The Growth and Residual Effects of Applied Rare Earth Elements (REEs) on Wheat

YAN BEIZHAN, LIANG TAO, ZHANG SHEN, WANG LIJUN, HU AITANG

Institute of Geography, CAS, Beijing, 100101, China; Nanjing Agricultural University, Nanjing, 210095, China

ABSTRACT

12 wheat samples soil-dressed with REEs and their controls were collected from 5 provinces of China. Different parts (root, leaf, straw and grain) were submitted to ICP-MS. Results showed that there were almost no REEs accumulations (just 5-10% higher) in applied samples compared with the controls. Pot experiments were carried out with increasing concentrations (0, 333, 667, 1000, 1333, 1667, 2000, 2500µg/g) of REEs applied in yellow brown soil (REEs 168.95µg/g) on winter wheat (determined by Instrumental Neutron Activation Analysis (INAA)). The Higher REEs concentrations used the higher REEs contents in whole plant (except grain). Similar REE distribution of different parts of applied wheat with those of applied fertilizer indicates that the applied REEs has been absorbed by roots and accumulated in these parts. Dry matter weights of these parts in three periods(seedling, jointing-booting, maturity) of each sample have also been measured. Results display that to these plants dealt with REEs from 333 to 1600 µg/g, in seedling period, the REEs inhibits the wheat growth; while in joint-booting period, the REEs promotes the wheat growth. The unrecoverable damage caused by REEs is about 2000-2500µg/g. Statistic analyses displayed that the lethal dose is about 3320µg/g.

Key words: Rare Earth Elements Wheat Soil-dressing Growth effect Residue

INTRODUCTION

In China, numerous studies have demonstrated that using REEs microfertilizers could increase the yields of various kinds of crops (GUO Bosheng, et al. 1985). In the past 20 years, REEs fertilizer (known as Nongle then Changle, an agent mainly containing REEs) has been widely used in farming. There were mainly three methods using REE fertilizers: soilñdressing, leaf-dressing and foliage-dressing. Now the soil-dressing is be widely accepted by farmer because of its easy operation (REEs was added to the other fertilizer and applied to soil directly during plowing)( Wang Lijun.et al. 1999). But the soil-dressing might cause more heavily residue for this method uses many times of REEs' amount than the others (Wang Lijun.et al., 1999). Although many research works on REEs environmental studies have also been made (LIU Shujuan, et al., 1997, SUN Jingxin et al. 1994) there leaves much to make certain whether this method could cause the REEs accumulation and residue in plants and soils and whether the grain's REE contents will be very high after continuous soil application of REEs. Does the high REE content cause environment problems? All these are very important to evaluate the REEs effects in agriculture. In this study, 12 applied samples (soil-dressing) and their controls were collected from 5 provinces. Pot experiments were also carried out to study the growth and residual effects of REEs on wheat under high REE contents. METHODS 1.1 Field study under normal application: The applied fertilizer( named REEP in this paper) was produced by Sichuang Guanghan Qingquan Corporation (a kind of fertilizer whose main producing procedure is to add REE solution into the general phosphorus fertilizer (Ca(HPO4)2..H2O), named GP). REEP was applied to the top layer (0-20cm) of applied samples during plowing, while GP used to the controls which was produced also by the same company. By investigating and choosing, 12 applied samples and their controls were selected from 5 provinces (detailed methodologies will be given elsewhere). The applied samples and controls should be adjacent and have the same soil type°¢the same wheat type and same input except the REEs. In the other words, all living conditions are the same except the different REEs input contents. The whole plants were collected at maturity period, and then divided into four parts with ceramic scissors: root°¢straw°¢leaf and seed. All parts were examined by ICP-MS (Elan 500 made by Perkin- Elmer Sciex Co.) after cleaning with deionized water. The parameter and conditions were in accordance with that described by LIANG Tao, et al (1999). 1.2 Pot experiments under high REE contents Detailed descriptions had been reported by other papers ( HU Aitang, et al. 1999), here is just the outline of the experiments: Pot experiments were carried out in a conservatory with increasing concentrations (0, 333, 667, 1000, 1333, 1667, 2000, 2500µg/g separately) of rare earth elements (in the cpd of REE(Cl)3)applied in yellow brown soil (168.95µg/g,. Nanjing,China). During the three periods( seedling, jointing-booting, and maturity) the samples were collected and divided to various parts. The weight of dry matter was measured and 8 kinds of REE were determined by Instrumental Neutron Activation Analysis (INAA). The method is detailed described by SUN jingxin et al(1994 ). 2.

RESULTS AND DISCUSSION

2.1 Samples from field study 24 samples were collected from 12 REEP applied sites with each site one applied and one control. Statistic analyses show that the contents of REEs don't follow normal but meet lognormal distribution(LIANG Tao et al, 2000). So geometric mean and standard deviation can properly represent the characteristics of size and scatter with contents of REEs in wheat. The statistic results will be described detailed in another paper(YAN BEIZHAN, Chinese Sciences Bulletin). Following the general method of geochemistry, contents of REEs in plants are standardized by contents of REEs in Chondrites. The distributions of REEs in various parts of the wheat are showed in Fig.1. There is some similarity in the distribution pattern between the plant samples with the soil (the average REE contents in the Chinese soils). That means the biological differentiation in the soil-crops system is not explicit under normal application methods. Results also indicate that there are no obvious accumulations between the control and applied samples. The applied samples have the same REEs distributions with the control. The REEs contents of various part (roots leaves straws, grains) of applying is about 5-10% higher than the control. We attribute this increase of REEs content of applied samples to the weak effect of REEP on the wheat, because the REEP might increase the soil REE concentrations (more analyses is being made to get the REEP effects on the soil). But all contents of applied samples are in the background of national standards of wheat (GB13107-91) so they are safe in the food chain.. 2.2 Pot experiments applied high contents 2.2.1 The effect of REEs on wheat growth. Fig.2.Growth response of winter wheat to REEs uptaken by roots Fig 2 clearly describes the relationship of biomass of whole plants with the applied REEs concentrations in different growing periods. Results show that during seedling period, the biomass has a linear relationship with the REEs contents: the higher the REEs contents the lower the biomass; While during joint-booting and maturity periods, the relationship is relatively complex. When the concentrations are between 400-1667ppm, the biomass of the applied is higher than that of the controls, when the concentration is higher than 1600, the biomass is lower than control. So REEs has different effects on the growth process in different periods. In the seedling period, the plants are sensitive to the changes of REEs concentrations. At this period, REEs restrain wheat growth (other reports also show same conclusion); while during the joint-booting periods, the REEs strongly stimulates the growth of wheat. At the contents of 400- 1600 PPM the wheat not only makes up the difference of biomass during seedling period, but also outstrips the control biomass about 10-20% percent. After booting until maturity, the growth rate is the same with the control except the sample at 2500ppm, which still shows restraint of growth, we call it theunrecoverable concentration >From above, it was realized that the suitable concentrations of applied REEs were very important. REEs have the two-blades on the wheat growth, at suitable concentration (0- 1600ppm), it stimulates the growth of wheat, if higher than 1600, it might arrests the growth. In different periods wheat also shows different growth response to the REEs (suitable concentration): seedling period checking; joint-booting stimulating. 2.2.2 The tolerance of wheat to REEs As mentioned earlier and shown in fig.4, during seedling period, biomass has a linear relationship with the concentration of applied REEs. By making trendline, a equation was got( Fig.3). Extrapolating the line to the zero value of the dry weight, the concentration is 3320. The concentration could be called lethal dose. 2.2.3 Distributions of REEs in applied wheat The REEs are frequently subdivided in two groups: the light REE (from La to Eu, LREE) and heavy REEs( HREE, from Gd to Lu). In the geochemistry field, the LREE/HREE (L/H) stands for the ratio of light REEs with heavy REEs. The contents and L/H of REEs of various parts at maturity period were given in table 1. Fig. 4 shows the distribution patterns of REEs in the leaves. The table indicates that the contents of REEs in the various parts (except grain) have a positive relationship with the REEs content applied to the soils. Fig.4 and table 1 also show that in the applied samples, LREE (the main components of the applied REE fertilizer with L/H ratio 558.43) were significantly enriched in each parts of the plants except grain, which also demonstrate the applied REEs have be absorbed by roots and accumulated in these parts. Grains have no response to the change of applied REE concentrations Table 1. The Total REEs concentrations and L/H in various parts of wheat Content 0 333 667 1000 2500 ÂR L/H ÂR L/H ÂR L/H ÂR L/H ÂR L/H Grain 1.231 37.00 0.654 36.27 0.762 17.33 0.286 16.58 0.728 22.80 Straw 0.758 2.85 1.221 7.06 1.786 7.64 2.409 9.65 18.19 93.00 Husk 1.210 3.07 2.018 13.54 1.693 6.87 2.249 7.32 5.588 31.07 Leaves 7.468 2.92 11.17 6.50 14.95 11.76 40.857 19.24 349.33 104.60 Root 20.573 8.50 118.136 8.09 397.037 79.39 958.440 146.30 3893.132 214.36 Note: ER is the total REES , L/H stands for the ratio of LREE and Heavy REE. Only 8 kind of REEs concentration are got from INAA( LA, Ce Nd, Sm, Eu, Tb, Yb, Lu), the other 7 elements are interpolated by being normalized to Chondrites. 2.3 the safety of the REE fertilizer: >From above, we know that if applied normally, there is no obviously REE residue in the plants. But after many years of continuous use, the soil might contain high contents of REE, and the redundant REE might enter into the food chain when the leaves and straws are used. The medical research have proved that high contents of REE in the environment could lower the children intelligence quotient (IQ)( Zhu Weifang, et al. 1996) and impair the living organism's Calcium's normal absorption( P. H. Brown, et al.1990). So, the overabundant usage of REES should be avoided.

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