Radioactive trace in semi natural grassland. Effect of 40K in soil and potential remediation
Liliana Eugenia del Carmen Ciuffo*
Financial support: This work was carried out as a part of the EC-project “Long term dynamics of radionuclides in semi-natural environments: derivation of parameters and modeling” in the EC-framework “Nuclear Fission Safety”. Research Contract # F14P-CT95-0022. European Commission - Nuclear Fission Safety Programme.
Keywords: 137Cs plant uptake, radioactive traces, radiocaesium, radiopotassium.
The uptake of radionuclides by plant roots constitutes the main pathway for the migration
of radiocaesium from soil to humans, via food chain. In this study
we assessed radiocaesium uptake by plant in order to piece together
information on factors affecting uptake processes, particularly K
supply and differential uptake among different plant species. Soil
contaminated by the
in soil are taken up by plants, thereby becoming available for further
redistribution within food chains. The understanding of mechanisms
that affect the radiocaesium uptake by plant species which growth
under field conditions, become a subject of increasing interest. Radiocaesium
bioavaibility in soil is strongly influenced by soil properties such
as K status, clay content, pH and soil organic matter (Absalom
et al. 2001). On the other hand, the radiocaesium availability
in soil is one of factors controlling ion uptake by plant roots. It
is generally accepted that, following the
are present in the environment either naturally or artificially. 137Cs
is an artificial radioelement, generated in the past by nuclear weapon
testing and the
K enters to the plant roots via ion channels or specific transporters, while Cs+ uptake takes place by means of K+ channels. On the other hand, 137Cs, an artificial radioelement and a non-strict analogue of K, is a competitor at the K+ uptake plant mechanisms (Smolders et al. 1996a; Smolders et al. 1996b; Marscher, 1998; Zhu and Smolders, 2000).
K+ and Cs+ ionic radius are 0.33 nm and 0.31 nm (hydrated ion) respectively, and the absorption rate are 26 μmol g-1 fresh wt per 3 hrs for potassium and 12 μmol g-1 fresh wt per 3 hrs for caesium. Despite its smaller diameter, Cs+ is taken up at a much lower rate than K+. Obviously, factors other than ionic diameter are involved in the uptake regulation, one of them being the affinity for membrane-bound carriers, or channels (Marscher, 1998). Consequently, high K concentration in soil acts as inhibitor and prevents Cs uptake. Radionuclide transfer from soil-to-plant is affected by a number of factors, summarized in Figure 1. It is well accepted that K+ uptake by plant roots is a consequence of two different, but parallel, mechanisms present on plasma membranes: a) high-affinity energized transporter system and b) low-affinity mechanism, usually associated with ion-channels (Smolders et al. 1996a; Smolders et al. 1996b; Zhu and Smolders, 2000). Thus, it becomes of great importance, to study the relationship between both elements in soil.
number of field studies in contaminated areas have been conducted
observations in semi-natural grassland suggest that the Transfer Factor
(TF) is a highly variable parameter that appears to be independent
of the radionuclide activity in soil (Ciuffo et al. 2002).
137Cs uptake was assessed for different plant species from
two sites in a semi-natural grassland, at the Giulia Alps,
present study assess the radiocaesium uptake by plants in order to
piece together information on factors affecting the uptake processes,
particularly K supply and differences among different plant species.
The study was conduced in a semi-natural grassland contaminated by
area of study was situated in the Tarvisio Woodlands, in
and grass were sampled simultaneously –within an area of one hectare
along three equidistant transect– on the same day (June 1999). Twenty-four
parallel samples of soil and grass were taken. Grass samples were
collected by cutting the total herbage growing at each 1 x 1-m plot,
samples: monoliths of 15 x
soil and grass samples were analyzed for 137Cs and 40K
by gamma spectrometry with a high purity Germanium detector (HPGe).
The 137Cs activity was decay-corrected to the date of the
Data were analyzed by using Statgraphic package.
performed an analysis of the natural 40K and artificial
137Cs radioelements present in soil from an area contaminated
To understand the effect of 40K soil content in 137Cs sorption by plant roots, we performed an analysis for the different plant species present in the sampled area. We are reporting data of the predominant plant species present: Taraxacum officinale, Trifolium spp., Graminaceae, Plantago sp.
We observed that different plant species behave in a different way with respect to 137Cs sorption against 40K soil content. The ANOVA evaluation of the 137Cs activity concentration in plants [Bq kg-1] with respect to the 40K activity present in superficial soil [Bq m-2], exhibited a statistically significant relationship between variables for Graminaceae and Taraxacum officinale (R= -0.67 and -0.56, respectively; P < 0.05; 95% confidence level). On the other hand, the ANOVA analysis indicates a relatively poor relationship between variables for Plantago sp. and Trifolium spp. (R = 0.43 and -0.09, respectively). Figure 2 shows the scattergram of 137Cs plant activity concentration for forage and different plants against 40K superficial activity concentration in soil.
Graminaceae and T. officinale, behave in a comparable way under field conditions, with increased K concentration in soil: higher K soil concentration causes low 137Cs uptake by plant roots, in agreement with previous results from other authors (Zhu and Shaw, 2000; Zhu and Smolders, 2000; Zhu et al. 2000). However, for forage samples, which comprise mixed species, a statically less significant relationship was observed than for individual species in particular (R= -0.47; P < 0.05; 95% confidence level). Evidently, when forage samples were considered, the individual effect of each species was dampered.
Several authors made a number of contributions, mainly by laboratory experimental studies (Smolders et al. 1996a; Smolders et al. 1996b; Zhu and Shaw, 2000), and demonstrate the inhibitory effect of potassium on the uptake of radiocaesium by plants. The effect is better expressed when 137Cs activity concentration in plant was related to the ratio of 137Cs/ 40K in soil.
To evaluate the effect of the ratio of 137Cs/40K in soil on plant uptake, we analyzed the effect of 40K concentration increment in soil on 137Cs absorption by plant, using field experimental values. Under field conditions, the ratio observed varied in a range of 0.5 to 1.3 (Figure 3). We should point out that total 137Cs and 40K activities were considered, the cationic fraction in the solution not was discriminated.
Figure 3 shows a comparison of the mobile means of 137Cs activity concentration in plant, for different plant species under study and forage samples against ratio of 137Cs/ 40K in soil. At higher 40K concentrations a lower absorption of 137Cs by plant was observed, an observation valid at low ratio values of 137Cs/ 40K in soil. However, for Plantago sp. and 137Cs/ 40K ratio values in soil higher than 0.8, we observed a sensible decrease in 137Cs absorption. This observation could be interpreted as a singular characteristic of Plantago sp.
The inhibitory pattern of K on Cs uptake by plant roots has been previously described by Sacchi et al. (1997). Several authors (Maathuis and Sanders, 1996; Ichida et al. 1999; Marscher, 1998; Zhu et al. 2000) provide evidence for the presence of two distinct pathways for root K+ uptake. One system of low selectivity for Cs+, Rb+ and K+, a carrier-mediated transporter, which operates at low external concentration of K+, probably driven by cation symport with H+. A second system highly selective for K+, channel mediated, which operates at high external K+ concentration (Maathuis and Sanders, 1996).
An inhibitory effect in the kinetics of Cs uptake, at low external K+ concentrations was described in the briophyte Riccia fluitans L. after preculture at different K concentrations. This observation and our present results agree with an inhibitory effect of K in Cs uptake at high affinity K channels (Casadesus et al. 2001).
Our results suggest that 137Cs plant uptake is better described when ratio of 137Cs/ 40K in soil is considered, because an increment of K concentration in soil inhibits 137Cs uptake by the plant. This observation suggests an inhibitory effect of potassium on radiocaesium uptake by plant roots.
Considering different plant species and uptake nutrients from soil, the analysis performed provides evidence that, under field conditions, K+ is more efficiently absorbed than Cs+.
We analyzed the influence of the ratio of 137Cs/ 40K in soil on 137Cs plant uptake. Under field conditions, the ratio 137Cs/ 40K varied in a range of 0.5 to 1.3. For most of the species, at higher 40K soil concentration a lower 137Cs uptake was observed, a fact that reflects the resulting effect of the complexity of factors controlling ion absorption from soil.
The present results suggest that K fertilization could prevent radiocaesium uptake under field conditions. Bioremediation is emerging as an alternative approach to reduce the contamination level of 137Cs, such as application of minerals or chemical fertilizers in agricultural production systems with low levels of contamination. However, further investigation is still needed to optimize these countermeasures.
Ciuffo thanks to the ICTP TRIL (
ABSALOM, J.P.; YOUNG, S.D.; CROUT N.M.J.; SANCHEZ A.; WRIGHT, S.M.; SMOLDERS, E.; NISBET, A.F. and GILLETT A.G. Predicting the transfer of radiocaesium from organic soils to plants using soil characteristics. Journal of Environment Radioactivity, 2001, vol. 52, no. 1, p. 31-43. [CrossRef]
BELLI, M.; BLASI, M.; MENEGON, S.; MICHELUTTI, G.; NASSIMBENI, P.; NAZZI, P.; PIVIDORI, G. and SANSONE, U. Radioecologia ed agricoltura nella pianura Friuliana. Notiziario dell’ENEA Sicurezza e Protezione, 1989, no. 21, p. 160.
BUNZL, K.; ALBERTS, B.; SCHIMMACK, W.; BELLI, M.; CIUFFO, L. and MENEGON, S. Examination of a relationship between 137Cs concentration in sloils and plant from alpine pastures. Journal of Environment Radioactivity, 2000, vol. 48, no. 2, p. 145-158. [CrossRef]
CASADESUS, J.; SAURAS, T.; GONZE, M.A.; VALLEJO, R. and BRÉCHIGNAC, F. A nutrient-based mechanistic model for predicting the root uptake of radionclides in Radiactive Polulants. In: Impact on the Environment, 2001, p. 209-239. ISBN 2-868883-544-9.
Liliana E.C.; BELLI, María; PASQUALE, Arthur; MENEGON, Sandro and
VELASCO, Hugo. 137Cs and 40K soil-to-plant
relatioshipin a natural grassland of the Giulie Alps,
CIUFFO, Liliana E.C.; VELASCO, Hugo; BELLI, María; SANSONE, Umberto. 137Cs sil-to-plant transfer for individual species in a semi-natural grassland. Influence of potassium soil content. Journal Radiation Research, 2003, vol. 44, no. 3, p. 277-283. [CrossRef]
ICHIDA, Audrey M.; BAIZABAL-AGUIRRE, Victor M. and SCHROEDER, Julian I. Genetic selection of inward-rectifying K+ channel mutants with reduced Cs+ sensitivity by random recombinant DNA shuffling mutagenesis and mutant selection in yeast. Journal of Experimental Botany, June 1999, vol. 50, special issue, p. 967-978. [CrossRef]
MAATHUIS, Frans J.M. and SANDERS, Dale. Characterization of csi52, a Cs+ resistant mutant of Arabidopsis thaliana altered in K+ transport. The Plant Journal, 1996, vol. 10, no. 4, p. 579-589. [CrossRef]
SACCHI, G.A.; ESPEN, L.; NOTILO, F.; COCUCCI, M. Cs+ uptake in subapical maize root segments: Mechanism and effect on H+ release, transmembrane electric potential and cell pH. Plant Cell Physiology, 1997, vol. 38, no. 3, p. 282-289.
SMOLDERS, E.; KIEBOOMS, L.; BUYSSE, J. and MERCKX, R. 137Cs uptake in spring wheat (Triticum aestivum L. Cv. Tonic) at varying K supply. I: A potted soil experiment. Plant and Soil, 1996a, vol. 18, no. 2, p. 205-209. [CrossRef]
SMOLDERS, E.; KIEBOOMS, L.; BUYSSE, J. and MERCKX, R. 137Cs uptake in spring wheat (Triticum aestivum L. Cv. Tonic) at varying K supply. II: The effect in solution culture. Plant and Soil, 1996b, vol. 18, no. 2, p. 211-220. [CrossRef]
THIRY, Y. and MYTTENAERE, C. Behaviour of radiocaesium in forest multilayeder soils. Journal of Environmental Radioactivity, 1993, vol. 18, no. 3, p. 247-257. [CrossRef]
ZHU, Yung G. and SHAW, George. Soil contamination with radionuclides and potential remediation. Chemosphere, 2000, vol. 41, no. 2, p. 121-128. [CrossRef]
ZHU, Y.G.; SHAW, G.; NISBET, A.F. and WILKINS, B.T. Effects of external potassium supply on compartmentation and flux characteristics of radiocaesium in intact spring wheat roots. Annals of Botany, 2000, vol. 85, no. 2, p. 293-298. [CrossRef]
ZHU, Yung and SMOLDERS, Erik. Plant uptake of radiocaesium: a review of mechanisms, regulation and application. Journal of Experimental Botany, October 2000, vol. 51, no. 351, p. 1635-1645. [CrossRef]
Note: Electronic Journal of Biotechnology is not responsible if on-line references cited on manuscripts are not available any more after the date of publication.