botanical composition of the peat of the studied bumpy swamps, which was based on Polytrichum strictum on the mounds and Sphagnum in the hollows. Other studied species are represented in the Botanical composition of peat to a lesser extent. Sampling coordinates – 67°03' S, 62°56' E. The underlying soils were characterized by an acidic reaction of the environment, the pH values varied in the range from 3.4 for the upper layers to 5.5 in the lower peat horizons.

Chemical and analytical studies of plants were carried out in the Central research center "Chromatography" Of the Komi Institute of biology, SC UrD RAS. The method for analyzing the mass fraction of PAHs in plants was described earlier (Яковлева и др., 2018). ASE-350 accelerated solvent extraction system (Dionex Corporation, USA) was used to extract PAHs from plants. A sample of the plant weighing 1 g was placed in an extraction cell and extracted three times with a mixture of methylene chloride: acetone (1:1) at a temperature of 100° C. Then the extracts were concentrated using the Kudern-Danish apparatus at a temperature of 70° C in the thermostat and the solvent was replaced with hexane. The 3 cm³ of the resulting sample concentrate was purified from inorganic impurities by column chromatography using Brockman grade II aluminum oxide. As an eluent, a 50 cm³ mixture of hexane : methylene chloride (4:1) was used. Eluate was concentrated using an Kuderna-Danish apparatus at a temperature in the thermostat of 85° C to a volume of 5 cm³, then 3 cm³ of acetonitrile was added and evaporated at a temperature of 90° C until the hexane was completely removed. The sample concentrate in acetonitrile was analyzed for PAH content by HPLC. The control of accuracy of the measurement results were performed using the standard sample Certified reference material BCR-683 (European commission community bureau of reference). Statistical processing to assess the reliability of discrepancies in average data was performed using the Student's t-test, P = 0.95. The Statistica-6 program was used for correlation analysis.

Results

11 individual PAHs were identified in the plants of bumpy swamps: naphthalene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, chrysene, benz[a]anthracene, benzo[b]fluoranthene, benzo[k]fluoranthene, benz[a]pyrene. PAHs in plants were mainly represented by low-molecular 2-4-nuclear structures, their share of the total amount of PAHs varied in the range of 96-100 % for different plant species. We assume that the presence of polyarenes in plants could be due to a number of factors: intracellular synthesis in plants, PAH absorption by plants from the soil, and the arrival of polyarenes from the atmosphere during long-range transport in forest tundra conditions. A high degree of correlation was found between the composition of PAHs in plants of different species, the coefficients varied from r = 0.63 to 1.00 (for n = 11, p < 0.05). The maximum correlation coefficients r = 0.99-1 (at n = 11, p < 0.05) were found between Sphagnum riparium, sedges and downy plants. Another group includes tree and shrub species: Betula nana, Picea abies shoots, Salix lapponum and Betula pubescens. The composition of the PAHs of the leaves of the Salix lapponum and Betula pubescens shrubs correlated with each other. The composition of PAHs in Comarum palustre and Equisetum fluviative was closely correlated, r = 0.99 (for n = 11, p < 0.05).

The maximum PAH content among swamp plants was found in Polytrichum strictum, Picea abies shoots, Betula pubescens branches, and to a lesser extent in Salix lapponum and Betula nana branches (Fig. 1).

 

Fig. 1. PAH content in plants of bumpy swamps, ng/g

 

These plants were characterized by an increased content of naphthalene, the mass fraction of which was from 85 to 91% of the total mass of PAHs in plants. For herbaceous plants, riverine horsetail (Equisetum fluviative), sphagnum (Sphagnum riparium), leaves of Lapland willow (Salix lapponum) and silver birch (Betula pubescens), the PAH content was close, 2-4 times lower compared to plants- accumulators, and the naphthalene content in these species varied from 31 to 53 %.

The accumulation of PAHs by different plant organs was studied (Fig. 2). The study of the accumulation of polyarenes in the living and dead parts of mosses showed that the total concentration of polyarenes in the dead part was 2 times higher in the case of Sphagnum riparium and 3 times higher in the case of Polytrichum strictum. This effect can be explained by the long-term accumulation of hydrocarbons in the dead part of mosses, which made up 70% of their total biomass. It should be noted that in previous studies, we identified similar trends in the accumulation of Pleurozium schreberi PAHs in the background areas of the southern tundra (Яковлева и др., 2016).

 

Fig. 2. Multiplicity of excess of the total content of PAHs in different plant organs. For mosses – the excess content of PAHs in the dead part over the content in the living one. For Comarum palustre: A – excess content in the leaves over the stems, B – excess content in the leaves over the roots. For Equisetum fluviatile – the excess content in the side branches over the stems. For other herbaceous plants – the excess content of PAHs in the underground part over the aboveground. For Betula nana: A – in the roots over the leaves, B – in the roots over the branches. For Salix lapponum and Betula pubescens – excess in the branches over the leaves.

 

 There was no significant excess of PAH content in the underground part over their mass fraction in the aboveground part for sedges and cotton grass, the multiplicity did not exceed 1.3. The same excess was found for the PAH content in the lateral branches of Equisetum fluviative over the mass fraction in the stem. We have showed different patterns of PAH accumulation in grassy plants in the background areas of the southern tundra. In Festuca ovina and Deschampsia caespitosa, the predominant accumulation of all PAHs in the underground part of plants was found in the background and polluted areas (Яковлева и др., 2016). The multiplicities of exceeding the total PAH content for the background plots were 2-2. 5 times. The fact of a smaller accumulation of PAHs by the roots of grassy plants in swamps can be explained by the large water-cutness of their place of growth, which excludes the subsidence of soil particles on the roots.

For Comarum palustre, the leaves, stems, and subterranean part were examined separately. The PAH content in the leaves and roots was close and 2 times higher than the mass fraction of PAH in the stems. Perhaps this is due to the fact that PAHs accumulated by leaves from the atmosphere and roots from the soil did not penetrate into the stems of the plant.

For Betula pubescens, the studied organs of which were branches and leaves, a three-fold excess of the PAH content in the branches above the leaves was found. To a lesser extent, this excess was found in the branches of Salix lapponum, it was 2 times compared to the leaves. For Betula nana, in the case of which the roots of the plant were also studied, it was also shown that the mass fraction of PAHs in the branches over the leaves exceeded 2 times. Betula nana roots had the highest PAH content, and their PAH concentrations were 1.3 times higher than in the branches. Our previous studies also showed predominantly root-derived PAHs in Betula nana in the background areas of south tundra (Яковлева, Габов, 2018). It was found that polyarenes can enter shrubs through the root system and concentrate on the surface of the bark, while the flow of PAHs inside the stems is minimal. The low PAH content in the leaves is due to both the leafiness of shrubs and the low level of atmospheric polyarenes in the background areas.

For shoots of Picea abies of the 1st year, the PAH content in the conifer above the branches was found to be 1.6 times higher. For shoots of 2-7 years, the reverse trend was revealed: the PAH content in the branches was higher than in the conifers. Multiplicities of excess reached 5 and naturally increased with increasing of shoots age. The predominance of PAHs in the branches is largely due to the presence of a powerful lipid layer on the branches of conifers, which is able to regulate the flow of PAHs due to their adsorption on the surface. A waxy coating is formed on the conifers, which prevents the penetration of PAHs inside. Probably, on the branches of the first year, the lipid layer, like the wax coating, is at the stage of formation, which leads to a greater accumulation of PAHs in the conifer. With age, an increase in the mass share of needles in the total biomass of shoots from 63% for shoots of the 1st year to 77% in shoots of the 7th year was also revealed. In shoots of Picea abies revealed the presence of only light polyarenes.

It should be noted that the total content of polyarenes for shoots of 2-7 years almost did not change depending on the age (Fig. 3).

    

Fig. 3. PAH content in the shoots of Picea abies, depending on age, ng/g. 1 – naphthalene mass fraction, 2 – total mass fraction of the rest PAH.

 

At the same time, the mass fraction of polyarenes in the 1st year shoots were 2 times higher. PAHs in Picea abies shoots were 92 % represented by naphthalene, which is characterized by affinity with plant vitamins and hormones. apparently, its large amount in the shoots of 1st year is associated with active growth processes.

Conclusions

1. The presence of 11 PAH structures was found in plants of background tundra swamps. Polyarenes in plants were represented mainly by light structures.
2. High levels of correlation were found that determine similar patterns of PAH accumulation between the composition of sphagnum mosses, cottongrass and sedge, as well as between tree and shrub species.
3. The maximum PAH content among swamp plants was found for Polytrichum strictum, Picea abies shoots, Betula pubescens, Salix lapponum and Betula nana branches.
4. Differences in the accumulation of PAHs by different plant organs were found. Close PAH content in the underground and aboveground parts of sedges and cottongrass are identified. For mosses, the increased content of PAH in the dead parts is shown, for large and small shrubs – in branches and roots. For shoots of Picea abies of the 1st year, the excess of PAH content in conifers over branches was found, for shoots of the 2nd-7th year, the reverse trend was revealed.
5. In the shoots of Picea abies, the maximum PAH content was found in shoots of 1st year, in the shoots of 2-7 years the mass share of polyarenes was the same.
6. Data on the content of polyamines in plants of natural tundra swamps can be used to monitor the contamination of wetlands with polyarenes. In particular, Polytrichum strictum and Sphagnum riparium mosses can be used to assess the level of PAH contamination as representatives of the hillock and hollow vegetation.
7. Based on the results obtained, it is possible to estimate the contribution of plants to the formation of the peat PAH composition at different stages of the formation of a peat Deposit.

References