Arbuscular mycorrhiza reduces phytoextraction of uranium, thorium and other elements from phosphate rock

Uranium transfer in grasses grown on mining waste and natural soil

The transfer of radionuclides from soil to the food chain often begins with uptake by plant root system. The roots of most angiosperms showed symbiosis with arbuscular mycorrhizal fungi (AMF) and to understand the transfer process of these toxic elements it is important to consider different physical, chemical and biological factors in soils. In the present study, three grass species (Poaceae), Zea mays, Chrysopogon zizanioides and Aristida setifolia were cultivated with and without organic fertilization in experimental blocks on natural soils, at Fazenda Vargem Formosa (VF) with low uranium (U) contents in the soil, and in the leached ore deposit at the Uranium Concentrate Unit Mine (URA) in Caetité (Uraniferous Province of Lagoa Real - Brazil). In the present study, the biomass production of plants, their rate of root colonization by AMF, the levels of U in soils, roots and leaves, as well as different physico-chemical parameters related to soil fertility were evaluated. The data analysis was performed using Artificial Neural Networks (ANNs), specifically Self-Organizing Maps (SOMs). The levels of available uranium in the soil ranges from 0.33 to 1.11 mg kg^-1 in VF and from 177.5 to 475.8 mg kg^-1 in URA. The results revealed high percentage of root AMF colonization, even in soils with high U contents. There was an inverse relationship between soil U content and its transfer to the plant organs, with U transfer rates being influenced by plant species and not by soil parameters. C. zizanioides had the lowest transfer factor to the shoot and the highest mass productivity under conditions of high U content in soil. The results indicate that C. zizanioides is an important species for use in the recovery of U mining areas.

Multiple environmental factors influence 238U, 232Th and 226Ra bioaccumulation in arbuscular mycorrhizal-associated plants

Ecological consequences of low-dose radioactivity from natural sources or radioactive waste are important to understand but knowledge gaps still remain. In particular, the soil transfer and bioaccumulation of radionuclides into plant roots is poorly studied. Furthermore, better knowledge of arbuscular mycorrhizal (AM) fungi association may help understand the complexities of radionuclide bioaccumulation within the rhizosphere. Plant bioaccumulation of uranium, thorium and radium was demonstrated at two field sites, where plant tissue concentrations reached up to 46.93 μg g^{-1 238}U, 0.67 μg g^{-1 232}Th and 18.27 kBq kg^{-1 226}Ra. High root retention of uranium was consistent in all plant species studied. In contrast, most plants showed greater bioaccumulation of thorium and radium into above-ground tissues. The influence of specific soil parameters on root radionuclide bioaccumulation was examined. Total organic carbon significantly explained the variation in root uranium concentration, while other soil factors including copper concentration, magnesium concentration and pH significantly correlated with root concentrations of uranium, radium and thorium, respectively. All four orders of Glomeromycota were associated with root samples from both sites and all plant species studied showed varying association with AM fungi, ranging from zero to >60% root colonisation by fungal arbuscules. Previous laboratory studies using single plant-fungal species association had found a positive role of AM fungi in root uranium transfer, but no significant correlation between the amount of fungal infection and root uranium content in the field samples was found here. However, there was a significant negative correlation between AM fungal infection and radium accumulation. This study is the first to examine the role of AM fungi in radionuclide soil-plant transfer at a community level within the natural environment. We conclude that biotic factors alongside various abiotic factors influence the soil-plant transfer of radionuclides and future mechanistic studies are needed to explain these interactions in more detail.

On the possible role of macrofungi in the biogeochemical fate of uranium in polluted forest soils

Interactions of macrofungi with U, Th, Pb and Ag were investigated in the former ore mining district of Příbram, Czech Republic. Samples of saprotrophic (34 samples, 24 species) and ectomycorrhizal (38 samples, 26 species) macrofungi were collected from a U-polluted Norway spruce plantation and tailings and analyzed for metal content. In contrast to Ag, which was highly accumulated in fruit-bodies, concentrations of U generally did not exceed 3mg/kg which indicates a very low uptake rate and efficient exclusion of U from macrofungi. In ectomycorrhizal tips (mostly determined to species level by DNA sequencing), U contents were practically identical with those of the non-mycorrhizal fine spruce roots. These findings suggest a very limited role of macrofungi in uptake and biotransformation of U in polluted forest soils. Furthermore, accumulation of U, Th, Pb and Ag in macrofungal fruit-bodies apparently does not depend on total content and chemical fractionation of these metals in soils (tested by the BCR sequential extraction in this study).