Accumulation of rare earth elements in maize plants (Zea mays L.) after application of mixtures of rare earth elements and lanthanum

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1 Plant and Soil 252: , Kluwer Academic Publishers. Printed in the Netherlands. 267 Accumulation of rare earth elements in maize plants (Zea mays L.) after application of mixtures of rare earth elements and lanthanum Xingkai Xu 1,2,4, Wangzhao Zhu 3, Zijian Wang 1 & Geert-Jan Witkamp 3 1 State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China. 2 State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China. 3 Laboratory for Process Equipment, Technical University of Delft, 2628 CA, Delft, The Netherlands. 4 Corresponding author Received 27 May Accepted in revised form 27 November 2002 Key words: Ce anomaly, Gd anomaly, ICP-MS, maize, rare earth elements, safety assessment Abstract Rare earth elements are applied in China to improve crop production, and the distribution patterns of individual rare earth elements in native plants have widely been reported. But our knowledge is still limited about the dosedependent accumulation of individual rare earth elements in agricultural crops after application of rare earth elements. Effects of lanthanum and mixtures of rare earth elements were studied in pot experiments on the accumulation of individual rare earth elements in maize plants. All plant samples were divided into plant tops and roots. On addition of mixtures of rare earth elements and lanthanum to the soil, a significant dose-dependent accumulation of individual rare earth element(s) was found in the roots and in the plant tops. Application of mixtures of rare earth elements at >10 mg kg 1 soil, resulted in a significant increase in contents of light rare earth elements in the roots, and at a dose of 50 mg kg 1 soil, a similar phenomenon was found in the plant tops. When mixtures of rare earth elements were replaced by lanthanum alone, at a dose higher than 10 mg La kg 1 soil, a significant increase in La content occurred in the roots and in the plant tops. The content ratio of La to Ce in maize plants appeared to increase as the application doses of rare earth element(s) increased. At a highest dose (50mgkg 1 soil), the transport of the absorbed La from the roots to the plant tops might be substantially reduced after treatment with lanthanum alone, compared with mixtures of rare earth elements. Increasing the application doses of rare earth element(s) appeared to cause a positive Gd and negative Ce anomaly in the roots and in the plant tops, and the anomaly was more obvious in the plant tops than in the roots. The results indicated that the Gd and Ce anomaly in corns might be considered as important parameters for the safety assessment of agricultural application of rare earth elements. Introduction Rare earth elements can be taken up through the leaf surface after spraying (Sun et al., 1994), but normally the uptake takes place exclusively via the roots. Contents of rare earth elements in plants seem to be extremely variable, and are dependent on the various species of plants and their growing conditions (Fu FAX No: xingka:_xu@hotmail.com et al., 2001; Ichihashi et al., 1992; Wyttenbach et al., 1998). Plants growing on the rare earth elementsenriched soil show very high contents (Miekeley et al., 1994). The distribution patterns of rare earth elements in native plants have widely been reported (Fu et al., 2001; Henke, 1977; Ichihashi et al., 1992; Miekeley et al., 1994; Wang et al., 1997; Wyttenbach et al., 1998). However, the dose-dependent accumulation of individual rare earth elements in crops after application of rare earth elements is less studied (Wang

2 268 et al., 2001; Xu et al., 2002). Furthermore, little is known about the mechanisms by which the rare earth elements accumulate in the plants. Contents of rare earth elements in plants have been measured mostly using instrumental neutron activation analysis (INAA) (Ni et al., 1999; Wang et al., 1997). However, the INAA technique can only measure eight rare earth elements (La, Ce, Nd, Sm, Eu, Tb, Yb and Lu) in plant samples. In recent years, high resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) has been used to effectively measure the contents of individual rare earth elements (including Sc, Y and 14 lanthanides) in biological and environmental samples (Xu et al., 2002; Zhu, 1999). This highly sensitive and dissociative technique will further promote the study on the behaviors of individual rare earth elements in biological samples. Since the early 1990s, some fertilizers containing rare earth elements have been applied in China to improve crop production and are estimated to cover approximately ha in 1993 and ha in 1995 (Dialoff et al., 1996; Ni et al., 1995). Since, up to now, Chinese farmers have used rare earth elements-containing fertilizers as base fertilizers (with N-fertilizers) to improve crop production (Wen et al., 2000; Yu and Chen, 1995), currently only studies are available on the combined effects of nitrogen and lanthanides (Xu and Wang, 2001; Xu et al., 2003). In addition, only little attention has been paid to the accumulation of rare earth elements in crops after years of application. For the safety assessment of agricultural application of rare earth elements, it is important for us to study the dose-dependent accumulation of individual rare earth elements in crops upon addition of such fertilizers, and the corresponding mechanisms by which the rare earth elements can enter the plants. To our knowledge this is the first report on the accumulation of individual rare earth elements in maize plants (Zea mays L.) upon application of mixtures of rare earth elements and lanthanum. The aim of the study is to understand the dose-dependent accumulation of individual rare earth elements in crops and their anomalies. We also wish to compare the effects of La alone with mixtures of rare earth elements, because most experimental data have been obtained on individual rare earth elements whereas mixtures have been applied in the field. Materials and methods Soil characteristics Soil for this study was collected from the top layer (0 20 cm) of an upland field in the suburbs of Beijing (China). The soil was classified, according to FAO taxonomy, as a Luvisol. Main physical and chemical properties of the soil, measured as described by Kim (1995), are: total N 980 mg kg 1 ; total P 8600 mg kg 1 ; total C mg kg 1 ; available N 67.5 mg kg 1 ; available P 72.5 mg kg 1 ; ph 7.25 (soil: water = 1:5); sand 240 g kg 1 ; silt 420 g kg 1 ; clay 340 g kg 1 andcec19.8cmolkg 1. The soil sample was dried, thoroughly mixed and then sieved (5 mm) before use. Background concentrations of individual rare earth elements in the soil were measured by ICP-MS (John et al., 1998) and are listed in Table 1. Preparation of solutions containing mixtures of rare earth elements or lanthanum alone The mixture of rare earth elements used in our study was a commercial product for agricultural use, and was supplied by the Research Center for Agricultural Application of Rare Earth Elements in China. It is a mixture of rare earth elements as their nitrates. Normally, this mixture is made by extracting the rare earth elements from their ores using nitric acid, and more than 60% of the mixture is thus nitrate and soluble (Brown et al., 1990). To remove nitrate from the mixture, a 40-g portion was dissolved in distilled water and the ph was adjusted to 5.5. Then 40 ml of 4 m oxalate solution was added, and the precipitate formed was filtered and washed with distilled water. The precipitated rare earths oxalate was transformed into oxides in a muffle furnace at 700 C for 4 h, and the oxides were then dissolved in 200 ml hydrochloric acid (1:1). After dissolution, the ph of the solution wasadjustedto5.5with0.1mhcland3mnaoh, and diluted to 1000 ml with distilled water. The prepared stock solution was stored at 4 C before use. The constituents and concentrations of rare earth elements in the stock solution were analyzed by inductively coupled plasma-mass spectrometry (VG PlasmaQuard III, Fisons Instruments, UK). The instrument was operated at a sampling dose of 1.0 ml min 1 with a measuring time of 40 s. Indium (In 115 ) was used as an internal standard for calibrating the instrument. The constituents and concentrations of rare earth elements in the stock solution are shown in Table 2.

3 269 Table 1. Background concentrations of individual rare earth elements in the soil tested (mg kg 1 soil) Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Total Table 2. Constituents and concentrations of individual rare earth elements in the stock solution (mg l 1 ) Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Total A g sample of lanthanum oxide (La 2 O 3, 99.95% purity, supplied by Changchun Institute of Applied Chemistry, Chinese Academy of Sciences) was dissolved with 20 ml hydrochloric acid (1:1). After dissolution, the ph of the solution was also adjusted to 5.5 with 0.1 M HCl and 3 M NaOH, and diluted to 1000 ml with distilled water. The final stock solution contained 1000 mg La l 1 and was stored at 4 Cbeforeuse. Experimental description and plant sampling A 2.5 kilogram sample of the air-dried soil was put in each pot; 0.5 g K 2 HPO 4 and 1.0 g urea were added as base fertilizers. Two hundred ml of a solution containing 12.5, 62.5, 125 or 625 mg l 1 mixed rare earth elements or La alone was added to the soil in different treatments, and 200 ml of distilled water was added in the control. The solutions containing rare earth element(s) were added drop-wise to moisten the whole soil, and the soil was thoroughly mixed. The control and the treatments each had four replicates, and corresponded to doses of 0, 1, 5, 10 or 50 mg kg 1 rare earth elements or La in dry soil. Each experimental pot contained 10 maize plants until the end of the experiment. The plants were cultured at a growth chamber at 25 C, with light intensity of 120 µmol m 2 s 1 for 12 h each day. During the entire experimental period, the same amount of distilled water was, at regular intervals, added to the soil in any pot. On day 45 after application of rare earth element(s), the whole plant in any pot was taken out, and each plant sample was divided into the plant tops and the roots. All plant samples were washed in the following order: tap water, 0.01% nitric acid, tap water and distilled water. After the final washing step, plant samples were dried in an electric oven at 80 C for 18 h to constant weight. The samples were then ground into powder for analysis. Measurement of individual rare earth elements in plant samples Before digestion, all plant samples were dried in an electric oven at 80 C for 6 h to remove the moisture. A portion of dry plant sample ( gram) was placed into a test tube, and reacted with a 4-mL solution of nitric acid (conc. 70%) overnight. After 24 h, the samples were heated at 80 Cinanoilboxfor24hand then cooled. Three ml of 30% H 2 O 2 was added to each to complete the digestion, and the final solutions were weighed. All reagents used were at least of analytical reagent grade. Calculation and statistical analysis All data are presented on basis of oven-dried weight of plant materials. Logarithms of the chondritenormalized abundances of individual rare earth elements in maize plants are plotted against the individual rare earth elements in the order of their atomic numbers. Chondritic normalization has two important functions. Firstly it eliminates the abundance variations between odd and even atomic number elements, and secondly it allows any fractionation of the rare earth elements group relative to chondritic meteorites to be identified. These meteorites are considered to represent the naturally occurring lanthanide abundances, undisturbed by geological and biological partitioning (Evensen et al., 1978). Logarithms of the chondrite-normalized abundances of individual rare earth elements in maize plants are expressed as Log(REm/REc), where REm is the content of individual rare earth elements in the plants, REc

4 270 Table 3. Analytical results (µg g 1 dry wt) of certified reference materials by HR-ICP-MS (± standard deviation of four determinations) Elements Sample CRM670 CRM0104 Determined Certified Determined Certified Sc 212±72 191± ± ±6.3 Y 412±6 462± ± ±26.7 La 426±6 487± ± ±14.3 Ce 914±9 987± ± ±41.4 Pr 106±2 121±8 26.6± ±0.7 Nd 441±8 473± ± ±13.1 Sm 95.8± ± ± ±2.4 Eu 23.7± ± ± ±1.59 Gd 100.2± ± ± ±4.0 Tb 13.6± ± ± ±0.27 Dy 71.9± ± ± ±1.2 Ho 14.5± ± ± ±0.3 Er 38.8± ± ± ±1.1 Tm 5.3± ± ± ±0.3 Yb 32.8± ± ± ±0.8 Lu 5.1± ± ± ±0.33 is the content of individual rare earth elements in the chondrite from Evensen et al. (1978). Not infrequently the plotted position of cerium lies off the general trend defined by the other elements on the diagram (Fu et al., 2001; Wyttenbach et al., 1998) and may set a cerium anomaly. Cerium anomalies may be quantified by comparing the measured content (Ce) with an expected content obtained by interpolating between the chondrite-normalized values of La and Pr (Ce ). Thus the ratio Ce/Ce is a measure for the cerium anomaly, and a value of larger than 1.0 indicates a positive anomaly whereas a value of less than 1.0 is a negative anomaly. In this study, Ce/Ce has been expressed as Ce N /[(La N ) (Pr N )] 1/2, where the chondrite-normalized values are denoted with a subscript N (e.g. La N and Ce N ) (Adrian et al., 1996). Similarly, Gd/Gd can be also expressed as Gd N /[(Eu N ) (Tb N )] 1/2. Means and standard deviations are calculated in all treatments. Significant differences between means are analyzed by t-test using STATISTICA software for windows (release 4.5), with a confidence interval of 95%. Results and discussion Analysis of measurement of individual rare earth elements in maize plants The precision of the measurements for individual rare earth elements in plants was assessed by analyzing different kinds of plant samples in duplicates. It was found that the relative standard deviation in any plant sample was below 10% for the various rare earth elements, with the exception of Sc. The accuracy was checked by analysis of certified reference materials CRM 670 and CRM 0104 (supplied by Community Bureau of Reference, The Netherlands); results deviated by less than 10% from accepted values (Table 3). Hence, the above digestion procedure is suitable for the measurement of individual rare earth elements in plant samples using HR-ICP-MS. Accumulation of individual rare earth elements in maize plants Logarithms of the chondrite-normalized abundances of individual rare earth elements in the maize plants after application of mixtures of rare earth elements are shown in Figure 1. From these patterns, it can be seen

5 271 Table 4. Total amounts of individual rare earth elements in maize plants after treatment with La alone and mixtures of rare earth elements a Different parts of maize plants Application doses of mixtures of rare earth elements or La alone (mg kg 1 soil) With mixtures of rare earth elements Roots (µg) 1.16(0.12)a 1.03(0.17)a 1.15(0.14)a 2.00(0.08)b 5.26(0.44)c Plant tops (µg) 0.83(0.01)a 0.70(0.05)a 0.76(0.09)a 0.73(0.03)a 1.78(0.15)b Whole plant (µg) 1.99(0.13)a 1.73(0.22)a 1.90(0.23)a 2.73(0.11)b 7.05(0.59)c With La alone Roots (µg) 1.16(0.12)a 1.33(0.11)a 1.40(0.34)a 2.23(0.45)b 3.94(0.35)c Plant tops (µg) 0.83(0.01)a 0.75(0.06)a 0.69(0.10)a 0.94(0.19)ab 1.12(0.10)b Whole plant (µg) 1.99(0.13)a 2.09(0.18)a 2.09(0.44)a 3.17(0.63)b 5.06(0.45)c a Values in the table are means of four replicates, and standard deviations are shown in parentheses. In each row means followed by different letters are significantly different at P=0.05. Total amount of individual rare earth elements in the plants includes Sc, Y and 14 lanthanides (from La to Lu). Table 5. Concentration ratios of the lighter lanthanides (from La to Eu) to the heavier lanthanides (from Gd to Lu) in the plants and in the soils after application of mixtures of rare earth elements and lanthanum alone a Application doses of mixtures of rare earth elements or La alone (mg kg 1 soil) After application of mixtures of rare earth elements Soil Plant tops 4.2(0.4)a 4.5(0.3)a 4.3(0.2)a 4.8(0.3)a 5.9(0.4)b Roots 8.2(1.1)a 7.4(1.4)a 8.5(1.2)a 9.7(1.2)a 15.6(1.8)b After application of lanthanum alone Soil Plant tops 4.2(0.4)a 4.5(0.2)a 4.5(0.2)a 5.2(0.4)a 5.0(0.4)a Roots 8.2(1.1)a 10.1(1.6)a 9.9(1.4)a 8.4(1.2)a 8.7(1.3)a a Means of four replicates (standard deviations in parentheses) are shown in the above table. In each row means followed by different letters are significantly different at P=0.05. that application of mixtures of rare earth elements below 10 mg kg 1 soil induced no obvious accumulation of individual rare earth elements in the plant tops or in the roots. When the application dose of rare earth elements was 50 mg kg 1 soil, a substantial accumulation of most lighter lanthanides and Gd was found in the plant tops (Figure 1a) and in the roots (Figure 1b), and the total content of rare earth elements increased significantly (P<0.05) (Table 4). The accumulation is much stronger for the lighter lanthanides (from La to Eu) than for the heavier lanthanides (from Gd to Lu), particularly at high application doses (Figure 1 and Table 5). This partly results from the high contents of lighter lanthanides in the fertilizer (Table 2). After addition of rare earth element(s) the concentration ratios of the lighter to the heavier lanthanides in the plant tops were much lower than in the roots (Table 5), thereby indicating a fractionation of the rare earth elements in the plants. Increasing the application doses of mixtures of rare earth elements appeared to reduce the concentration ratio of the lighter to the heavier lanthanides in the soil. However, at a highest dose (50 mg rare earth elements kg 1 soil), a substantial increase in the concentration ratio was found in the plant tops and in the roots (P<0.05) (Table 5). Hence, application of the mixtures at high doses might result in a preferred accumulation of the lighter compared to the heavier lanthanides in the corns. Although increasing the application doses of lanthanum alone could enhance the concentration ratios of the lighter to the heavier lanthanides in the plants, no significant variations were found in all treatments (Table 5).

6 272 Figure 2. Chondrite-normalized abundances of rare earth elements in the plant tops (a) and in the roots (b) upon La application. Figure 1. Chondrite-normalized abundances of rare earth elements in the plant tops (a) and in the roots (b) after treatment with mixtures of rare earth elements. This indicated that La application at high doses might to some extent enhance the uptake of the heavier lanthanides by the roots. For our previous field experiment (Xu et al., 2002), mixtures of rare earth elements were added to the soil with irrigation water, when maize plants entered their vigorous vegetation growth stage (e.g. early stem-elongation stage). We assume the incorporation of the rare earth elements into a 20-cm depth surface soil. At a dose of more than 10 kg rare earths ha 1 (corresponding to 5 mg rare earths kg 1 soil ), a substantial accumulation of individual rare earth elements occurred in the plants even at maturity stage (P<0.05) (more than 2 months after addition of rare earth elements) (Xu et al., 2002). In the laboratory study, the solutions containing rare earth element(s) were added to the soil, and the soil was thoroughly mixed before the incubation. After 45 days maize plants have entered their vigorous vegetation growth stage. A significant accumulation of individual rare earth elements in the plants only occurred at a dose of 50 mg kg 1 soil (Figure 1). Apparently, the dosedependent accumulation of individual rare earth elements in corns depended on the application methods of rare earth elements and plant growth stages. When mixtures of rare earth elements were replaced by lanthanum alone, at a dose higher than 10 mg La kg 1 soil, a substantial increase in La content occurred in the plant tops (Figure 2a) and in the roots (Figure 2b). In comparison with mixtures of rare earth elements (Figure 1), La application at a relatively smaller dose, resulted in a substantial accumulation of La in maize plants (P<0.05). This indicates that the uptake of rare earth elements by the roots and the transport of the absorbed elements from the roots to the plant tops are possibly variable with different rare earth elements. Rare earth elements can enter the roots in ionic forms from the soil, via first being absorbed into epidermal, cortical, or endodermal cells and then

7 273 moving through the cytoplasmic continuum to the vascular tissue of the roots (Nagahashi et al., 1974). The rare earth elements in the vascular tissue may transfer to the plant tops via the water flow of transpiration. Of course, these elements can be adsorbed by some nondiffusible anions (e.g. COOH ) inside the vascular tissue, and exchanged by other cations (Lauchli and Bieleski, 1983). In addition, the bonding functions of rare earth elements inside the vascular tissue are variable with their atomic numbers. These phenomena possibly explain the differences in the dose-dependent accumulation of rare earth element(s) in corns after addition of mixtures of rare earth elements and La alone. Increasing the application doses of mixtures of rare earth elements appeared to enhance the content ratio of La to Ce in the maize plants (Figure 3). When the application doses of rare earth elements increased up to 50 mg kg 1 soil, the ratio value in the plants increased significantly (P<0.05). The ratio for any treatment in the plant tops and in the roots had no apparent variations (Figure 3b). This is apparently different from the ratio of La to Ce in the plants upon La application (Figure 3a). When the application doses of La alone increased up to 50 mg La kg 1 soil, the content ratio of La to Ce was larger in the roots than in the plant tops (P<0.05) (Figure 3a); this ratio in the roots was much larger than for application of mixtures of rare earth elements (Figure 3b). La application at high doses could enhance the permeability of cell membrane (Brown et al., 1990; Chang, 1991), leading to more accumulation of rare earth elements in the roots than in the plant tops (Table 4). The transport of the absorbed La from the roots to the plant tops would be to some extent reduced, due to a plant-protection function from adverse effects of lanthanum (Diatloff et al., 1995). Using lanthanum in combination with electron microscopy, Nagahashi et al. (1974) reported that the Casparian strip of corn roots can provide a barrier to the diffusion of La 3+ in the apoplast from the cortex to the vascular tissue. In corn roots, La 3+ is believed to bind to, and (or) displace Ca 2+ from, membrane sites normally occupied by Ca 2+. These can perhaps result in a higher La/Ce in the roots than in the plant tops at the dose of 50 mg La kg 1 soil. Ce and Gd anomaly in maize plants Figure 4 shows the Ce and Gd anomaly in the maize plants upon application of mixtures of rare earth elements. In the control, the ratio Ce/Ce in the plant tops and in the roots was 0.85 and 0.99, respectively, indicating a minor negative Ce anomaly in the plant tops. Increasing the application doses of rare earth elements reduced the ratio Ce/Ce in the plant tops and in the roots (Figure 4). Hence, under the experimental conditions, application of mixtures of rare earth elements, especially at high doses, resulted in an apparent negative Ce anomaly in the plants. The negative anomaly states that Ce contents in maize plants are less than expected from contents of its neighboring elements (e.g. La and Pr). This is due to the fact that in soils Ce 3+, in contrast to all other rare earth elements, can assume a valence of +4 (e.g. CeO 2 ), and that this compound has some chemical attributes differing from the trivalent rare earth elements (Brookins, 1989). Normally, CeO 2 in soils can not be absorbed by the roots. Upon addition of rare earth element(s), some variations of oxidation-reduction and the complexation in the rhizosphere of corns might affect a reduced availability and/or uptake of Ce into the plants (Wyttenbach et al., 1998). Through an incubation experiment of rice seedlings, Zheng et al. (2002) reported that La 3+ might substantially affect the gradient of transmembrane proton and membrane potential in the roots. In the control, the ratio Gd/Gd in the plant tops and in the roots was 9.15 and 2.28, respectively. This indicates a positive Gd anomaly in the maize plants, particularly in the plant tops. An increase in the ratio Gd/Gd occurred in the plant tops and in the roots as the application doses of rare earth elements increased (Figure 4). Hence, upon application of mixtures of rare earth elements, an apparent accumulation of Gd content may occur in the plant tops (Figure 1a) and in the roots (Figure 1b). Under field experimental conditions, a positive Gd anomaly in maize plants was also observed after application of the fertilizer containing rare earth elements (Xu et al., 2002). This partly results from the rare earths-containing fertilizers with a high content of gadolinium. Because positive Gd anomalies were discovered in sewage treatment plants, Knappe et al. (2001) showed that using the REE gadolinium was considered as a new tracer for sewage influence in aqueous urban systems. Although the reason for the so-called Gd anomaly in the plants is so far not clear, the Gd anomaly can be considered as an important parameter for the safety assessment of agricultural application of rare earth elements-containing fertilizers.

8 274 Figure 3. Content ratio of La to Ce in maize plants after treatment with lanthanum (a) and mixtures of rare earth elements (b). Vertical bars indicate standard deviations (n=4). The solid triangles show the content ratio of La to Ce in the soils after treatment with lanthanum and mixtures of rare earth elements. When mixtures of rare earth elements were replaced by La alone, the same phenomenon as shown in Figure 4 were found for the Ce and Gd anomaly in maize plants (Figure 5). When mixtures of rare earth elements or La alone was added to the soil, we could observe a positive Gd and negative Ce anomaly in the roots and in the plant tops, and these anomalies were more obvious in the plant tops than in the roots (P<0.05) (Figures 4 and 5). Thus means that La indirectly influences the uptake of other rare earth elements.

9 275 Figure 4. Ce and Gd anomalies in maize plants after application of mixtures of rare earth elements. Vertical bars indicate standard deviations (n=4). Figure 5. Ce and Gd anomalies in maize plants upon La application. Vertical bars indicate standard deviations (n=4).

10 276 Total amount of individual rare earth elements in maize plants Table 4 shows the total amount of individual rare earth elements in maize plants (including Sc, Y and 14 lanthanides). When the application doses of rare earth element(s) increased up to 50 mg kg 1 soil, a significant increase in the total amount of rare earth elements was observed in the plant tops (P<0.05). Furthermore, at a dose of 10 mg kg 1 soil, a substantial increase in the total amount occurred in the roots and in the whole plant (P<0.05). At a dose smaller than 10 mg kg 1 soil, no apparent variations in the total amount were found between mixtures of rare earth elements and lanthanum alone. However, at a highest dose (50 mg kg 1 soil), the total amount in the plants was smaller upon La application than upon application of mixtures of rare earth elements (Table 4). As shown in Table 4, total amount of rare earth elements among all treatments in the roots was higher than in the plant tops. With an increase in the application doses of rare earth element(s), the increase in the total amount was higher in the roots than in the plant tops (P<0.05) (Table 4). The dose-dependent accumulation of individual rare earth elements in the roots was more obvious than in the plant tops, when rare earth elements were added to the soil. Conclusions When mixtures of rare earth elements and La alone were added to the soil, a significant dose-dependent accumulation of individual rare earth element(s) was found in the roots and in the plant tops. This accumulation in the roots was larger than in the plant tops. Increasing the application doses of rare earth element(s) induced a positive Gd and negative Ce anomaly in the roots and in the plant tops, and the anomaly was more obvious in the plant tops than in the roots. The content ratio of La to Ce in the maize plants appeared to increase as the application doses of rare earth element(s) increased. At high doses, the transport of the absorbed La from the roots to the plant tops was substantially reduced after treatment with lanthanum, compared with mixtures of rare earth elements. The results indicated that the chondritenormalized pattern of individual rare earth elements in crops should be analyzed for the safety assessment of agricultural application of rare earth elements. Acknowledgements This project was financially supported jointly by the National Natural Science Foundation of China (Grant No ) and by the Fengqiu Agro-Ecological Experimental Station from the Chinese Academy of Sciences (Grant No ) and by the Royal Netherlands Academy of Arts and Sciences (00CDP005). K. C. Wong Education Foundation (Hong Kong) also financially supported the post-doctoral work. References Adrian P J, Wall F and Williams C T 1996 Rare Earth Minerals: Chemistry, Origin and Ore Deposits. Chapman & Hall, UK. 129 pp. Brookins D G 1989 Aqueous geochemistry of rare earth elements. In Geochemistry and Mineralogy of Rare Earth Elements. Eds. Lipin B R and G A McKay. pp Mineralogical Society of America, Washington. Brown P H, Rathjen A H, Graham R D and Tribe D E 1990 Rare Earth Elements in Biological Systems. In Handbook on the Physics and Chemistry of Rare Earths, Vol 13. Eds. Gschneidner Jr K A and L Eyring. pp North-Holland, Amsterdam. Chang J 1991 Effects of lanthanum on the permeability of root plasmalemma and the absorption and accumulation of nutrients in rice and wheat. Plant Physiol. Commun. 27, Diatloff E, Asher C J and Smith F W 1996 Concentration of rare earth elements in some Australian soils. Aust. J. Soil Res. 34, Diatloff E, Smith F W and Asher C J 1995 Rare earth elements and plant growth: I. Effects of lanthanum and cerium on root elongation of corn and mungbean. J. Plant Nutr. 18, Evensen N M, Hamilton P J and O Nions R K 1978 Rare earth abundances in chondritic meteorites. Geochim. Cosmochim. Acta 42, Fu FengFu, Akagi T, Yabuki S and Iwaki M 2001 The variation of REE (rare earth elements) patterns in soil-grown plants: a new proxy for the source of rare earth elements and silicon in plants. Plant Soil 235, Henke G 1977 Activation analysis of rare earth elements in opium and cannabis samples. J. Radioanal. Nucl. Chem. 39, Ichihashi H, Morita H and Tatsukawa R 1992 Rare earth elements in naturally grown plants in relation to their variation in soils. Environ. Pollut. 76, John R D, Owen B, Andrew F, Louise M G, Malcolm S C, Peter W and Mark C 1998 Atomic spectrometry update-environmental analysis. J. Anal. At. Spectrom. 13, Knappe A, Fritz B, Pekdeger A, Moller P, Dulski P and Hubberter H W 2001 Using the REE gadolinium as a new tracer for sewage influence in aqueous urban systems. In Water-Rock Interaction, Vol Ed. Cidu R. pp AA Balkema, Rotterdam. Kim H T 1995 Soil Sampling, Preparation and Analysis. Marcel Dekker, New York. 115 pp. Lauchli A and Bieleski R L 1983 Inorganic Plant Nutrition. Spring- Verlag, Berlin. 23 pp. Miekeley N, Casartelli E A and Dotto R M 1994 Concentration levels of rare earth elements and thorium in plants from the Morro do Ferro environment. J. Radioanal. Nucl. Chem. 182,

11 277 Nagahashi G, Thomson W W and Leonard R T 1974 The Casparian strip as a barrier to the movement of lanthanum in corn roots. Science 183, Ni B F, Tian W Z, Nie H N, Wang P S and He G K 1999 Study on air pollution in Beijing s major industrial areas using multielements in biomonitors and NAA techniques. Biol. Trace Elem. Res. 71/72, Ni G 1995 Bioinorganic Chemistry of Rare Earth Elements. Science Press, Beijing. 58 pp. Sun J, Zhao H and Wang Y 1994 Study of the contents of trace rare earth elements and their distribution in wheat and rice samples. J. Radioanal. Nucl. Chem. 179, Wang Y Q, Sun J X, Chen H M and Guo F Q 1997 Determination of the contents and distribution characteristics of REE in natural plants by NAA. J. Radioanal. Nucl. Chem. 219, Wang Z, Liu D F, Lu P and Wang C X 2001 Accumulation of rare earth elements in corn after agricultural application. J. Environ. Qual. 30, Wen H Y, Peng R Z and Chen X W 2000 Application of rare earth compound fertilizer in some crops in central Yunnan. Chinese Rare Earths 21, Wyttenbach A, Furrer V, Schleppi P and Tobler L 1998 Rare earth elements in soil and in soil-grown plants. Plant Soil 199, Xu X K and Wang Z J 2001 Effect of lanthanum and mixtures of rare earths on ammonium oxidation and mineralization of nitrogen in soil. Eur. J. Soil Sci. 52, Xu X K, Wang Z J, Wang Y S and Inubushi K 2003 Urea hydrolysis and inorganic-n in a luvisol after application of fertilizer containing rare-earth elements. Aust. J. Soil Res. 41(4)(in press) Xu X K, Zhu W Z, Wang Z J and Witkamp G J 2002 Distributions of rare earths and heavy metals in field-grown maize after application of rare earth-containing fertilizer. Sci. Total Environ. 293, Yu Z and Chen M 1995 Rare Earth Elements and their Application. Metallurgical Industry Press, Beijing. 55 p. Zhang H L, Zhang C G, Zhao Z Q, Ma J H, Li X Q and Li L 2002 Effects of La 3+ on H + transmembrane gradient and membrane potential in rice seedling roots. J. Chin. Rare Earth Soc. 20, Zhu W 1999 Advanced Inductively Coupled Plasma Mass Spectrometry Analysis of Rare Earth Elements: Environmental Application. Ph Dissertation. Technical University of Delft, AA Balkema Publishers, Rotterdam. 60 pp. Section editor: A.J.M. Baker