A contribution on the bio-actions of rare earth elements in the soil/plant environment

Abstract

Data on the biological effects of REEs are scarce and contradictory. There are no indications that REEs are essential to humans and animals. For plants, no data concerning essentiality are available either. It has been suggested that REEs may increase the yield of crop plants. However, the reported effects of application of REEs as fertilizers ranged from stimulation to no role in increasing agricultural plant production up to reduction of growth, apparently as a function of concentration, speciation and bioavailability. The main objective of the present study was to investigate the effect of the REEs (La, Ce, Pr, and Nd) in a soil substrate on morphological, agronomic and physiological parameters of oilseed rape and maize, and soil microbial parameters under controlled greenhouse conditions. The research strategy was based on the comparison between the effects of REEs compared to that of another heavy metal, Cu, and Ca as Ca may be replaced by the presumably more effective La in plant metabolism. Two agricultural crops, maize (Zea mays L.) and oilseed rape (Brassica napus L.) were tested. The investigations were conducted in pot experiments under controlled greenhouse conditions. Each pot (capacity 1 litre) contained 900 g of soil substrate (dry weight basis) and was seeded with 6 maize seeds and 10 oilseed rape seeds on April 29th and Mai 14th and harvested on July 5^th and July 17^th in 2005 and 2006, respectively. Several treatments have been performed using five different REE-fertilizer application rates (REE0: control, REE1: 2.7 µg g^-1, REE2: 27 µg g^-1, REE3: 135 µg g^-1 and REE4: 270 µg g^-1 added as RECl3 x xH2O). REE-fertilizer (La, ce, Pr, Nd), La, Ce, Ca and Cu treatments were applied at rates being multiples of their plant available content in soils (1-fold, 10-fold, 50-fold, 100-fold). In case of Ca graded rates were based on its plant available concentration in the first year of experimentation and adjusted to that of rare earth elements in the second year of experimentation. Essential nutrients (N, P, K, Mg, and S) were mixed homogenously with the soil before sowing in order to fully satisfy the nutrient demand. Each treatment was carried out with 4 and 6 replicates. The most important findings of the research work presented here were:
1) With graded REE-fertilizer application rates, the soil enzyme activities (dehydrogenase and alkaline phosphatase) decreased. In general, the dehydrogenase activity was 78 % higher in 2005 and 96% higher in 2006 in vegetated (maize) soil compared to nonvegetated soil. The corresponding values for oilseed rape were 84% in 2005 and 96% in 2006. With graded Ca (at rates based on its plant available concentration) and Cu application rates, the soil enzyme activities (dehydrogenase and alkaline phosphatase) decreased, whereby this effect can be attributed to the toxic effect of Cu to soil microorganisms. The strongest Ca effect was observed in 2005 when maize was cultivated on the dehydrogenase activity with a reduction of 24%. In comparison, Cu yielded a reduction of the dehydrogenase activity of 56% when maize was grown and 62% when oilseed was cultivated. Ca reduced the alkaline phosphatase activity at maximum by 25% when oilseed rape was grown in 2005. Cu reduced the alkaline phosphatase activity by about 17% in both crops. Generally, soil enzyme activity values (dehydrogenase and alkaline phosphatase) of oilseed rape were between 25 and 38% higher than for maize.
2) Low rates of La, Ce and REE-fertilizer applications resulted regularly in a higher number of selected microbial counts, whereby this effect was more pronounced for maize. Also Ca rates equivalent to that of La and Ce yielded a significant increase in the number of heterotrophic bacteria and actinomycetes in maize pots.
3) Graded REE-application rates promoted (at low levels) and inhibited (at high levels) the soil microbial communities and this observation is well in accordance with the literature. Graded REE-fertilizer decreased significantly the number of fungi in pots grown with oilseed rape from 1.5·106 to 6.9·105, while this effect was not significant for maize. Graded rates of Cu reduced all three microbial parameters irrespective of the crop. This may be attributed that graded Cu application rates are toxic at these levels to soil microbial communities. Fungi, among soil microbial communities, were more sensitive to changes in soil characteristics. This may be related the negative and significant correlative relationships between the number of fungi and soil EC and pH and some soil enzyme activities.
4) Graded REE applications, in general, decreased the germination rate from 100% to 83% and plant height from 73 cm to 52 cm in case of maize. In case of oilseed rape, plant height was reduced from 29.7 cm to 22.1 cm. The results were consistent in each year. The results revealed that the effect on germination rate (maize) and plant height (oilseed rape) was significant when the highest rate of REE fertilizers (270 μg g^-1) were applied.
5) It was observed that graded REE-fertilizer application rates promoted the total biomass production up to levels of 2.7 µg g^-1 for maize in 2005 and 2006, and 27 µg g^-1 in 2005 and 2.7 µg g^-1 for oilseed rape. So the biomass of maize increased in 2005 from 15.5 g pot^-1 to 24.9 g pot^-1 and in 2006 from 14.0 g pot^-1 to 31.5 g pot^-1. In case of oilseed rape the biomass increased from 10.0 g pot^-1 to 20.9 g pot^-1 in 2005 and 12.5 g pot^-1 to 13.8 g pot^-1 in 2006. Biomass production of both crops was reduced at rates of 270 µg g^-1 by up to 47% in maize and 52% in oilseed rape. This observation is well in accordance with results reported in literature. These and other findings suggest that next to the concentration, the composition of REEs influence the impact on plant growth.
6) The REE content of roots and shoots increased with increasing REE applications (La, Ce and REE-fertilizer). The highest concentration of REEs was found in roots when compared to shoots of oilseed rape and maize. The La, Ce, Pr and Nd content was at maximum 120, 180, 17.9 and 56.7 μg g-1 in roots and 1.7, 1.8, 0.18 and 0.58 μg g^-1 in shoots of maize. The upper values for oilseed rape were 163, 235, 21.9 and 67.2 μg g^-1 in roots and 3.7, 5.7, 0.61 and 2.0 μg g^-1 in shoots. On average the La, Ce, Pr and Nd concentrations were 100 times higher in roots than in shoots of maize. Differences in the shoot concentrations between both crops were even more pronounced for Ca and Ce, which were about 10 times higher in oilseed rape than maize. The Pr and Nd concentration in oilseed rape shoots was with 0.61 and 2.0 μg g-1 about 27 and 2.5 times higher than in maize. This may be attributed to the fact that roots of dicots (oilseed rape) release and take up more REEs in the soil from its compounds than monocots (maize). The results reveal that next to crop also element-specific differences in root uptake of REEs exist and that translocation of individual REEs within the plant seem to be controlled by different transporter systems for oilseed rape and maize.
7) Accumulation of REEs in different parts of plants decreased in the following order: root > shoots and REEs in the order: Ce > La > Nd >Pr for each part of plants and for each crop. With increasing application rates of La, Ce and REE-fertilizer the concentration and uptake of La, Ce and REEs (La, Ce, Pr and Nd) increased in both roots and shoots of each crop.
8) Highly significant and significant correlation coefficients (r) were found between graded REE-application rates and the REE uptake of shoots and roots in maize and oilseed rape. The relationship between Ca concentration in maize roots and REE concentrations in maize roots proved to be not significant. In contrast, these relationships were significant between REE concentrations in maize shoots and Ca concentrations in the same plant part.
9) In general, with the exception of K, graded REE-fertilizer applications increased the concentration of essential nutrients from about 15% for S to up to 45% for Zn in roots of maize. This trend was not consistent for oilseed rape. With graded REE-fertilizer application rates, the concentrations of K, Fe, and Zn decreased in shoots of maize and oilseed rape. In case of roots, the highest uptake values were found for all essential macro and micro-nutrients in maize, whereas that for oilseed rape were least affected by graded REE-fertilizer application rates. Inverse results were determined for shoots where the S, Ca and Mn uptake was highest for oilseed rape and distinctly lower in maize.
10) The individual transfer factors (TFs) of REEs decreased with graded REE-fertilizer application rates for roots and shoots of maize and oilseed rape. The highest application rate of REE fertilizer reduced the TFsoil/roots in pots vegetated with maize from 4.24 to 1.19 (La), 11.0 to 2.2 (Ce), 4.5 to 0.9 (Pr) and 4.7 to 0.8 (Nd); the TFsoil/shoots decreased from 0.096 to 0.017 (La), 0.283 to 0.022 (Ce) and 0.084 to 0.008 (Nd). The highest application rate of REE fertilizer reduced the TFsoil/roots in pots vegetated with oilseed rape from 9.54 to 1.52 (La), 23.9 to 2.9 (Ce), 9.4 to 1.1 (Pr) and 9.7 to 0.95 (Nd); the TFsoil/shoots decreased from 0.225 to 0.037 (La), 0.468 to 0.07 (Ce), 0.51 to 0.031 (Pr) and 0.182 to 0.029 (Nd). Generally, for individual REEs the TF values decreased in the order Ce > La > Nd > Pr in roots and shoots of oilseed rape and maize. The TF for total biomass (roots or shoots) decreased with graded REE-fertilizer application rates for both crops. In general, TFs for individual REEs and the sum of REEs were higher when oilseed rape was grown than maize; this result was consistent for roots and shoots.
11) When compared to maize, oilseed rape plants contained the highest values of both Alpha–tocopherol (248 μg g^-1 dw) and total chlorophyll (13.8 μmol g^-1 dw) when treated with REEs. This indicates that monocotyledonous plants (like maize) react differently to heavy metal stress than dicotyledonous plants. Graded REE-fertilizer application rates increased the Alpha-tocopherol content in maize from 59 to 95 μg g^-1 dw, too but this effect was not significant for any crop. The total chlorophyll content, in maize and oilseed rape leaves decreased with increasing of graded REE application rates.