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Interactions Between Exogenous Rare Earth Elements and Phosphorus Leaching in Packed Soil Columns
Pedosphere 20(5): 616–622, 2010 ISSN 1002-0160/CN 32-1315/P c 2010 Soil Science Society of China Published by Elsevier Limited and Science Press
Interactions Between Exogenous Rare Earth Elements and Phosphorus Leaching in Packed Soil Columns∗1 LIANG Tao1,∗2 , SONG Wen-Chong1 , WANG Ling-Qing1 , P. J. A. KLEINMAN2 and CAO Hong-Ying1 1 Institute
of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101 (China) Pasture Systems and Watershed Management Research Unit, University Park, PA 16802-3702 (USA)
2 USDA-ARS,
(Received December 10, 2009; revised July 9, 2010)
ABSTRACT Rare earth elements (REEs) increasingly used in agriculture as an amendment for crop growth may help to lessen environmental losses of phosphorus (P) from heavily fertilized soils. The vertical transport characteristics of P and REEs, lanthanum (La), neodymium (Nd), samarium (Sm), and cerium (Ce), were investigated with addition of exogenous REEs at various doses to packed soil columns (20 cm deep). Vertical transfers of REEs and P were relatively small, with transport depths less than 6 cm for most REEs and P. Export of applied REEs in leachate accounted for less that 5% of inputs. The addition of Ce, Nd and Sm to soil columns significantly decreased concentrations of extractable soil P up to a depth of 4 cm, with soil P concentrations unaffected at depths > 4 cm. In general, REEs had little effect on the vertical leaching of P in packed soil columns. Key Words:
extractable soil P, manure, mobility, translocation, vertical transport
Citation: Liang, T., Song, W. C., Wang, L. Q., Kleinman, P. J. A. and Cao, H. Y. 2010. Interactions between exogenous rare earth elements and phosphorus leaching in packed soil columns. Pedosphere. 20(5): 616–622.
INTRODUCTION Exogenous rare earth elements (REEs) can be rapidly adsorbed and immobilized on the solid surfaces of soils after application (Xu, 2005). It is well established that REEs have strong effects on soil fertility, composition of soil solution, soil microorganisms, soil urea activity, and the amount of available nitrogen (Ding et al., 2005). Rare earth elements can also alter the adsorption of ions onto soil particle surfaces as well as change the cation exchange capacity of soils, thus playing an important role in soil conservation and the availability of nutrients, especially anions such as phosphate, to the soil solution and plant roots (Xu, 2005). Phosphorus is a reactive solute but primarily exists in insoluble phosphate complexes in soils. Phosphates can be readily adsorbed by soil minerals (clay edges and amorphous oxides with amphoteric charge), precipitated by common soil cations (Al, Ca, Fe), or immobilized by microorganisms after a series of chemical, biological and physical transformations. As a result, the utilization efficiency of applied P by for crops can be as low as 10%–25%. Increasingly, the environmental fate of P is of concern as P is a key limiting nutrient of freshwater eutrophication (Jarvie et al., 2006). Phosphorus transport, while dominantly occurring in surface runoff, can also be significant in subsurface flow (Sims et al., 1998). New environmental management practices to minimize off-site P transport have been developed, some of which involve the application of materials to convert phosphate in soils and soil amendments (e.g., manures) into insoluble forms to minimize dissolved P in runoff water (Callahan et al., 2002). Previous REE studies have focused on the improvement of crop yield and quality (Liang et al., 2007), ∗1 Supported
by the National Natural Science Foundation of China (Nos. 40871225 and 40571146) and the Short-Term Visiting Program for Advanced Scholars, Chinese Academy of Sciences. ∗2 Corresponding author. E-mail: [email protected].
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differentiation of REEs in plants (Ding et al., 2005), REE distribution, adsorption and desorption (Zhu et al., 1996), speciation and biological availability (Sun et al., 1998) and the alteration of soil properties after the application of agricultural rare earth fertilizers (Liang et al., 2007). However, the vertical transport of REEs and P in soil profiles and their interactions have not been assessed. Indeed, REEs have the potential to adsorb or precipitate phosphate anions, particularly when applied in halide form to soils (Sun et al., 1998). In addition, the formation of REE-P compounds provides the opportunity to better understand particulate P transport, using specific REEs as reactive tracers. The purpose of this study was to investigate the vertical transport of P as affected by exogenous REEs via a benchtop leaching experiment. A total of 4 REEs, lanthanum (La), neodymium (Nd), samarium (Sm), and cerium (Ce), were added to soil columns at different rates to assess their effects on vertical P transport. MATERIALS AND METHODS Materials Composite soil samples for the column experiment were collected from surface soils (0–25 cm deep) at the National Xiaotangshan Precision Agriculture Experimental Base, Beijing, China. The soils were classified as Haplic Greyxems according to legend of the FAO-UNESCO soil map of the world and had been under wheat-corn rotational management without applications of REEs and manures for 25 years. The soil has an organic matter content of 2.15%, an electrical conductivity (EC) of 108 μS cm−1 , total P and Olsen P of 365 and 22 mg kg−1 , and total La, Nd, Ce and Sm contents of 33.2, 35.9, 74.6 and 7.1 mg kg−1 , respectively. Samples for the chicken (Gallus domesticus) manure, which is richest in P, were collected from egglaying chickens of the Organic Agriculture Demonstration Area in Liuminying, Beijing, with a total P content of 14.557 g kg−1 after air drying. Both soil and fertilizer samples were sieved (following air drying) through a 2-mm nylon mesh prior to the column experiment. Rare earth chlorides were selected for the study. Chloride crystals of La, Ce, Nd and Sm (RECl3 ·6H2 O, purity up to 99.95%) were passed through a 100-mesh sieve, and hermetically preserved in desiccators prior to their use in the column experiment. Experimental design and chemical analyses Polyvinyl chloride (PVC) tubes (external diameter = 11 cm, internal diameter = 10.5 cm, height = 20 cm) were used in the column experiment. A layer of medium-speed qualitative filter paper was placed at the bottom of each tube, using a perforated PVC disk (9 holes of 1 cm diameter). At the bottom of each column, a total of 900 g of soil (column height = 8 cm) was used to fill the lower 8 cm (bulk density = 1.299 g cm−3 ). Then, a total of 225 g soil were mixed with 6.84 g chicken manure (equivalent to addition of 200 kg ha−1 total P), plus different doses of exogenous REEs. The doses of exogenous REEs (Table I) were designed as 0, 4, 8, 12, 16 and 20 times the background value of REEs in the soil which was determined using inductively coupled plasma-mass spectrometry (ICP-MS, ELAN TABLE I Doses of exogenous rare earth elements (REEs) in the soil columns Rare earth chloride
LaCl3 ·6H2 O SmCl3 ·6H2 O NdCl3 ·6H2 O CeCl3 ·6H2 O
Times the REE background value in soil 4
8
12
16
20
0.019 0.062 0.322 0.679
0.076 0.124 0.644 1.359
g 0.152 0.186 0.965 2.039
0.228 0.248 1.287 2.718
0.304 0.311 1.609 3.397
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DRC-e, PerkinElmer ELAN DRC-e, PerkinElmer SCIEXTM , Canada) from the initial analysis of soil properties. The soil/manure mixture was then placed on top of the unamended soil in each PVC tube to simulate a surface horizon that had been amended with manure and REEs with an approximate bulk density of 1.339 g cm−3 . Leaching experiments were conducted to assess trends in P and REEs translocation in the packed soil columns over a 12 d period. For the initial irrigation, 170 mL deionized water was applied to each column and allowed to drain for 48 h. Care was taken to ensure that irrigation water was applied slowly and did not pond, ensuring aerobic conditions during the experiment. Following the initial irrigation and 48 h drainage period, columns were irrigated with 200 mL of deionized water every 48 h over a 10 d period, resulting in a total of 1 000 mL of additional irrigation water, equivalent to 11.56 cm depth. The PVC tubes were covered with plastic material after each irrigation to minimize evaporation. Leachate was collected every 48 h immediately prior to additional irrigation and stored at 4 ◦ C for analysis. Following the completion of the 12 d leaching experiment, soil samples were sliced into 2 cm sections, dried, ground and sieved through a 100-mesh sieve for analyses. Laboratory and statistical analyses Soil and manure samples were digested with HNO3 -HF-HClO4 (Liang et al., 2007) to determine total P and rare earth elements. Extractable P and soil organic matter (SOM) were determined on select samples by the loss on ignition method of Bao (Bao, 2000). Electrical conductivity of soil samples was measured at 1:5 soil:water ratio (Liu et al., 2006). Phosphorus concentrations in digests and water samples were measured using inductively coupled plasma-optical emission spectrometry, and REE concentrations in digests and water samples were measured using ICP-MS. All samples were analyzed in duplicate, and the relative errors of the results were less than 1% on average. Differences in REEs and P between treatment groups were assessed by Student’s t-test in SPSS 13.0 for Windows (SPSS Inc., Chicago, USA). Associations between REEs and P were evaluated by Pearson’s correlation coefficient. Statistical inferences discussed below were considered significant at P < 0.05. RESULTS Concentrations of REEs and P in leachate Leachate samples were first collected on the 4th day of the leaching experiment, as no leachate was generated until then. At that point, approximately 570 mL had been applied to the columns. Most of the La concentrations were below the detection limit (0.001 mg L−1 ) and are not reported. In general, REE concentrations declined with subsequent leaching events, even in samples collected from the unamended control, although some inconsistencies were notable (e.g., Ce trends at the dose of 20 times the background value). Phosphorus concentrations in leachate also declined with time, although to a lesser degree than REEs. Relative differences in mean P concentrations in leachate point to the influence of exogenous REEs on P in leachate. However, the low concentrations of P in all leachate samples might obscure statistically significant differences. The concentrations of Sm and P in first batch were much higher than the others (Fig. 1). Except for the first batches of leachate for Ce (the 4th day), concentrations of Ce, Nd and P were all quite low, even when the exogenous REEs were at a high dosage of 20 times above their background values. Rare earth elements and P were variably correlated in leachate, with Sm, Ce and P concentrations significantly correlated (r = 0.61–0.69). No significant correlations were observed between Nd and P concentrations in leachate (r < 0.28). Concentrations of REEs and P in soil columns Trends in the vertical distribution of all four REEs were similar. The highest concentrations were observed at the surface (0–2 cm) where the exogenous REEs were added. At the layer of 2–4 cm, REE
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Fig. 1 Concentrations of rare earth elements (REEs) and P in leachates of soil columns with different doses of exogenous REEs.
concentrations decreased rapidly, and they reached the level for the soil column without exogenous REEs at the depth of 4 cm and below. Results for P were similar, with concentrations reaching background levels by the 6–8 cm sample (Fig. 2). DISCUSSION Relationship between REEs and P in leachate The concentrations of REEs and P in the leachate decreased gradually with the increase of leaching time. The results showed that the cumulative removal of REEs ranged from 0.39% to 0.40%, 0.43% to 0.50%, and 0.48% to 0.51% for Sm, Nd and Ce with the mean values of 0.40%, 0.44%, and 0.49% of applied sources, respectively. Although there was some difference in cumulative removal among the exogenous La, Ce, Nd and Sm, probably caused by the different adsorption capability of REEs in soils, it is obvious that vertical transfers of REEs were relatively difficult. Most of REEs and P were likely quickly adsorbed and fixed by the solid particles when they were added to soils. In addition, the use of packed soil columns, which lacked tortuous macropore pathways connecting the soil surface and column bottom, undoubtedly limited preferential flow. It is well established that preferential flow is the dominant transport pathway for P and soil particles to the subsoil in undisturbed soils (Bao, 2000; Xu, 2005; Liu et al., 2006). Even so, some exogenous REEs and P did leach from the columns, diminishing in concentration with time, perhaps as a function of dilution with increasing volumes of irrigation water and depletion of mobile sources. Interactions between REEs and P in soil columns Results of the laboratory leaching experiment demonstrated the limited mobility of exogenous REEs
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Fig. 2
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Concentrations of rare earth elements (REEs) and P in soil columns with different doses of exogenous REEs.
(La, Nd, Ce and Sm) and P via matrix flow through soils. Even when the dosages of the exogenous REEs were relatively high (e.g., 20 times their background values), very few REEs were leached. Previous studies showed that more than 99.5% of exogenous REEs could be adsorbed by soil particles after being added to soils, with only a very small portion remaining dissolved in the soil solution (Jones, 1997; Bao, 2000; Xu, 2005). Adsorption of REEs is affected by various factors including clay minerals, pH, oxidationreduction potential, organic matter, and cation exchange capacity (Ran and Liu, 1993). Meanwhile, when the soils were saturated with water, the transport time of leachate through the columns was short. Thus contact times between water and soils were insufficient for the dissolution of precipitated forms of REEs. Zhu et al. (1996) suggested that exogenous Ce could transport downward through soils at a rate of approximately 1.0 cm annually under acidic conditions, 0.2 cm in neutral conditions and negligibly in alkaline conditions. Zhang et al. (1996) studied the chemistry of 141 Ce and 147 Nd in a leaching experiment with 15-cm deep packed soil columns, and found that even when the exogenous
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REEs reached 50% of the adsorption capacity of soils and when the simulated rainfall reached 1 000 mm, the maximum depth that Ce and Nd could be leached from nine different soils was 4 cm. In a few cases, the REEs were leached to a depth of 10 cm, but no 141 Ce and 147 Nd were detected in the leachate. As with REEs, the transport of P through soils with poor structure, as in the packed soil column experiment, is impeded by a variety of processes (Sims et al., 1998). There are two principal mechanisms for vertical P transport in soil: permeation flux, which occurs with interactions with soil particles at a slow speed; and preferential flow, which happens when water passes through the soil pores at a high speed (Cogger and Duxbury, 1984). The leaching of P in soils is affected by many factors which are complicated and closely related, including climatic factors (rainfall, evaporation), soil factors (soil structure, content of organic matter, biological activity, pH, oxidation/reduction and permeability capacity for P), hydrographical factors (slope, groundwater level) as well as agricultural management factors (fertilizers and their quantity and application method, irrigation and rotation system) (Hooda et al., 2000; Sharpley and Tunney, 2000; Welch et al., 2009). The results of the laboratory leaching experiment showed that the vertical transport of P in the soil columns was difficult, and the depth that it could reach was less than 8 cm. The results of the laboratory leaching experiment also demonstrated that the influence of exogenous REEs on the vertical transport of P was limited. It was found that the addition of exogenous La enhanced the vertical leaching of P at the first stage, but the influence rapidly reached a balance and the transport depth for P was no more than 8 cm in the soil columns. The other exogenous REEs (Ce, Nd and Sm) did not show an obvious influence on the vertical transport of P, even when the doses of exogenous REEs reached a high level of 20 times their background values. CONCLUSIONS During the leaching experiment, La could not be detected in the leachate samples, and P in the leachate was also found at a low level of less than 0.001 mg L−1 . In the leachate samples from soil columns with exogenous Nd, Sm and Ce, concentrations of REEs and P were also very low. The REEs could hardly be leached even when the dosages of exogenous REEs reached a high level of 20 times the background values of the soil. The downward transport of La, Nd, Sm and Ce in the soil columns was very difficult, even when the exogenous REEs reached a high level, and they could easily be immobilized by soil particles. In the soil columns with exogenous La, the depth that La and P could be leached to was less than 6–8 cm. In the soil columns with exogenous Ce, Nd, and Sm, the REE and P concentrations decreased sharply at the depth of 2–4 cm, and their concentrations reached their background levels at the depth of 4–6 cm. Meanwhile the exogenous REEs did not have an obvious influence on the vertical transport of P. REFERENCES Bao, S. D. 2000. Soil and Agricultural Chemistry Analysis (in Chinese). China Agriculture Press, Beijing. Callahan, M. P., Kleinman, P. A., Sharpley, A. N. and Stout, W. L. 2002. Assessing the efficacy of alternative phosphorus sorbing soil amendments. Soil Sci. 167: 539–547. Cogger, C. and Duxbury, J. M. 1984. Factors affecting phosphorus losses from cultivated organic soils. J. Environ. Qual. 13: 111–114. Ding, S. M., Liang, T. and Yan, J. C., Zhang, Z. L. and Sun, Q. 2005. Effects of organic ligands on accumulation and fractionation of rare earth elements REEs in wheat. Acta Ecol. Sin. (in Chinese). 25(11): 2888–2894. Jarvie, H. P., Neal, C. A and Withers, P. J. A. 2006. Sewage-effluent phosphorus: A greater risk to river eutrophication than agricultural phosphorus? Sci. Total Environ. 306: 243–253. Hooda, P. S., Rendell, A. R., Edwards, A. C., Withers, P. J. A., Aitken M. N. and Truesdale V. W. 2000. Relating soil phosphorus indices to potential phosphorus release to water. J. Environ. Qual. 29: 1166–1171. Jones, D. L. 1997. Trivalentmetal (Cr, Y, Rh, La, Pr, Gd) sorption in two acid soils and its consequences for bioremediation. Eur. J. Soil Sci. 48: 697–702. Liang, T., Ding, S. M., Song, W. C., Chong, Z. Y. and Chen, Y. 2007. Advances of rare earth elements fractionations and
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mechanisms studies in plants and their significance. J. Chin. Rare Earth Soc. (in Chinese). 25(2): 129–132. Liu, G. M., Yang, J. S. and Yao, R. J. 2006. Electrical conductivity in soil extracts: Chemical factors and their intensity. Pedosphere. 16(1): 100–107. Ran, Y. and Liu, Z. 1993. Adsorption and desorption of rare earth elements on soils and synthetic oxides. Acta Sci. Circum. (in Chinese). 13(3): 782–794. Sharpley, A. and Tunney, H. 2000. Phosphorus research strategies to meet agricultural and environmental challenges of 21st century. J. Environ. Qual. 29: 176–181. Sims, J. T., Simard, R. R. and Joern, B. C. 1998. Phosphorus losses in agricultural drainage: Historical perspective and current research. J. Environ. Qual. 27: 277–293. Sun, H., Wang, X. R., Wang, Q., Wu, H., Yang, L. R., Chen, Y. J., Dai, L. M. and Cao, M. 1998. Study on the species and the bioavailabilities of rare earth elements in soil and wheat system. China Environ. Sci. (in Chinese). 18(3): 268–271. Welch, S. A., Christy, A. G., Isaacson, L. and Kirste, D., 2009. Mineralogical control of rare earth elements in acid sulfate soils. Geochim. Cosmochim. Acta. 73: 44–64. Xu, X. K. 2005. Research advances in the behavior and fate of rare earth elements in soil-plant systems. J. Agro-Environ. Sci. (in Chinese). S1: 315–319. Zhang, L. G., Zhu, W. M., Zhang, J. Z. and Chen, Z. Y. 1996. Determination of RE adsorption, desorption, and diffusion by isotopic tracer. J. Chin. Rare Earth Soc. (in Chinese). 14(3): 249–253. Zhu, W. M., Zhang, J. Z., Zhang, L. G. and Chen, Z. Y. 1996. Numerical simulation of RE migration in soils. J. Chin. Rare Earth Soc. (in Chinese). 14(4): 341–346.
Interactions Between Exogenous Rare Earth Elements and Phosphorus Leaching in Packed Soil Columns∗1 LIANG Tao1,∗2 , SONG Wen-Chong1 , WANG Ling-Qing1 , P. J. A. KLEINMAN2 and CAO Hong-Ying1 1 Institute
of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101 (China) Pasture Systems and Watershed Management Research Unit, University Park, PA 16802-3702 (USA)
2 USDA-ARS,
(Received December 10, 2009; revised July 9, 2010)
ABSTRACT Rare earth elements (REEs) increasingly used in agriculture as an amendment for crop growth may help to lessen environmental losses of phosphorus (P) from heavily fertilized soils. The vertical transport characteristics of P and REEs, lanthanum (La), neodymium (Nd), samarium (Sm), and cerium (Ce), were investigated with addition of exogenous REEs at various doses to packed soil columns (20 cm deep). Vertical transfers of REEs and P were relatively small, with transport depths less than 6 cm for most REEs and P. Export of applied REEs in leachate accounted for less that 5% of inputs. The addition of Ce, Nd and Sm to soil columns significantly decreased concentrations of extractable soil P up to a depth of 4 cm, with soil P concentrations unaffected at depths > 4 cm. In general, REEs had little effect on the vertical leaching of P in packed soil columns. Key Words:
extractable soil P, manure, mobility, translocation, vertical transport
Citation: Liang, T., Song, W. C., Wang, L. Q., Kleinman, P. J. A. and Cao, H. Y. 2010. Interactions between exogenous rare earth elements and phosphorus leaching in packed soil columns. Pedosphere. 20(5): 616–622.
INTRODUCTION Exogenous rare earth elements (REEs) can be rapidly adsorbed and immobilized on the solid surfaces of soils after application (Xu, 2005). It is well established that REEs have strong effects on soil fertility, composition of soil solution, soil microorganisms, soil urea activity, and the amount of available nitrogen (Ding et al., 2005). Rare earth elements can also alter the adsorption of ions onto soil particle surfaces as well as change the cation exchange capacity of soils, thus playing an important role in soil conservation and the availability of nutrients, especially anions such as phosphate, to the soil solution and plant roots (Xu, 2005). Phosphorus is a reactive solute but primarily exists in insoluble phosphate complexes in soils. Phosphates can be readily adsorbed by soil minerals (clay edges and amorphous oxides with amphoteric charge), precipitated by common soil cations (Al, Ca, Fe), or immobilized by microorganisms after a series of chemical, biological and physical transformations. As a result, the utilization efficiency of applied P by for crops can be as low as 10%–25%. Increasingly, the environmental fate of P is of concern as P is a key limiting nutrient of freshwater eutrophication (Jarvie et al., 2006). Phosphorus transport, while dominantly occurring in surface runoff, can also be significant in subsurface flow (Sims et al., 1998). New environmental management practices to minimize off-site P transport have been developed, some of which involve the application of materials to convert phosphate in soils and soil amendments (e.g., manures) into insoluble forms to minimize dissolved P in runoff water (Callahan et al., 2002). Previous REE studies have focused on the improvement of crop yield and quality (Liang et al., 2007), ∗1 Supported
by the National Natural Science Foundation of China (Nos. 40871225 and 40571146) and the Short-Term Visiting Program for Advanced Scholars, Chinese Academy of Sciences. ∗2 Corresponding author. E-mail: [email protected].
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differentiation of REEs in plants (Ding et al., 2005), REE distribution, adsorption and desorption (Zhu et al., 1996), speciation and biological availability (Sun et al., 1998) and the alteration of soil properties after the application of agricultural rare earth fertilizers (Liang et al., 2007). However, the vertical transport of REEs and P in soil profiles and their interactions have not been assessed. Indeed, REEs have the potential to adsorb or precipitate phosphate anions, particularly when applied in halide form to soils (Sun et al., 1998). In addition, the formation of REE-P compounds provides the opportunity to better understand particulate P transport, using specific REEs as reactive tracers. The purpose of this study was to investigate the vertical transport of P as affected by exogenous REEs via a benchtop leaching experiment. A total of 4 REEs, lanthanum (La), neodymium (Nd), samarium (Sm), and cerium (Ce), were added to soil columns at different rates to assess their effects on vertical P transport. MATERIALS AND METHODS Materials Composite soil samples for the column experiment were collected from surface soils (0–25 cm deep) at the National Xiaotangshan Precision Agriculture Experimental Base, Beijing, China. The soils were classified as Haplic Greyxems according to legend of the FAO-UNESCO soil map of the world and had been under wheat-corn rotational management without applications of REEs and manures for 25 years. The soil has an organic matter content of 2.15%, an electrical conductivity (EC) of 108 μS cm−1 , total P and Olsen P of 365 and 22 mg kg−1 , and total La, Nd, Ce and Sm contents of 33.2, 35.9, 74.6 and 7.1 mg kg−1 , respectively. Samples for the chicken (Gallus domesticus) manure, which is richest in P, were collected from egglaying chickens of the Organic Agriculture Demonstration Area in Liuminying, Beijing, with a total P content of 14.557 g kg−1 after air drying. Both soil and fertilizer samples were sieved (following air drying) through a 2-mm nylon mesh prior to the column experiment. Rare earth chlorides were selected for the study. Chloride crystals of La, Ce, Nd and Sm (RECl3 ·6H2 O, purity up to 99.95%) were passed through a 100-mesh sieve, and hermetically preserved in desiccators prior to their use in the column experiment. Experimental design and chemical analyses Polyvinyl chloride (PVC) tubes (external diameter = 11 cm, internal diameter = 10.5 cm, height = 20 cm) were used in the column experiment. A layer of medium-speed qualitative filter paper was placed at the bottom of each tube, using a perforated PVC disk (9 holes of 1 cm diameter). At the bottom of each column, a total of 900 g of soil (column height = 8 cm) was used to fill the lower 8 cm (bulk density = 1.299 g cm−3 ). Then, a total of 225 g soil were mixed with 6.84 g chicken manure (equivalent to addition of 200 kg ha−1 total P), plus different doses of exogenous REEs. The doses of exogenous REEs (Table I) were designed as 0, 4, 8, 12, 16 and 20 times the background value of REEs in the soil which was determined using inductively coupled plasma-mass spectrometry (ICP-MS, ELAN TABLE I Doses of exogenous rare earth elements (REEs) in the soil columns Rare earth chloride
LaCl3 ·6H2 O SmCl3 ·6H2 O NdCl3 ·6H2 O CeCl3 ·6H2 O
Times the REE background value in soil 4
8
12
16
20
0.019 0.062 0.322 0.679
0.076 0.124 0.644 1.359
g 0.152 0.186 0.965 2.039
0.228 0.248 1.287 2.718
0.304 0.311 1.609 3.397
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DRC-e, PerkinElmer ELAN DRC-e, PerkinElmer SCIEXTM , Canada) from the initial analysis of soil properties. The soil/manure mixture was then placed on top of the unamended soil in each PVC tube to simulate a surface horizon that had been amended with manure and REEs with an approximate bulk density of 1.339 g cm−3 . Leaching experiments were conducted to assess trends in P and REEs translocation in the packed soil columns over a 12 d period. For the initial irrigation, 170 mL deionized water was applied to each column and allowed to drain for 48 h. Care was taken to ensure that irrigation water was applied slowly and did not pond, ensuring aerobic conditions during the experiment. Following the initial irrigation and 48 h drainage period, columns were irrigated with 200 mL of deionized water every 48 h over a 10 d period, resulting in a total of 1 000 mL of additional irrigation water, equivalent to 11.56 cm depth. The PVC tubes were covered with plastic material after each irrigation to minimize evaporation. Leachate was collected every 48 h immediately prior to additional irrigation and stored at 4 ◦ C for analysis. Following the completion of the 12 d leaching experiment, soil samples were sliced into 2 cm sections, dried, ground and sieved through a 100-mesh sieve for analyses. Laboratory and statistical analyses Soil and manure samples were digested with HNO3 -HF-HClO4 (Liang et al., 2007) to determine total P and rare earth elements. Extractable P and soil organic matter (SOM) were determined on select samples by the loss on ignition method of Bao (Bao, 2000). Electrical conductivity of soil samples was measured at 1:5 soil:water ratio (Liu et al., 2006). Phosphorus concentrations in digests and water samples were measured using inductively coupled plasma-optical emission spectrometry, and REE concentrations in digests and water samples were measured using ICP-MS. All samples were analyzed in duplicate, and the relative errors of the results were less than 1% on average. Differences in REEs and P between treatment groups were assessed by Student’s t-test in SPSS 13.0 for Windows (SPSS Inc., Chicago, USA). Associations between REEs and P were evaluated by Pearson’s correlation coefficient. Statistical inferences discussed below were considered significant at P < 0.05. RESULTS Concentrations of REEs and P in leachate Leachate samples were first collected on the 4th day of the leaching experiment, as no leachate was generated until then. At that point, approximately 570 mL had been applied to the columns. Most of the La concentrations were below the detection limit (0.001 mg L−1 ) and are not reported. In general, REE concentrations declined with subsequent leaching events, even in samples collected from the unamended control, although some inconsistencies were notable (e.g., Ce trends at the dose of 20 times the background value). Phosphorus concentrations in leachate also declined with time, although to a lesser degree than REEs. Relative differences in mean P concentrations in leachate point to the influence of exogenous REEs on P in leachate. However, the low concentrations of P in all leachate samples might obscure statistically significant differences. The concentrations of Sm and P in first batch were much higher than the others (Fig. 1). Except for the first batches of leachate for Ce (the 4th day), concentrations of Ce, Nd and P were all quite low, even when the exogenous REEs were at a high dosage of 20 times above their background values. Rare earth elements and P were variably correlated in leachate, with Sm, Ce and P concentrations significantly correlated (r = 0.61–0.69). No significant correlations were observed between Nd and P concentrations in leachate (r < 0.28). Concentrations of REEs and P in soil columns Trends in the vertical distribution of all four REEs were similar. The highest concentrations were observed at the surface (0–2 cm) where the exogenous REEs were added. At the layer of 2–4 cm, REE
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Fig. 1 Concentrations of rare earth elements (REEs) and P in leachates of soil columns with different doses of exogenous REEs.
concentrations decreased rapidly, and they reached the level for the soil column without exogenous REEs at the depth of 4 cm and below. Results for P were similar, with concentrations reaching background levels by the 6–8 cm sample (Fig. 2). DISCUSSION Relationship between REEs and P in leachate The concentrations of REEs and P in the leachate decreased gradually with the increase of leaching time. The results showed that the cumulative removal of REEs ranged from 0.39% to 0.40%, 0.43% to 0.50%, and 0.48% to 0.51% for Sm, Nd and Ce with the mean values of 0.40%, 0.44%, and 0.49% of applied sources, respectively. Although there was some difference in cumulative removal among the exogenous La, Ce, Nd and Sm, probably caused by the different adsorption capability of REEs in soils, it is obvious that vertical transfers of REEs were relatively difficult. Most of REEs and P were likely quickly adsorbed and fixed by the solid particles when they were added to soils. In addition, the use of packed soil columns, which lacked tortuous macropore pathways connecting the soil surface and column bottom, undoubtedly limited preferential flow. It is well established that preferential flow is the dominant transport pathway for P and soil particles to the subsoil in undisturbed soils (Bao, 2000; Xu, 2005; Liu et al., 2006). Even so, some exogenous REEs and P did leach from the columns, diminishing in concentration with time, perhaps as a function of dilution with increasing volumes of irrigation water and depletion of mobile sources. Interactions between REEs and P in soil columns Results of the laboratory leaching experiment demonstrated the limited mobility of exogenous REEs
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Concentrations of rare earth elements (REEs) and P in soil columns with different doses of exogenous REEs.
(La, Nd, Ce and Sm) and P via matrix flow through soils. Even when the dosages of the exogenous REEs were relatively high (e.g., 20 times their background values), very few REEs were leached. Previous studies showed that more than 99.5% of exogenous REEs could be adsorbed by soil particles after being added to soils, with only a very small portion remaining dissolved in the soil solution (Jones, 1997; Bao, 2000; Xu, 2005). Adsorption of REEs is affected by various factors including clay minerals, pH, oxidationreduction potential, organic matter, and cation exchange capacity (Ran and Liu, 1993). Meanwhile, when the soils were saturated with water, the transport time of leachate through the columns was short. Thus contact times between water and soils were insufficient for the dissolution of precipitated forms of REEs. Zhu et al. (1996) suggested that exogenous Ce could transport downward through soils at a rate of approximately 1.0 cm annually under acidic conditions, 0.2 cm in neutral conditions and negligibly in alkaline conditions. Zhang et al. (1996) studied the chemistry of 141 Ce and 147 Nd in a leaching experiment with 15-cm deep packed soil columns, and found that even when the exogenous
REE INTERACTIONS WITH P LEACHING
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REEs reached 50% of the adsorption capacity of soils and when the simulated rainfall reached 1 000 mm, the maximum depth that Ce and Nd could be leached from nine different soils was 4 cm. In a few cases, the REEs were leached to a depth of 10 cm, but no 141 Ce and 147 Nd were detected in the leachate. As with REEs, the transport of P through soils with poor structure, as in the packed soil column experiment, is impeded by a variety of processes (Sims et al., 1998). There are two principal mechanisms for vertical P transport in soil: permeation flux, which occurs with interactions with soil particles at a slow speed; and preferential flow, which happens when water passes through the soil pores at a high speed (Cogger and Duxbury, 1984). The leaching of P in soils is affected by many factors which are complicated and closely related, including climatic factors (rainfall, evaporation), soil factors (soil structure, content of organic matter, biological activity, pH, oxidation/reduction and permeability capacity for P), hydrographical factors (slope, groundwater level) as well as agricultural management factors (fertilizers and their quantity and application method, irrigation and rotation system) (Hooda et al., 2000; Sharpley and Tunney, 2000; Welch et al., 2009). The results of the laboratory leaching experiment showed that the vertical transport of P in the soil columns was difficult, and the depth that it could reach was less than 8 cm. The results of the laboratory leaching experiment also demonstrated that the influence of exogenous REEs on the vertical transport of P was limited. It was found that the addition of exogenous La enhanced the vertical leaching of P at the first stage, but the influence rapidly reached a balance and the transport depth for P was no more than 8 cm in the soil columns. The other exogenous REEs (Ce, Nd and Sm) did not show an obvious influence on the vertical transport of P, even when the doses of exogenous REEs reached a high level of 20 times their background values. CONCLUSIONS During the leaching experiment, La could not be detected in the leachate samples, and P in the leachate was also found at a low level of less than 0.001 mg L−1 . In the leachate samples from soil columns with exogenous Nd, Sm and Ce, concentrations of REEs and P were also very low. The REEs could hardly be leached even when the dosages of exogenous REEs reached a high level of 20 times the background values of the soil. The downward transport of La, Nd, Sm and Ce in the soil columns was very difficult, even when the exogenous REEs reached a high level, and they could easily be immobilized by soil particles. In the soil columns with exogenous La, the depth that La and P could be leached to was less than 6–8 cm. In the soil columns with exogenous Ce, Nd, and Sm, the REE and P concentrations decreased sharply at the depth of 2–4 cm, and their concentrations reached their background levels at the depth of 4–6 cm. Meanwhile the exogenous REEs did not have an obvious influence on the vertical transport of P. REFERENCES Bao, S. D. 2000. Soil and Agricultural Chemistry Analysis (in Chinese). China Agriculture Press, Beijing. Callahan, M. P., Kleinman, P. A., Sharpley, A. N. and Stout, W. L. 2002. Assessing the efficacy of alternative phosphorus sorbing soil amendments. Soil Sci. 167: 539–547. Cogger, C. and Duxbury, J. M. 1984. Factors affecting phosphorus losses from cultivated organic soils. J. Environ. Qual. 13: 111–114. Ding, S. M., Liang, T. and Yan, J. C., Zhang, Z. L. and Sun, Q. 2005. Effects of organic ligands on accumulation and fractionation of rare earth elements REEs in wheat. Acta Ecol. Sin. (in Chinese). 25(11): 2888–2894. Jarvie, H. P., Neal, C. A and Withers, P. J. A. 2006. Sewage-effluent phosphorus: A greater risk to river eutrophication than agricultural phosphorus? Sci. Total Environ. 306: 243–253. Hooda, P. S., Rendell, A. R., Edwards, A. C., Withers, P. J. A., Aitken M. N. and Truesdale V. W. 2000. Relating soil phosphorus indices to potential phosphorus release to water. J. Environ. Qual. 29: 1166–1171. Jones, D. L. 1997. Trivalentmetal (Cr, Y, Rh, La, Pr, Gd) sorption in two acid soils and its consequences for bioremediation. Eur. J. Soil Sci. 48: 697–702. Liang, T., Ding, S. M., Song, W. C., Chong, Z. Y. and Chen, Y. 2007. Advances of rare earth elements fractionations and
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