Changes in radiocaesium uptake and distribution in wheat during plant development: a solution culture study

Spring wheat plants were grown in a ¹³⁷Cs labelled nutrient solution, either in the presence or absence of NH₄ as a secondary N source. Between 11 and 64 days after sowing (DAS), plants were harvested on nine occasions. The plants supplied with NH₄ and NO₃ had lower root ¹³⁷Cs Activity Concentrations (AC) than those supplied with NO₃ only. Shoot AC were equal in both nutrition treatments. Shoot and root ¹³⁷Cs AC (dry weight basis) showed the same trends with plant age in both nutrition treatments. Shoot AC almost doubled between 11 and 28 DAS after which they gradually decreased concomitant with a similar decrease in K concentrations. Root AC were always higher than shoot AC and increased to a maximum at 35 DAS after which they fluctuated. Expressed on a tissue water basis, the ¹³⁷Cs AC varied less during plant age than did dry weight based AC. Furthermore, root and shoot AC expressed on a tissue water basis were almost equal. It is shown that the initial increase in ¹³⁷Cs AC in both root and shoot can largely be explained by the initial dilution of absorbed ¹³⁷Cs in the unlabelled seedling tissues. No correlation was found between K and ¹³⁷Cs distribution among ears, leaves, stems and roots in 64 old wheat plants. NH₄ as a secondary N source in a nitrate nutrient solution marginally affected ¹³⁷Cs distribution.Abbreviations AC activity concentrations - DAS days after sowing FAX no corresponding author: +3216321997     

The EDTA Amendment in Phytoextraction of 134Cs From Soil by Indian Mustard (Brassica juncea)

Soil contamination with radiocaesium is a significant problem at any countries when a nuclear accident occurred. Recently, phytoextraction technique is developed to remediate the contaminated environment. However, the application is limited by the availability of the contaminant for root uptake. Therefore, a green house trial experiment of soil amendment with ethylene diamine tetraacetic acid (EDTA) has been conducted to examine ¹³⁴Cs availability for root uptake. Two groups of Indian mustard (Brassica juncea) were cultivated in ¹³⁴Cs contaminated soil. The soil in the first group was treated with EDTA amendment, while the other was not. Plant growth was observed gravimetrically and the ¹³⁴Cs concentration in soil as well as plants were determined using gamma spectrometry. The plant uptake capacity was determined as transfer factor (F_v), and the F_v values of 0.22 ± 0.0786 and 0.12 ± 0.039 were obtained for the soil treated with and without EDTA amendment, respectively. The phytoextraction efficiency of the plant cultivated in ¹³⁴Cs contaminated soil both with and without EDTA amendment was low. The EDTA amendment to the soil seems to enhance the ¹³⁴Cs availability for root uptake of Indian mustard and can still be considered to assist the field phytoremediation of contaminated soil.     

Root uptake and distribution of radiocaesium from contaminated soils and the enhancement of Cs adsorption in the rhizosphere

Uptake by roots from contaminated soil is one of the key steps in the entry of radiocaesium into the food chain. We have measured the uptake by roots of radiocaesium and its transfer to shoots of a heathland grass, sheep fescue (Festuca ovina L.) from two contrasting agricultural soils, a sandy podzol and a clayey calcareous soil. A culture device which keeps the roots separate from the soil was used thus allowing rhizosphere soil to be obtained easily and enhancing the effect of root action. Biomass production and ¹³⁷Cs in shoots and roots were recorded. Cs adsorption was studied on both the initial, nonrhizosphere soil and on rhizosphere soil in dilute soil suspension. Cs desorption was measured by resuspending subsamples of contaminated soil in solutions containing various concentrations of stable Cs. The proportion of Cs fixed, i.e. not readily desorbable, was calculated by comparison of the adsorption and desorption isotherms. Uptake by roots varied considerably between soils and did not appear to be diffusion limited. Root-to-shoot transfer did not differ for the two soils studied. Root action considerably enhanced Cs adsorption on the soils, particularly the in sandy podzol with a low Cs affinity. The value of K_d was increased by up to an order of magnitude. A large proportion of adsorbed Cs was found to be fixed, the K_d was up to seven times greater on desorption than adsorption, indicating that up to 80% of adsorbed Cs was not readily exchangeable. Root action had little effect on the fixed fraction. This revised version was published online in June 2006 with corrections to the Cover Date.     

Radiocaesium in an organic soil and the effect of treatment with the fungicide ‘Captan’

Soil fungi accumulate radiocaesium from contaminated soil and it has been hypothesised that this may alter the plant availability and movement of the radionuclide in soil. The effect of twice-monthly addition of an aqueous suspension of the fungicide ‘Captan’ on the changes in a peaty podzol soil at 2 sites, contaminated 2 or 3 years earlier by the injection of ¹³⁴Cs, has been quantified. The sites had different soil acidity and vegetation cover. The less acid soil (pH_water 5.0) had been improved by the addition of lime and fertilizer and was reseeded with grass and clover. The more acid soil (pH_water 3.8) was under hill grasses, herbs and heather. On both sites the addition of fungicide did not alter the amount or concentration of radiocaesium in plant material sampled monthly or the depth distribution of radiocaesium in the soil profile. The concentration of the fungal constituent, ergosterol, in the soil, measured monthly, was unaffected by the fungicide treatment but evidence was obtained from a pot experiment to show that ergosterol decomposes slowly in cold, wet soils. On the more acid soil, two weeks after the last application of fungicide, there was a decline in active fungi as measured by fluorescein diacetate staining. Chloroform fumigation of the more acid soil resulted in a small increase in the amount of ¹³⁴Cs exchangeable with 1 M ammonium acetate. Radiocaesium in seven different fungi grown in pure culture was found to be almost entirely extractable (> 95%) with 1 M ammonium acetate. Another, Amanita rubescens, showed some retention and 88% was extractable. These findings do not preclude the fungal biomass as an important soil component controlling plant availability of radiocaesium from acid, organic soils by maintaining radiocaesium in a biological cycle, but make it unlikely that any fixation by fungi in a chemical sense is involved.     

Sorption and release of radiocaesium from particulate matter of the Baltic coastal zone

Laboratory expriments have demonstrated that radiocaesium can be released in different proportions from Baltic sediments, depending on the type and origin of sediment, contact time and solid-to-liquid phase ratio. Rapidly accumulating sediments in areas affected by river discharge have much higher percentage of exchangeable radiocaesium than slowly accumulating marine sediments. The latter have been shown to {uptake radiocaesium from overlying sea water at high suspended loads. Pronounced radiocaesium gradients at sediment-water interface in Gda\'nsk Bay can be explained by either diffusion from pore water or desorption from sediment particles uprised by waves and/or bottom currents, or a combination of both. Desorption is likely to decrease with age of the sediment.     

Root uptake and translocation of radiocaesium from agricultural soils by various plant species

Plant uptake of radiocaesium from soil is an important pathway for the entry of this pollutant into the human food chain and so contributes to any assessment of the radiation dose following contamination. Large differences in soil–plant transfer factors have been reported for plant species grown on the same soils. Few studies have attempted to distinguish between differences in root uptake and root-to-shoot translocation. We have investigated the root uptake of radiocaesium from artificially contaminated soils and the subsequent translocation to shoots for various plant species grown on three agricultural soils. The effects of short contact times and potassium starvation or enrichment have been studied. The Cs adsorption properties of rhizosphere soils have been compared with those of the initial soils. The proportion of activity removed from soil is largely soil dependent. Root uptake properties have less effect, but appear to be species determined, and not influenced by soil properties. Differences in soil-to-shoot transfer factor arise from species-dependent differences in root-to-shoot translocation. Root-to-shoot activity ratios are not soil dependent. There was little effect of soil potassium status. Root action slightly enhanced Cs adsorption on one soil, probably due to mineral weathering associated with the release of nonexchangeable potassium.     

Genotypic effects in phytoavailability of radiocaesium are pronounced at low K intensities in soil

The differences in radiocaesium uptake between species were analysed in a series of solution culture and pot trials. Since radiocaesium uptake is very sensitive to the solution potassium (K) concentration, it was hypothesised that species depleting K in the rhizosphere to a larger extent, will have a higher radiocaesium uptake. Five species (bean, lettuce, winter barley, ryegrass and bentgrass) were grown for 18–21 days in nutrient solution spiked with ¹³⁷Cs and at 4 K concentrations between 0.025 and 1.0 mM. Shoot ¹³⁷Cs activities all decreased between 17- and 81-fold with increasing K supply. Shoot ¹³⁷Cs activities were 4-fold different between species at the lowest K supply and 3.4-fold different at high K supply. The same five species were grown in two ¹³⁴Cs spiked soils with contrasting exchangeable K but similar clay content. Shoot ¹³⁴Cs activities were up to 19-fold higher in the soil with lowest exchangeable K. Differences in shoot activity concentrations between the species were only 4.5-fold in the high K soil, but were 15-fold in the low K soil. Bulk soil solution ¹³⁴Cs and K concentration data were combined with radiocaesium uptake characteristics measured in solution culture to predict radiocaesium uptake from soil. Predictions were within 1.6-fold of observations in the high K soil but largely underestimated ¹³⁴Cs uptake in lettuce, ryegrass and barley in the low K soil. A solute transport model was used to estimate K and radiocaesium concentrations in the rhizosphere. These calculations confirmed the assumption that higher radiocaesium uptake is found for species that deplete K in the rhizosphere to a larger extent.     

Short rotation coppice as alternative land use for Chernobyl-contaminated areas of Belarus

Field experiments were conducted in the Chernobyl-affected area to assess if short rotation coppice (SRC) for energy production is a feasible alternative for contaminated land. Four willow clones were planted on sandy and peaty soil and the radiocaesium (137Cs) and radiostrontium (90Sr) transfer factors (TF) and yield relevant parameters were recorded during four growing seasons. The 137Cs and 90Sr soil-to-willow wood TF on sandy soil (second growing season) were on average 1.40+/-1.06 x 10(-3) m2 kg(-1) and 130+/-74 x 10(-3) m2 kg(-1), respectively. The 137Cs TF recorded for the peaty soil (fourth growing season or end of the first rotation cycle) was on average 5.17+/-1.59 x 10(-3) m2 kg(-1). The 90Sr-TF was on average 2.61+/-0.44 x 10(-3) m2 kg(-1). No significant differences between clones for the 137Cs and 90Sr-TF were observed. Given the high TFs and the high deposition levels, Belarus exemption levels for fuel wood were highly exceeded. The annual average biomass production for one rotation cycle on the peaty soil ranged from 7.8 to 16.0 t ha(-1) y(-1) for one of the clones, comparable with average annual yield figures obtained for western Europe. On the sandy soils, first-year yields were 0.25 t ha(-1) y(-1). These soils are not suitable for SRC production and should better be dedicated to pine forests or drought-resistant grasses.