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Uptake of 134Cs in the shoots of Amaranthus tricolor and Amaranthus cruentus
Environmental Pollution 125 (2003) 305–312 www.elsevier.com/locate/envpol
Uptake of
134
Cs in the shoots of Amaranthus tricolor and Amaranthus cruentus
Shirong Tang*, Ziyuan Chen, Hongyan Li, Jiemin Zheng Institute of Nuclear Agricultural Sciences, Zhejiang University, Huajiachi Campus, Hangzhou 310029, PR China Received 27 November 2000; accepted 24 March 2003
‘‘Capsule’’: Amaranthus tricolor and Amaranthus cruentus responded differently to 134Cs and addition of (NH4)2SO4. Abstract Amaranthus tricolor L. and Amaranthus cruentus L. were grown in pots containing 7.5 kg soils artificially contaminated with three levels of 134Cs activity: 5.55105 Bq pot 1, 1.11106 Bq pot 1, and 1.665106 Bq pot 1, respectively. Forty-nine days after sowing and growth, plants were harvested. The plants growing in soils with increasing 134Cs concentrations showed increasing concentration of this radionuclide in shoots. There were significant differences in uptake of 134Cs applied to soils between and within the plant species, depending on the initial 134Cs concentrations. The plant species showed different responses to the addition of (NH4)2SO4 to soils. Biomass production of both species was reduced in pots treated with (NH4)2SO4. (NH4)2SO4 application decreased the uptake of 134Cs by A. tricolor but increased the accumulation of 134Cs by A. cruentus, showing that chemicals with the highest efficiency to enhance the desorption of 134Cs might play an unexpected role in transferring the radionuclide to shoots. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Uptake; 134Cs; Amaranthus tricolor; Amaranthus cruentus
1. Introduction Soils may become contaminated with radionuclides as a result of proximity to radio-mineral outcrops, or anthropogenically as a result of the development of nuclear technology. Contamination caused by longlived radionuclides, particularly 137Cs and 90Sr, poses a long-term environmental problem. Remediation technologies are needed to avert risk to humans or the environment from radionuclide-contaminated soils. Many chemical and physical approaches used to treat large volumes of soils marginally contaminated with concentrations of long half-life radionuclides have proved to be cost-prohibitive and time-consuming (Entry et al., 1996). As an emerging strategy, phytoextraction is considered to be cost-effective for clean-up of sites contaminated with low levels of long half-life radionuclides. Before this remediation technology is widely accepted, further research to elucidate mechan-
* Corresponding author. +86-571-86971528; fax: 86971421. E-mail address: [email protected] (S. Tang).
0086-571-
isms of phytoremediation is needed to enable the development of this potentially valuable technique. Problems such as how to screen a series of plants with appropriate biological characteristics that can rapidly accumulate significant quantities of radionuclides, and how to find a way to enhance the bioavailability of radionuclides to the plants, should be addressed. Many plants with the ability to accumulate radiocesium abnormally have been widely documented outside China (Entry & Watrud 1998; Lasat et al., 1997, 1998; Broadley & Willey, 1997; Willey & Martin, 1997; Entry et al., 1993, 1996; Sawidis, 1988; Wallace & Romney, 1972) but only a few species have been reported within China (Qiu, 1988). During the past few decades, Chinese scientists have focused their attention on searching for K-rich plant species in order to solve crop problems of potassium deficiency. As a result, a wide range of plant species were found to have an ability to accumulate potassium in the shoots, of which many are in the family Amaranthaceae (Tu et al., 1999; Li & Jin, 1990; Hu et al., 1980). The high proportion of potassium-rich species and chemical similarity of Cs to K led us to speculate that some strong radiocesium accumulators with high biomass and fast growth rate could be
0269-7491/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0269-7491(03)00124-6
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screened from those K-rich plants discovered so far. However, little research has been carried out on their ability to accumulate radiocesium and on their potentials for application into phytoextraction of radionuclide contaminated soils so far. A review of the literature shows that a wide variety of chemical extractants have been investigated during the past few years for their potential to desorb radiocesium (Lasat et al., 1997, 1998; Field et al., 1993; Gombert, 1993; Kirk and Staunton, 1989; Francis and Brinkley, 1976). Because the extent to which radiocesium is desorbed is affected significantly by soil characteristics, it is difficult to compare radiocesium release efficiency resulting from the application of some chemical extractants in these different studies. Thus, further research is still needed to characterize suitable chemical extractants for particular soil types. Two species, Amaranthus tricolor and A. cruentus, were selected for this study since they have contrasting characteristics in terms of mineral uptake. Both A. tricolor and A. cruentus are annual herbs and are widely distributed in South and North China. A. tricolor is about 50–150 cm high with an erect stem, flowering from June to August and bearing fruits from July to September. It is considered to be an important vegetable in summer since it is characteristic of abundant vitamin C, various amino acids, and calcium. A. cruentus is about 50–200 cm high with an erect stem, flowering from July to August and bearing fruits from September to October. Its characteristics of high biomass, high concentrations of minerals in the shoots, ability to absorb ‘‘insoluble nutritions’’, and compatibility with main agricultural crops, have made it widely used in agriculture as a green manure, and in stockbreeding as a feedstuff for animals. The objectives of this study are (1) investigation of 134Cs accumulation in shoots of A. tricolor and A. cruentus from the Amaranthaceae grown in pots treated with different levels of 134Cs activity; (2) identification of some appropriate chemical treatments with potential for desorbing 134Cs from the soil tested and study of their potential for enhancement of 134Cs uptake by both species.
2. Materials and methods The experiment was arranged in a randomized factorial design. A paddy soil (Li, 1992) with a silty loam texture sampled from the experimental agricultural land of Huajiachi Campus, Zhejiang University and two above mentioned plant species were used for the study. The entire experiment was replicated four times. A greenhouse pot experiment was conducted using 7.5 kg of the soil. Table 1 shows some properties of the soil tested. Soil organic matter, pH, CEC, soil texture were measured using methods described previously (Tang et
al., 1999). We tested this soil type because it is widely distributed in Southeast China and similar to the soil surrounding the Qin-shan Nuclear Power Station, also known as Qin-shan Phase I Nuclear Power Station, located at Hai Yan, Zhejiang Province, on the northern shore of the Hangzhou Bay, about 90 km from Hangzhou and 120 km from Shanghai. Fresh soil was sieved to pass a 3-mm mild sieve and kept in the darkness before use. The soil used for this experiment was artificially contaminated with different levels of 134Cs. The control treatment had no 134Cs added. The following treatments were applied to the soil: low 134Cs activity concentrations (AC) 5.55105 Bq/pot (LAC), medium 134Cs AC 1.11106 Bq/pot (MAC), and high 134Cs AC 1.665106 Bq/pot (HAC), respectively. Fifty ml solution containing the required 134 Cs AC was added to the soil, and well mixed. The mixed soil was then transferred into pots under which a suitable size plastic basin was placed at the bottom. Each pot was filled with water until the maximum water holding capacity (WHCmax) was reached. As soon as the water in pots had naturally evaporated dry (moisture content less than 10%), the pot was filled again with water until it reached the WHCmax. Two pots of the soil artificially contaminated with 1.665106 134Cs Bq/ pot in the above mentioned way were used to identify chemical extractants that enhance 134Cs desorption from soil to soil solution. The effectiveness of soil amendments in enhancing soil 134Cs desorption was dependent upon determined 134Cs activity concentration in soil solutions in response to the added soil amendments. The method used for this purpose was after Lasat et al. (1997). Soil treated with different soil amendments was watered to field capacity and kept at room temperature for 24 h before extracting the solution. Then, 0.1 mol/L solutions of the following extractants were used for study of 134Cs desorption: (1) weak organic acids; (2) ammonium salts; (3) potassium salts; Table 1 Physical and chemical characteristics of the soil used for this study Analysis items Total N (%) Total P(%) Available P (mg kg 1) Available K (mg kg 1) Available Cu (mg kg 1g) Available Zn (mg kg 1) Organic matter (%) CEC (cmol/100g soil) pH(H2O) pH(CaCl2) Soil texture Clay Silt Sand Soil texture (USDA)
0.06 0.24 58.10 22.74 7.33 19.81 1.50 7.27 7.14 6.89 9.7 74.7 15.6 Silty loam
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Fig. 1. Relative efficiency of 26 soil amendments in enhancing soil 134Cs desorption. 1=KOH; 2=C4H6O5; 3=L-C6H9N3O2; 4=H2C2O4; 5=Cu(CH3COO2)2; 6=CaCl2; 7=Ca(NO3)2; 8=(NH4)3BO3; 9=KH2PO4; 10=CuSO4; 11=Ca(H2PO4)2; 12=CuCl2; 13=Ca(CH3COO)2; 14=KCl; 15=deionized water; 16=K2CO3; 17=H2NCH2COOH; 18=CuNO3; 19=NH4H2PO4; 20=NH4HCO3; 21=K2SO4; 22=(NH4)NO3; 23=(NH4)2C2O4; 24=(NH4)2CO3; 25=KNO3; 26=NH4Cl; 27=(NH4)2SO4.
Fig. 2. 134Cs desorption in response to (NH4)2SO4 added to the contaminated soil. 134Cs activity concentration increased with transient increase of (NH4)2SO4 concentration by 0.1.
(4) calcium salts; (5) copper salts (Fig. 1). On the basis of screening results, we identified the most efficient extractant and compared the relative efficiency of this soil amendment in enhancing the desorption of 134Cs with different concentration treatments (Fig. 2). The exact procedures were as follows: a 300 g aliquot of the < 2 mm soil fraction and 100 ml of the chemical extractant were mixed well in an open-mouth cup. The mixture was transferred into a flat-bottom funnel with two Whatman No. 42 filter papers placed on the bottom. After 24 h, the mixture was extracted for soil solution using a vacuum pump. The resulting filtrate was then vacuum-passed through a 0.45 mm millipore
membrane. The filtrate was collected in a 100-ml plastic bottle and analyzed for determination of 134Cs activity concentrations. Ammonium sulphate was used to extract successively the soil sample with gradually increasing concentrations (0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 mol l 1). Three replicates were used for all treatments. Other pots waited for soil equilibration (6 weeks) following the addition of 134Cs before seeds of A. tricolor and A. cruentus were sown directly in the soil. After germination and 18 day’s growth, seedlings were thinned down to four plants per pot for A. tricolor and five for A. cruentus. Soil moisture content and maximum water holding capacity (WHCmax) were determined
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prior to the experiment. A 100-ml liquid fertilizer was supplied weekly to the plants at N, 130; P, 30; K, 10 mg kg 1 soil. The K fertilizer supply was low because previous research work showed that K+ can compete with casium during root uptake (Lasat et al., 1997; Shaw and Bell, 1991) and that low K concentrations can be favorable to radiocaesium uptake by plants (Lasat et al., 1997; Buysse et al., 1996). The soil was watered using deionized water as needed but with the same volume of water for each pot each time during the experimental period. Each 134Cs and (NH4)2SO4 treatment was replicated four times. The experiment was carried out in a greenhouse without climate control and at ambient temperature with natural illumination (light intensity 2500–4500 lux, and a photoperiod 15–16 h light/9–8 h dark). The experiment was performed during June and August 2000. After 42 day’s growth, 250 ml 0.4 mol/l (NH4)2SO4 was added to the treated pots and the same amount of deionized water to the control pots. Plants were harvested one week after the application of (NH4)2SO4. At harvest, the plants were cut 1 cm above the soil, and the shoots were washed with deionized water, recorded for biomass (fresh w.), dried at 70 C, and measured for constant dry matter yields (DM). Plant and soil solution samples were assayed for 134Cs activity in a high-resolution gamma ray spectrometer. The spectrometer was equipped with a well-type 33 inches. NaI (Tl) detector surrounded by heavy multishielding to reduce background radiation. After being amplified, the detector’s signals were fed to a single channel analyzer (FH-1901). A certified 134Cs solution (The Chinese Academy of Measurement Sciences, May 2000) was used as a standard for efficiency calibration in the above-mentioned counting system. The standard solution filled a glass test tube like those used for the samples. Each sample was counted repeatedly for 10min at least three times. If the repeated measurements of a sample had an accumulated standard error of less than 5%, the measurements were terminated. Background 134Cs was determined by averaging the results of replicate 10 min counts of blank vials. Net 134Cs values were obtained by subtracting this background value from sample values. All results were corrected for inefficiencies of counting, and if necessary, for decay. The lower limit of detection was calculated at the 95% confidence level from the above mentioned system background. K in the plant tissues was analyzed in a H2SO4–H2O2 digest by atomic absorption spectrophotometry (Spectra AA 220) using LiCl to suppress interference. All data were subjected to a three-way ANOVA in the SAS version 6.0 statistical package to allow comparison of treatment means. Differences among treatment means were considered at P < 0.005 using the UNIQUE sums of squares.
3. Results and discussion 3.1. Desorption of 134
134
Cs from contaminated soils
Cs activity concentrations in soil solution, on which the efficiency of soil amendments in enhancing soil 134Cs desorption was dependent, were examined 24 h after chemicals application to the contaminated soil. The most effective desorption of 134Cs was observed in + soil treated with NH+ containing salts. This 4 and K + + may be due to the ability of NH+ to 4 , K , and Cs form a homologous series with a great degree of physiochemical similarities (Finston and Kinsley, 1961) or due to concentration-dependent desorption of radiocesium (Lasat et al., 1997). Although our results show that ammonium salts have high efficiency desorbing 134Cs from the artificially contaminated soils, there still exists some difference in the efficiency among different ammonium salts. Lasat et al. (1997) suggested that NH4NO3 was the most efficient chemical in facilitating desorption of 137Cs from soil to soil solution. Our results showed that among the amendments tested, (NH4)2SO4 was the most effective in enhancing 134Cs desorption (Fig. 1), followed by NH4Cl. The higher the concentrations of (NH4)2SO4 were applied to the soil, the more 134Cs was released from the contaminated soils (Fig. 2). The high efficiency with (NH4)2SO4 treatment was also reported in Dushenkov et al. (1999). It is known that soil composition and texture, including pH, CEC, the K status in soil, the content and type of organic matter, and a clay concentration, were considered to be of importance in determining the transfer of radiocesium (Roca et al., 1997; Lembrechts, 1993; Cremers et al., 1990). Tyler and Olsen (2001) investigating the concentrations of 60 elements in the soil solution as related to the soil acidity concluded that pH value could play a crucial role in ions availability to plants, with availability at pH 5.2 being about 3 fold higher than at pH 7.0. Results from this study demonstrated that the extent by which (NH4)2SO4 addition reduced the soil pH is dependent on (NH4)2SO4 concentrations, ranging from 0.5 to 1.0 unit. The decrease in pH value after (NH4)2SO4 addition may suggest more 134 Cs availability and might explain the increase released 134Cs with increasing concentrations of (NH4)2SO4. Although Field et al. (1993) reported that some organic acids could successfully release 137Cs from contaminated soils, our results show that the organic acids were ineffective in enhancing the desorption of 134 Cs from contaminated soils (Fig. 1). Similar findings were reported by Lasat et al. (1997). However, inducing hyperaccumulation of heavy metals in plants was reported following organic acid addition (Peter, 1999; Anderson et al., 1998; Blaylock et al., 1997; Huang et al., 1997), suggesting that heavy metals have a response different from radiocesium to organic acid addition.
S. Tang et al. / Environmental Pollution 125 (2003) 305–312
This is probably because most of the organic acids are present in the solution as un-ionised molecules, which results in less influence on the desorption of radiocesium from its fixation into clay minerals. 3.2. Plant growth There were some variations in biomass production between A. tricolor and A. cruentus (Table 2). Both plant species yielded more shoot DM without (NH4)2SO4 than with (NH4)2SO4 treated (Table 2), showing addition of (NH4)2SO4 at 0.4 mol kg 1 soil depressed biomass production in both plant species. It is likely that this effect resulted from ammonium toxicity or from low pH value due to high (NH4)2SO4 levels. The responses to the addition of 0.4 mol of (NH4)2SO4 to the soil for A. tricolor and A. cruentus were different, as shown by the extent to which both plant species were affected by (NH4)2SO4 (Table 2). This effect may be due to the difference in tolerance to ammonium toxicity. A. cruentus may be more susceptible to ammonium toxicity than A. tricolor. For both plant species, (NH4)2SO4 addition caused a 10 to 20% reduction in the shoot biomass. 3.3.
134
Cs activity concentrations in the shoots
On the basis of the results of soil 134Cs desorption, we investigated the uptake of 134Cs in the shoots of A. tricolor and A. cruentus. Regardless of (NH4)2SO4 treatments, 134Cs activity concentrations in the shoots of both species were significantly affected by 134Cs activity in soil (Table 2). In general, the higher 134Cs activity in the soil, the more 134Cs was accumulated in the shoots. The accumulation of 134Cs by A. tricolor was dependent upon the initial application of 134Cs concentrations in
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soil in the case of no (NH4)2SO4 application. In control (no (NH4)2SO4) pots, A. cruentus accumulated progressively more 134Cs in its shoots with 134Cs activity increase in the soil (from LAC to MAC), but took up less with 134Cs activity of 1.665106 added to the soil (Table 2). It is interesting to note that A. tricolor and A. cruentus had different responses to (NH4)2SO4 addition. Addition of 0.4 mol of (NH4)2SO4 decreased the uptake of 134 Cs by A. tricolor but enhanced the accumulation of 134 Cs by A. cruentus grown in pots treated with low and medium 134Cs activity (from LAC to MAC) (Table 2). This suggests that NH+ 4 may have a negative effect on 134 Cs uptake by A. tricolor but may have a positive effect on 134Cs uptake by A. cruentus. Lasat et al. (1997) investigating the effect of (NH4)NO3 application on 137 Cs bioavailability and its accumulation in plants found that (NH4)NO3 application could slightly increase the uptake of 137Cs by Amaranthus retroflexus and Phaseolus acutifolius. Our results showed that (NH4)2SO4 application decreased the uptake of 134Cs by A. tricolor but increased the accumulation of 134Cs by A. cruentus in general, suggesting that there is a different response to particular soil amendment for different species. Regardless of (NH4)2SO4 application, the accumulation of 134Cs in shoots of both species was dependent upon initial 134Cs activity concentrations in the soil. Similar results were reported in Lasat et al. (1998), Demirel et al. (1994), and Shaw and Bell (1991). Greatest amounts of 134Cs were concentrated in shoots of A. tricolor grown in pots treated with high 134Cs activity concentration and with no soil amendment added but maximum 134Cs concentrations were accumulated in the shoots of A. cruentus grown in medium 134Cs activity treated pots with (NH4)2SO4 application. This suggests
Table 2 Biomass, 134Cs concentration in shoots, and bioconcentration ratios (AverageSD), Species
134 Cs added (Bq per pot)
A. tricolor
Control 5.55105 5.55105 1.11106 1.11106 1.665106 1.665106 Control 5.55105 5.55105 1.11106 1.11106 1.665106 1.665106
A. cruentus
a
(NH4)2SO4 added
Treatment
Yes No Yes No Yes No
LACA LACNOA MACA MACNOA HACA HACNOA
Yes No Yes No Yes No
LACA LACNOA MACA MACNOA HACA HACNOA
Biomassa (g/per pot, dry weight) 8.771.63 7.521.28A,a 10.040.94A,b 9.131.18A,a 11.532.65A,b 11.042.09A,a 11.680.74A,b 8.731.86A 7.050.85A,a 10.421.09A,b 4.483.81A,a 7.711.60A,b 7.722.35A,a 8.651.71A,b
134 Cs concentration in shootsa (Bq g 1, dry weight)
Bioconcentration ratiosa
26598A,a 375104A,b 612107B,a 73423B,b 966114C,a 1235171C,b
3.41 0.95A,a 2.41 0.89A,b 3.34 0.10A,a 2.78 0.49A,b 2.93 0.35A,a 3.74 0.73A,b
28054A,a 258106A,b 70864B,a 64848B,b 662118C,a 547414C,b
2.55 0.49A,a 2.35 0.09A,b 3.22 0.29A,a 2.94 0.22A,b 2.01 0.36A,a 1.66 1.25A,b
Within each column, values followed by the same letter are not significantly different as determined by UNIQUE sums of squares (p< 0.05) n=12 for all values. A, B, C . . .=134Cs added; a, b, c. . .=soil amendments. Bioconcentration ratio=Bq 134Cs g 1 in shoot/Bq 134Cs in soil.
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that inter-species differences and initial radiocesium concentration in the soil still have an influence on accumulation of 134Cs in shoots. For control (ammonium free) pots, A. tricolor generally had significantly higher 134Cs bioaccumulation ratio (BR) than A. cruentus (Table 2). (NH4)2SO4 addition at 0.4 mmol kg 1 soil significantly (P < 0.05) increased the bioaccumulation ratios in A. tricolor but decreased the ratios in A. cruentus, suggesting a different response to (NH4)2SO4 addition for different species. Such a difference in response to (NH4)2SO4 addition may be due to the difference in biological characteristics of the species. The greatest BR (3.56) was obtained with A. tricolor grown in pots treated with high 134Cs activity concentrations and (NH4)2SO4. In control (ammonium free) pots, the 134Cs bioaccumulation ratios varied between 2.41 and 2.79 for A. tricolor, between 2.20 and 3.22 for A. cruentus, respectively, while in pots treated with (NH4)2SO4, the value ranged between 3.34 and 3.56 for A. tricolor, and between 2.35 and 2.94 for A. cruentus, respectively (Table 2). Lasat et al. (1997) reported that plant species grown in ammonium free pots had the 137Cs bioaccumulation ratios ranging from 0 to 1.0. Dushenkov et al. (1999) showed that the bioaccumulation ratios of all Amaranthus species varied from 0.30 to 2.03. Olive and orange trees grown on calcareous and acid soils had bioaccumulation ratios of 134 Cs usually less than 0.05 (Skarlou et al., 1999). Our results showed that the two species from the Amaranthaceae grown in pots treated with or without (NH4)2SO4 had higher bioaccumulation ratios than the above mentioned ones, being at the high range of reported radiocesium bioaccumulation ratios (Coughtry et al., 1983; Dushenkov et al., 1999).
Lasat et al. (1997) reported that (NH4)2SO4 addition increased the bioaccumulation ratios in Phalaris arundinacea, Brassica juncea, Phaseolus acutifolius, and B. oleracea var. capitata, showing a positive response. However, Dushenkov et al. (1999) investigating the effect of soil amendments on 137Cs accumulation in Indian mustard and sunflower showed that the EDTA and DTPA addition increased the bioaccumulation ratios of Helianthus annuus but decreased the ratios of Brassica juncea. Our results also show that not all plant species show a positive response in uptake of radiocesium to (NH4)2SO4 addition, being similar to Dushenkov et al. (1999). 3.4. Potassium concentrations in the shoots The K concentrations in the shoots of A. tricolor and A. cruentus were statistically analyzed using ANOVA and were significantly (P < 0.05) affected by plant species. 134Cs concentrations added into the soil and (NH4)2SO4 addition did not significantly affect the K concentration in the shoots of both species. A. cruentus generally accumulated more K than A. tricolor. The K concentration in the shoots of A. cruentus ranged between 5.12 and 3.60% with an average of 4.42% and standard deviation of 0.50 while A. tricolor has K concentration varying from 4.88 to 2.70% with an average of 3.76% and standard deviation of 0.68 in the shoots. A. cruentus was known to be a K-rich species (Tu et al., 1999) but A. tricolor was not. Such a difference might explain why A. cruentus had more K in the shoots than A. tricolor even in our case where the K fertilizer supply was low. A literature review shows that K concentration in the growth media was an important factor influencing
Fig. 3. Relationship between K and 134Cs concentrations in shoots of A. tricolor and A. cruentus (Cs=1278.71–158.23K, P <0.05).
S. Tang et al. / Environmental Pollution 125 (2003) 305–312 134
Cs absorption and movement within plant species (Lasat et al., 1997; Buysse et al., 1996) because of its similarity to cesium in chemistry. Strandberg et al. (1998) reported that plants grown in K treated soil had decreasing uptake of radiocesium. Rosen (1991) also reported similar effects from agricultural ecosystems where potassium fertilizing (100 kg ha 1) maximally reduced the cesium uptake by the crops by a factor of 10. However, Broadley and Willey (1997) investigating differences in root uptake of radiocesium by 30 plant taxa concluded that there was a strong relationship between mean shoot Cs and K concentration within the Gramineae and Chenopodiaceae families. Our experiment performed in the pot experiment demonstrated a weak relationship between shoot 134Cs activity and K concentrations in A. tricolor and A. cruentus (Fig. 3).
4. Conclusions We found that both initial 134Cs activity concentrations in the soil and inter-species differences have an influence on the 134Cs accumulation in the shoots of A. tricolor and A. cruentus. It was also found that there were different responses to the addition of 0.4 mol of (NH4)2SO4 to the soil for A. tricolor and A. cruentus. (NH4)2SO4 application decreased the uptake of 134Cs by A. tricolor but increased the accumulation of 134Cs by A. cruentus, suggesting that chemicals with the highest efficiency to enhance the desorption of 134Cs might play an unexpected role in transferring the radionuclides to shoots when applied into the soils. Potassium distribution in shoots is significantly affected by plant species and slightly by 134Cs treatments.
Acknowledgements This work was supported by a project from the Ministry of Science and Technology, P.R. China (Grant Number: NKBRSFG 1999011808). Dr. Tang wants to thank Professor Alan Baker from the School of Botany, University of Melbourne, Professor Slavik Dushenkov from Biotech Center, New Jersey, Professor Nicholas Lepp from Liverpool John Moores University, and three anonymous reviewers for their constructive advice.
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Li, Q.K., 1992. Paddy Soils of China. Science Press, Beijiang. Peter, R.W., 1999. Chelant extraction of heavy metals from contaminated soils. Journal of Hazardous Materials 66, 151–210. Qiu, T.C., 1988. Exploring a way to use plants for clean-up of 90Sr and 137 Cs contaminated soils. Agro-environmental Protection 7 (5), 14– 17. (in Chinese). Roca, M.C., Vallejo, V.R., Roig, M., Tent, J., Vidal, M., Rauret, G., 1997. Prediction of cesium-134 and strontium-85 crop uptake based on soil properties. J. Environ. Qual. 26, 1354–1362. Rosen, K., 1991. Effects of potassium fertilization on caesium transfer to grass, barley, and vegetables after Chernobyl. In: Moberg, L. (Ed.), The Chernobyl Fallout in Sweden. Swedish Radiation Protection Institute, Artprint, Stockholum. Sawidis, T., 1988. Uptake of radionuclides by plants after the Chernobyl accident. Environ. Pollut. 50, 317–324. Shaw, G., Bell, J.N.B., 1991. Competitive effects of potassium and ammonium on caesium uptake kinetics in wheat. J. Environ. Radioact. 13, 283–296. Skarlou, V., Nobeli, C., Anoussis, J., Haidouti, C., Papanicolaou, E., 1999. Transfer factors of 134Cs for olive and orange trees grown on different soils. J. Environ. Radioact. 45, 139–147.
Strandberg, M., Johansson, M., 1998. 134Cs in heather seed plants grown with and without mycorrhiza. J. Environ. Radioactivity 40 (2), 175–184. Tang, S.R., Wilke, B.M., Huang, C.Y., 1999. The uptake of copper by plants dominantly growing on copper mining spoils along the Yangtze River, the People’s Republic of China. Plant and Soil 209 (2), 225–232. Tu, S.X., Sun, J.H., Guo, Z.H., 1999. The root exudation of grain amaranth and its role in release of mineral potassium. Acta Agriculture Nucleatae Sinica 13 (5), 305–311. (in Chinese with English summary). Tyler, G., Olsen, T., 2001. Concentration of 60 elements in the soil solution as related to the soil acidity. European Journal of Soil Science 52, 151–165. Wallace, A., and Romney, E. M., 1972. Radioecology and ecophysiology of desert plants at the Nevada Test Site. Environmental Radiation Division, Laboratory of Nuclear Medicine University of California, Riverside, 432pp. Willey, N.J., Martin, M.H., 1997. A comparison of stable casium uptake by six grass species of contrasting growth strategy. Environmental Pollution 95 (3), 311–317.
Uptake of
134
Cs in the shoots of Amaranthus tricolor and Amaranthus cruentus
Shirong Tang*, Ziyuan Chen, Hongyan Li, Jiemin Zheng Institute of Nuclear Agricultural Sciences, Zhejiang University, Huajiachi Campus, Hangzhou 310029, PR China Received 27 November 2000; accepted 24 March 2003
‘‘Capsule’’: Amaranthus tricolor and Amaranthus cruentus responded differently to 134Cs and addition of (NH4)2SO4. Abstract Amaranthus tricolor L. and Amaranthus cruentus L. were grown in pots containing 7.5 kg soils artificially contaminated with three levels of 134Cs activity: 5.55105 Bq pot 1, 1.11106 Bq pot 1, and 1.665106 Bq pot 1, respectively. Forty-nine days after sowing and growth, plants were harvested. The plants growing in soils with increasing 134Cs concentrations showed increasing concentration of this radionuclide in shoots. There were significant differences in uptake of 134Cs applied to soils between and within the plant species, depending on the initial 134Cs concentrations. The plant species showed different responses to the addition of (NH4)2SO4 to soils. Biomass production of both species was reduced in pots treated with (NH4)2SO4. (NH4)2SO4 application decreased the uptake of 134Cs by A. tricolor but increased the accumulation of 134Cs by A. cruentus, showing that chemicals with the highest efficiency to enhance the desorption of 134Cs might play an unexpected role in transferring the radionuclide to shoots. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Uptake; 134Cs; Amaranthus tricolor; Amaranthus cruentus
1. Introduction Soils may become contaminated with radionuclides as a result of proximity to radio-mineral outcrops, or anthropogenically as a result of the development of nuclear technology. Contamination caused by longlived radionuclides, particularly 137Cs and 90Sr, poses a long-term environmental problem. Remediation technologies are needed to avert risk to humans or the environment from radionuclide-contaminated soils. Many chemical and physical approaches used to treat large volumes of soils marginally contaminated with concentrations of long half-life radionuclides have proved to be cost-prohibitive and time-consuming (Entry et al., 1996). As an emerging strategy, phytoextraction is considered to be cost-effective for clean-up of sites contaminated with low levels of long half-life radionuclides. Before this remediation technology is widely accepted, further research to elucidate mechan-
* Corresponding author. +86-571-86971528; fax: 86971421. E-mail address: [email protected] (S. Tang).
0086-571-
isms of phytoremediation is needed to enable the development of this potentially valuable technique. Problems such as how to screen a series of plants with appropriate biological characteristics that can rapidly accumulate significant quantities of radionuclides, and how to find a way to enhance the bioavailability of radionuclides to the plants, should be addressed. Many plants with the ability to accumulate radiocesium abnormally have been widely documented outside China (Entry & Watrud 1998; Lasat et al., 1997, 1998; Broadley & Willey, 1997; Willey & Martin, 1997; Entry et al., 1993, 1996; Sawidis, 1988; Wallace & Romney, 1972) but only a few species have been reported within China (Qiu, 1988). During the past few decades, Chinese scientists have focused their attention on searching for K-rich plant species in order to solve crop problems of potassium deficiency. As a result, a wide range of plant species were found to have an ability to accumulate potassium in the shoots, of which many are in the family Amaranthaceae (Tu et al., 1999; Li & Jin, 1990; Hu et al., 1980). The high proportion of potassium-rich species and chemical similarity of Cs to K led us to speculate that some strong radiocesium accumulators with high biomass and fast growth rate could be
0269-7491/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0269-7491(03)00124-6
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screened from those K-rich plants discovered so far. However, little research has been carried out on their ability to accumulate radiocesium and on their potentials for application into phytoextraction of radionuclide contaminated soils so far. A review of the literature shows that a wide variety of chemical extractants have been investigated during the past few years for their potential to desorb radiocesium (Lasat et al., 1997, 1998; Field et al., 1993; Gombert, 1993; Kirk and Staunton, 1989; Francis and Brinkley, 1976). Because the extent to which radiocesium is desorbed is affected significantly by soil characteristics, it is difficult to compare radiocesium release efficiency resulting from the application of some chemical extractants in these different studies. Thus, further research is still needed to characterize suitable chemical extractants for particular soil types. Two species, Amaranthus tricolor and A. cruentus, were selected for this study since they have contrasting characteristics in terms of mineral uptake. Both A. tricolor and A. cruentus are annual herbs and are widely distributed in South and North China. A. tricolor is about 50–150 cm high with an erect stem, flowering from June to August and bearing fruits from July to September. It is considered to be an important vegetable in summer since it is characteristic of abundant vitamin C, various amino acids, and calcium. A. cruentus is about 50–200 cm high with an erect stem, flowering from July to August and bearing fruits from September to October. Its characteristics of high biomass, high concentrations of minerals in the shoots, ability to absorb ‘‘insoluble nutritions’’, and compatibility with main agricultural crops, have made it widely used in agriculture as a green manure, and in stockbreeding as a feedstuff for animals. The objectives of this study are (1) investigation of 134Cs accumulation in shoots of A. tricolor and A. cruentus from the Amaranthaceae grown in pots treated with different levels of 134Cs activity; (2) identification of some appropriate chemical treatments with potential for desorbing 134Cs from the soil tested and study of their potential for enhancement of 134Cs uptake by both species.
2. Materials and methods The experiment was arranged in a randomized factorial design. A paddy soil (Li, 1992) with a silty loam texture sampled from the experimental agricultural land of Huajiachi Campus, Zhejiang University and two above mentioned plant species were used for the study. The entire experiment was replicated four times. A greenhouse pot experiment was conducted using 7.5 kg of the soil. Table 1 shows some properties of the soil tested. Soil organic matter, pH, CEC, soil texture were measured using methods described previously (Tang et
al., 1999). We tested this soil type because it is widely distributed in Southeast China and similar to the soil surrounding the Qin-shan Nuclear Power Station, also known as Qin-shan Phase I Nuclear Power Station, located at Hai Yan, Zhejiang Province, on the northern shore of the Hangzhou Bay, about 90 km from Hangzhou and 120 km from Shanghai. Fresh soil was sieved to pass a 3-mm mild sieve and kept in the darkness before use. The soil used for this experiment was artificially contaminated with different levels of 134Cs. The control treatment had no 134Cs added. The following treatments were applied to the soil: low 134Cs activity concentrations (AC) 5.55105 Bq/pot (LAC), medium 134Cs AC 1.11106 Bq/pot (MAC), and high 134Cs AC 1.665106 Bq/pot (HAC), respectively. Fifty ml solution containing the required 134 Cs AC was added to the soil, and well mixed. The mixed soil was then transferred into pots under which a suitable size plastic basin was placed at the bottom. Each pot was filled with water until the maximum water holding capacity (WHCmax) was reached. As soon as the water in pots had naturally evaporated dry (moisture content less than 10%), the pot was filled again with water until it reached the WHCmax. Two pots of the soil artificially contaminated with 1.665106 134Cs Bq/ pot in the above mentioned way were used to identify chemical extractants that enhance 134Cs desorption from soil to soil solution. The effectiveness of soil amendments in enhancing soil 134Cs desorption was dependent upon determined 134Cs activity concentration in soil solutions in response to the added soil amendments. The method used for this purpose was after Lasat et al. (1997). Soil treated with different soil amendments was watered to field capacity and kept at room temperature for 24 h before extracting the solution. Then, 0.1 mol/L solutions of the following extractants were used for study of 134Cs desorption: (1) weak organic acids; (2) ammonium salts; (3) potassium salts; Table 1 Physical and chemical characteristics of the soil used for this study Analysis items Total N (%) Total P(%) Available P (mg kg 1) Available K (mg kg 1) Available Cu (mg kg 1g) Available Zn (mg kg 1) Organic matter (%) CEC (cmol/100g soil) pH(H2O) pH(CaCl2) Soil texture Clay Silt Sand Soil texture (USDA)
0.06 0.24 58.10 22.74 7.33 19.81 1.50 7.27 7.14 6.89 9.7 74.7 15.6 Silty loam
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307
Fig. 1. Relative efficiency of 26 soil amendments in enhancing soil 134Cs desorption. 1=KOH; 2=C4H6O5; 3=L-C6H9N3O2; 4=H2C2O4; 5=Cu(CH3COO2)2; 6=CaCl2; 7=Ca(NO3)2; 8=(NH4)3BO3; 9=KH2PO4; 10=CuSO4; 11=Ca(H2PO4)2; 12=CuCl2; 13=Ca(CH3COO)2; 14=KCl; 15=deionized water; 16=K2CO3; 17=H2NCH2COOH; 18=CuNO3; 19=NH4H2PO4; 20=NH4HCO3; 21=K2SO4; 22=(NH4)NO3; 23=(NH4)2C2O4; 24=(NH4)2CO3; 25=KNO3; 26=NH4Cl; 27=(NH4)2SO4.
Fig. 2. 134Cs desorption in response to (NH4)2SO4 added to the contaminated soil. 134Cs activity concentration increased with transient increase of (NH4)2SO4 concentration by 0.1.
(4) calcium salts; (5) copper salts (Fig. 1). On the basis of screening results, we identified the most efficient extractant and compared the relative efficiency of this soil amendment in enhancing the desorption of 134Cs with different concentration treatments (Fig. 2). The exact procedures were as follows: a 300 g aliquot of the < 2 mm soil fraction and 100 ml of the chemical extractant were mixed well in an open-mouth cup. The mixture was transferred into a flat-bottom funnel with two Whatman No. 42 filter papers placed on the bottom. After 24 h, the mixture was extracted for soil solution using a vacuum pump. The resulting filtrate was then vacuum-passed through a 0.45 mm millipore
membrane. The filtrate was collected in a 100-ml plastic bottle and analyzed for determination of 134Cs activity concentrations. Ammonium sulphate was used to extract successively the soil sample with gradually increasing concentrations (0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 mol l 1). Three replicates were used for all treatments. Other pots waited for soil equilibration (6 weeks) following the addition of 134Cs before seeds of A. tricolor and A. cruentus were sown directly in the soil. After germination and 18 day’s growth, seedlings were thinned down to four plants per pot for A. tricolor and five for A. cruentus. Soil moisture content and maximum water holding capacity (WHCmax) were determined
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prior to the experiment. A 100-ml liquid fertilizer was supplied weekly to the plants at N, 130; P, 30; K, 10 mg kg 1 soil. The K fertilizer supply was low because previous research work showed that K+ can compete with casium during root uptake (Lasat et al., 1997; Shaw and Bell, 1991) and that low K concentrations can be favorable to radiocaesium uptake by plants (Lasat et al., 1997; Buysse et al., 1996). The soil was watered using deionized water as needed but with the same volume of water for each pot each time during the experimental period. Each 134Cs and (NH4)2SO4 treatment was replicated four times. The experiment was carried out in a greenhouse without climate control and at ambient temperature with natural illumination (light intensity 2500–4500 lux, and a photoperiod 15–16 h light/9–8 h dark). The experiment was performed during June and August 2000. After 42 day’s growth, 250 ml 0.4 mol/l (NH4)2SO4 was added to the treated pots and the same amount of deionized water to the control pots. Plants were harvested one week after the application of (NH4)2SO4. At harvest, the plants were cut 1 cm above the soil, and the shoots were washed with deionized water, recorded for biomass (fresh w.), dried at 70 C, and measured for constant dry matter yields (DM). Plant and soil solution samples were assayed for 134Cs activity in a high-resolution gamma ray spectrometer. The spectrometer was equipped with a well-type 33 inches. NaI (Tl) detector surrounded by heavy multishielding to reduce background radiation. After being amplified, the detector’s signals were fed to a single channel analyzer (FH-1901). A certified 134Cs solution (The Chinese Academy of Measurement Sciences, May 2000) was used as a standard for efficiency calibration in the above-mentioned counting system. The standard solution filled a glass test tube like those used for the samples. Each sample was counted repeatedly for 10min at least three times. If the repeated measurements of a sample had an accumulated standard error of less than 5%, the measurements were terminated. Background 134Cs was determined by averaging the results of replicate 10 min counts of blank vials. Net 134Cs values were obtained by subtracting this background value from sample values. All results were corrected for inefficiencies of counting, and if necessary, for decay. The lower limit of detection was calculated at the 95% confidence level from the above mentioned system background. K in the plant tissues was analyzed in a H2SO4–H2O2 digest by atomic absorption spectrophotometry (Spectra AA 220) using LiCl to suppress interference. All data were subjected to a three-way ANOVA in the SAS version 6.0 statistical package to allow comparison of treatment means. Differences among treatment means were considered at P < 0.005 using the UNIQUE sums of squares.
3. Results and discussion 3.1. Desorption of 134
134
Cs from contaminated soils
Cs activity concentrations in soil solution, on which the efficiency of soil amendments in enhancing soil 134Cs desorption was dependent, were examined 24 h after chemicals application to the contaminated soil. The most effective desorption of 134Cs was observed in + soil treated with NH+ containing salts. This 4 and K + + may be due to the ability of NH+ to 4 , K , and Cs form a homologous series with a great degree of physiochemical similarities (Finston and Kinsley, 1961) or due to concentration-dependent desorption of radiocesium (Lasat et al., 1997). Although our results show that ammonium salts have high efficiency desorbing 134Cs from the artificially contaminated soils, there still exists some difference in the efficiency among different ammonium salts. Lasat et al. (1997) suggested that NH4NO3 was the most efficient chemical in facilitating desorption of 137Cs from soil to soil solution. Our results showed that among the amendments tested, (NH4)2SO4 was the most effective in enhancing 134Cs desorption (Fig. 1), followed by NH4Cl. The higher the concentrations of (NH4)2SO4 were applied to the soil, the more 134Cs was released from the contaminated soils (Fig. 2). The high efficiency with (NH4)2SO4 treatment was also reported in Dushenkov et al. (1999). It is known that soil composition and texture, including pH, CEC, the K status in soil, the content and type of organic matter, and a clay concentration, were considered to be of importance in determining the transfer of radiocesium (Roca et al., 1997; Lembrechts, 1993; Cremers et al., 1990). Tyler and Olsen (2001) investigating the concentrations of 60 elements in the soil solution as related to the soil acidity concluded that pH value could play a crucial role in ions availability to plants, with availability at pH 5.2 being about 3 fold higher than at pH 7.0. Results from this study demonstrated that the extent by which (NH4)2SO4 addition reduced the soil pH is dependent on (NH4)2SO4 concentrations, ranging from 0.5 to 1.0 unit. The decrease in pH value after (NH4)2SO4 addition may suggest more 134 Cs availability and might explain the increase released 134Cs with increasing concentrations of (NH4)2SO4. Although Field et al. (1993) reported that some organic acids could successfully release 137Cs from contaminated soils, our results show that the organic acids were ineffective in enhancing the desorption of 134 Cs from contaminated soils (Fig. 1). Similar findings were reported by Lasat et al. (1997). However, inducing hyperaccumulation of heavy metals in plants was reported following organic acid addition (Peter, 1999; Anderson et al., 1998; Blaylock et al., 1997; Huang et al., 1997), suggesting that heavy metals have a response different from radiocesium to organic acid addition.
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This is probably because most of the organic acids are present in the solution as un-ionised molecules, which results in less influence on the desorption of radiocesium from its fixation into clay minerals. 3.2. Plant growth There were some variations in biomass production between A. tricolor and A. cruentus (Table 2). Both plant species yielded more shoot DM without (NH4)2SO4 than with (NH4)2SO4 treated (Table 2), showing addition of (NH4)2SO4 at 0.4 mol kg 1 soil depressed biomass production in both plant species. It is likely that this effect resulted from ammonium toxicity or from low pH value due to high (NH4)2SO4 levels. The responses to the addition of 0.4 mol of (NH4)2SO4 to the soil for A. tricolor and A. cruentus were different, as shown by the extent to which both plant species were affected by (NH4)2SO4 (Table 2). This effect may be due to the difference in tolerance to ammonium toxicity. A. cruentus may be more susceptible to ammonium toxicity than A. tricolor. For both plant species, (NH4)2SO4 addition caused a 10 to 20% reduction in the shoot biomass. 3.3.
134
Cs activity concentrations in the shoots
On the basis of the results of soil 134Cs desorption, we investigated the uptake of 134Cs in the shoots of A. tricolor and A. cruentus. Regardless of (NH4)2SO4 treatments, 134Cs activity concentrations in the shoots of both species were significantly affected by 134Cs activity in soil (Table 2). In general, the higher 134Cs activity in the soil, the more 134Cs was accumulated in the shoots. The accumulation of 134Cs by A. tricolor was dependent upon the initial application of 134Cs concentrations in
309
soil in the case of no (NH4)2SO4 application. In control (no (NH4)2SO4) pots, A. cruentus accumulated progressively more 134Cs in its shoots with 134Cs activity increase in the soil (from LAC to MAC), but took up less with 134Cs activity of 1.665106 added to the soil (Table 2). It is interesting to note that A. tricolor and A. cruentus had different responses to (NH4)2SO4 addition. Addition of 0.4 mol of (NH4)2SO4 decreased the uptake of 134 Cs by A. tricolor but enhanced the accumulation of 134 Cs by A. cruentus grown in pots treated with low and medium 134Cs activity (from LAC to MAC) (Table 2). This suggests that NH+ 4 may have a negative effect on 134 Cs uptake by A. tricolor but may have a positive effect on 134Cs uptake by A. cruentus. Lasat et al. (1997) investigating the effect of (NH4)NO3 application on 137 Cs bioavailability and its accumulation in plants found that (NH4)NO3 application could slightly increase the uptake of 137Cs by Amaranthus retroflexus and Phaseolus acutifolius. Our results showed that (NH4)2SO4 application decreased the uptake of 134Cs by A. tricolor but increased the accumulation of 134Cs by A. cruentus in general, suggesting that there is a different response to particular soil amendment for different species. Regardless of (NH4)2SO4 application, the accumulation of 134Cs in shoots of both species was dependent upon initial 134Cs activity concentrations in the soil. Similar results were reported in Lasat et al. (1998), Demirel et al. (1994), and Shaw and Bell (1991). Greatest amounts of 134Cs were concentrated in shoots of A. tricolor grown in pots treated with high 134Cs activity concentration and with no soil amendment added but maximum 134Cs concentrations were accumulated in the shoots of A. cruentus grown in medium 134Cs activity treated pots with (NH4)2SO4 application. This suggests
Table 2 Biomass, 134Cs concentration in shoots, and bioconcentration ratios (AverageSD), Species
134 Cs added (Bq per pot)
A. tricolor
Control 5.55105 5.55105 1.11106 1.11106 1.665106 1.665106 Control 5.55105 5.55105 1.11106 1.11106 1.665106 1.665106
A. cruentus
a
(NH4)2SO4 added
Treatment
Yes No Yes No Yes No
LACA LACNOA MACA MACNOA HACA HACNOA
Yes No Yes No Yes No
LACA LACNOA MACA MACNOA HACA HACNOA
Biomassa (g/per pot, dry weight) 8.771.63 7.521.28A,a 10.040.94A,b 9.131.18A,a 11.532.65A,b 11.042.09A,a 11.680.74A,b 8.731.86A 7.050.85A,a 10.421.09A,b 4.483.81A,a 7.711.60A,b 7.722.35A,a 8.651.71A,b
134 Cs concentration in shootsa (Bq g 1, dry weight)
Bioconcentration ratiosa
26598A,a 375104A,b 612107B,a 73423B,b 966114C,a 1235171C,b
3.41 0.95A,a 2.41 0.89A,b 3.34 0.10A,a 2.78 0.49A,b 2.93 0.35A,a 3.74 0.73A,b
28054A,a 258106A,b 70864B,a 64848B,b 662118C,a 547414C,b
2.55 0.49A,a 2.35 0.09A,b 3.22 0.29A,a 2.94 0.22A,b 2.01 0.36A,a 1.66 1.25A,b
Within each column, values followed by the same letter are not significantly different as determined by UNIQUE sums of squares (p< 0.05) n=12 for all values. A, B, C . . .=134Cs added; a, b, c. . .=soil amendments. Bioconcentration ratio=Bq 134Cs g 1 in shoot/Bq 134Cs in soil.
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that inter-species differences and initial radiocesium concentration in the soil still have an influence on accumulation of 134Cs in shoots. For control (ammonium free) pots, A. tricolor generally had significantly higher 134Cs bioaccumulation ratio (BR) than A. cruentus (Table 2). (NH4)2SO4 addition at 0.4 mmol kg 1 soil significantly (P < 0.05) increased the bioaccumulation ratios in A. tricolor but decreased the ratios in A. cruentus, suggesting a different response to (NH4)2SO4 addition for different species. Such a difference in response to (NH4)2SO4 addition may be due to the difference in biological characteristics of the species. The greatest BR (3.56) was obtained with A. tricolor grown in pots treated with high 134Cs activity concentrations and (NH4)2SO4. In control (ammonium free) pots, the 134Cs bioaccumulation ratios varied between 2.41 and 2.79 for A. tricolor, between 2.20 and 3.22 for A. cruentus, respectively, while in pots treated with (NH4)2SO4, the value ranged between 3.34 and 3.56 for A. tricolor, and between 2.35 and 2.94 for A. cruentus, respectively (Table 2). Lasat et al. (1997) reported that plant species grown in ammonium free pots had the 137Cs bioaccumulation ratios ranging from 0 to 1.0. Dushenkov et al. (1999) showed that the bioaccumulation ratios of all Amaranthus species varied from 0.30 to 2.03. Olive and orange trees grown on calcareous and acid soils had bioaccumulation ratios of 134 Cs usually less than 0.05 (Skarlou et al., 1999). Our results showed that the two species from the Amaranthaceae grown in pots treated with or without (NH4)2SO4 had higher bioaccumulation ratios than the above mentioned ones, being at the high range of reported radiocesium bioaccumulation ratios (Coughtry et al., 1983; Dushenkov et al., 1999).
Lasat et al. (1997) reported that (NH4)2SO4 addition increased the bioaccumulation ratios in Phalaris arundinacea, Brassica juncea, Phaseolus acutifolius, and B. oleracea var. capitata, showing a positive response. However, Dushenkov et al. (1999) investigating the effect of soil amendments on 137Cs accumulation in Indian mustard and sunflower showed that the EDTA and DTPA addition increased the bioaccumulation ratios of Helianthus annuus but decreased the ratios of Brassica juncea. Our results also show that not all plant species show a positive response in uptake of radiocesium to (NH4)2SO4 addition, being similar to Dushenkov et al. (1999). 3.4. Potassium concentrations in the shoots The K concentrations in the shoots of A. tricolor and A. cruentus were statistically analyzed using ANOVA and were significantly (P < 0.05) affected by plant species. 134Cs concentrations added into the soil and (NH4)2SO4 addition did not significantly affect the K concentration in the shoots of both species. A. cruentus generally accumulated more K than A. tricolor. The K concentration in the shoots of A. cruentus ranged between 5.12 and 3.60% with an average of 4.42% and standard deviation of 0.50 while A. tricolor has K concentration varying from 4.88 to 2.70% with an average of 3.76% and standard deviation of 0.68 in the shoots. A. cruentus was known to be a K-rich species (Tu et al., 1999) but A. tricolor was not. Such a difference might explain why A. cruentus had more K in the shoots than A. tricolor even in our case where the K fertilizer supply was low. A literature review shows that K concentration in the growth media was an important factor influencing
Fig. 3. Relationship between K and 134Cs concentrations in shoots of A. tricolor and A. cruentus (Cs=1278.71–158.23K, P <0.05).
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Cs absorption and movement within plant species (Lasat et al., 1997; Buysse et al., 1996) because of its similarity to cesium in chemistry. Strandberg et al. (1998) reported that plants grown in K treated soil had decreasing uptake of radiocesium. Rosen (1991) also reported similar effects from agricultural ecosystems where potassium fertilizing (100 kg ha 1) maximally reduced the cesium uptake by the crops by a factor of 10. However, Broadley and Willey (1997) investigating differences in root uptake of radiocesium by 30 plant taxa concluded that there was a strong relationship between mean shoot Cs and K concentration within the Gramineae and Chenopodiaceae families. Our experiment performed in the pot experiment demonstrated a weak relationship between shoot 134Cs activity and K concentrations in A. tricolor and A. cruentus (Fig. 3).
4. Conclusions We found that both initial 134Cs activity concentrations in the soil and inter-species differences have an influence on the 134Cs accumulation in the shoots of A. tricolor and A. cruentus. It was also found that there were different responses to the addition of 0.4 mol of (NH4)2SO4 to the soil for A. tricolor and A. cruentus. (NH4)2SO4 application decreased the uptake of 134Cs by A. tricolor but increased the accumulation of 134Cs by A. cruentus, suggesting that chemicals with the highest efficiency to enhance the desorption of 134Cs might play an unexpected role in transferring the radionuclides to shoots when applied into the soils. Potassium distribution in shoots is significantly affected by plant species and slightly by 134Cs treatments.
Acknowledgements This work was supported by a project from the Ministry of Science and Technology, P.R. China (Grant Number: NKBRSFG 1999011808). Dr. Tang wants to thank Professor Alan Baker from the School of Botany, University of Melbourne, Professor Slavik Dushenkov from Biotech Center, New Jersey, Professor Nicholas Lepp from Liverpool John Moores University, and three anonymous reviewers for their constructive advice.
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