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137CS AND 40K IN DOMINATING PLANT GROUPS OF THE TUNDRA LANDSCAPES IN THE LOWER YENISEY REACHES
KOROBOVA, ELENA M. Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences.
Ukraintseva, Natalia G. Moscow State University, Geographical Faculty.
Peculiarity of the northern (tundra) ecosystems of the Siberian estuarine environment is in their development under severe natural conditions with snow cover existing for a long period, a short period of active life and a thin active soil layer over permafrost table. The latter determines formation of shallow soil containing weakly decomposed organic matter often saturated with water. Such conditions lead to conservation of air pollution in the top soil layers and its long-term retention and circulation in the local biogeochemical cycles, higher sensitivity of tundra ecosystems to contamination and their vulnerability. Flood plain areas in the tundra zone are characterized by better conditions due to river warming effect and accumulation of nutrient-enriched deposits. However river transports contaminants from the local sources (e.g. Krasnoyarsk Chemical and Mining Combine in the Yenisey river basin) that accumulate on flood plain and cause local contamination of soils and ecosystems in addition to the global fallout .
The main goal of the study was to evaluate modern contamination of the dominating plant groups of the lower Yenisey flood plain and watershed tundra landscapes caused the global fallout and local discharge, to compare it with the natural 40K background and to find possible relation between 40K and 137Cs transfer to plants.
Study sites were located in the Yenisey upper and middle delta and estuary on the island and right bank coastal flood plain and the adjacent high ancient terraces, characterizing typical and southern tundra landscapes. Vegetation was sampled at 1-3 m2 plots (averaged sample per plot) located over the soil profiles. Air dried and homogenized soil and plant samples were analyzed for gamma-emitters with the help of CANBERRA spectrometer with HP Ge-detector (performed by A. Borisov, GEOKHI, and S. Kirov, SPA "Radon"). Determination error did not exceed 25%.
Vegetation cover in general consisted of mosses, lichens and rare grasses with dwarf birch and willow on watersheds, willow and alder shrubs on slopes of the ridges and gullies, wet meadows and mires on the low- and medium-level flood plains, grassy and shrubby communities on high ridges and cores of the high-level flood plain. Phytomass considerably varied in structure and amount (from 0.2 to 3.5 kg/m2, dw). depending upon geomorphological position and the underlain rocks, and increased in southward direction.
Mean concentration of 137Cs in the main studied plant groups varied from 31 to 100 Bq/kg and increased in groups in the following order: grasses
Our data proved lower moss parts to be relatively enriched in radiocesium. Therefore in case of forage deficiency the ration of grazing reindeers can be enrich in radiocesium. 40K accumulation in the studied plants was significantly higher compared to 137Cs and increased in plant groups in the other order being most variable in green mosses, willow leaves and horsetail.
137Cs TF were within the range of that obtained for the Chernobyl fallout for natural meadows [1-3], but mean values appeared to be relatively high (Fig. 1, TF values relative to the top 0-10 cm soil layer, they did not change much when calculated relative to the deeper layer except for the cases of buried deep contaminated horizons of the flood plain soils.), corresponding more to the values for peaty swampy soils. These results conform to those obtained for peaty soils in the countries of the Arctic region and are comparable with TF obtained for mushrooms in temperate zones developing on the forest weakly decomposed litter . In our opinion this has the following explanations: 1)conservation of contamination in plants and the top soil layer; 2)low radionuclide concentration in soil that usually leads to higher element transfer; 3)higher availability for plants growing on organic peaty weakly decomposed matter.
Although no statistically significant correlation was found for most groups between 137Cs and 40K content and their TF values (except for mosses, see below), for the boundary groups there is a tendency of lower radiocesium transfer to plants with higher potassium uptake and vice versa (e.g. horsetail and mosses) and for alder and willow. Horsetail and willow were noted for higher 40K accumulation and lower 137Cs TF compared to mosses and alder respectively. This means that mosses and alder are more sensitive indicators of 137Cs contamination compared to horsetail and willow despite relatively high concentration of radiocesium in the latter.
Cs137 concentration in green moss (Hylocomium splendens) collected in different landscape conditions varied from 19 to 96 Bq/kg (DM) and showed weak decreasing tendency in its level in seaward direction (Fig 2). Higher Cs-137 concentration was detected on watersheds and in their depressions and fissures with maximum value in the sample collected on the south-eastern slope to the watershed lake (KR2-3) covered by thick alder shrubs. Natural 40K concentration in mosses was 2-14 times that of technogenic 137Cs. Although moss cover is treated as a natural collector of atmospheric contamination there was inverse correlation between 137Cs concentration and 40K/137Cs ratio (r=-0.695, n=8) suggesting secondary uptake of radiocesium discriminated by potassium.
Radiocesium content in willow leaves was maximum in accumulative watershed landscapes and was not affected much by radiocesium burried in flood plain alluvium layers. Among sampled species Salix lanata showed maximum accumulation ability.
Calculation of 137Cs and 40K content in the main components of biomass per square meter confirmed the significant role of mosses (their litter in particular) in radiocesium accumulation by tundra vegetation. The tendency of inverse 137Cs and 40K accumulation by plant biomass per square was also found.
Concentration of 137Cs in plants growing the Yenisey estuarine zone is comparatively low and corresponds to the levels found earlier in the regions subjected to global and regional fallout due to distant aerial radionuclide migration. Mosses and their lower parts in particular appeared to be more enriched in radiocesium compared to lichens growing at the same plots.
Tundra plants have remarkably high TF values comparable to those obtained for peaty and swampy soils of the temperate zone and tundra areas contaminated after the Chernobyl fallout. This confirms vulnerability of tundra landscapes in case of radionuclide contamination.
Concentration of radionuclide in plants depends upon landscape conditions and plant species. Maximum 137Cs accumulation has been found in the lower parts of mosses grown on watersheds in accumulative elementary landscapes collecting surface run-off during melting period and incorporating perched water. 137Cs accumulation in shrubs (willow and alder leaves) especially growing on soils rich in organic and of coarse texture can considerably contribute to reindeer meat contamination in summer period. Salix lanata showed maximum accumulation ability among sampled species.
Local enhanced concentration of radiocesium in buried alluvium on flood plain area enriched in fine sediment may not lead to higher radionuclide accumulation in plants due to strong 137Cs fixation by fine particles.
Horsetail and willow accumulating 40K to a higher extent had lower 137Cs TF compared to mosses and alder. Therefore mosses and alder are believed to be most sensitive indicators of 137Cs contamination in tundra landscapes and more significant for the local food chains compared to lichens, horsetail, and willow.
Acknowledgements. The work was supported by the project ICA-CT-2000-10008 "ESTABLISH". Authors are extremely grateful to the head of expedition Dr. Stepanets, the captain and the crew of RV ?Akademik Boris Petrov? for assistance in the field work.
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Figure 1. Cs-137 and K-40 transfer factors (TF, m2/kg) for different plant groups (x ? mean value)
Figure 2. Cs-137 in green mosses (Bq/kg, dw) in different Yenisey estuary sections and landscapes. SK - gulf area, VR - inlet area, KR - delta area, UP - upper delta area. 1 - eluvial landscape (watershed); 2 - transitional landscape (slope); 3 - accumulative landscape (depression); 4 - super aqueous landscape (flood plain)
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