Environ. Sci. Technol. 2008, 42, 7885–7890
Arsenic in Rice (Oryza sativa L.) Related to Dynamics of Arsenic and Silicic Acid in Paddy Soils KATJA BOGDAN* AND MANFRED K. SCHENK Institute of Plant Nutrition, Leibniz Universita¨t Hannover, Herrenhaeuser Str. 2, 30419 Hannover, Germany
Received April 30, 2008. Revised manuscript received August 19, 2008. Accepted August 21, 2008.
Paddy rice is a global staple food which in some circumstances can contain high levels of the toxic element arsenic (As). In order to elucidate factors influencing As dissolution in the soil solution during paddy rice cultivation, rice (Oryza sativa L. “Selenio”) was cultivated to maturity in six paddy soils in the greenhouse in 2005 and 2006. Concentrations of Mn, Fe, As, P, and silicic acid in soil solution and As concentrations in rice straw and polished rice grain were determined. There was a close relationship between Fe and As concentrations in the soil solution, suggesting that the major part of dissolved As originated from reduced iron-(hydr)oxide. However, in addition to the factors causing As dissolution in the soil, other factors influenced the uptake of As by rice. The inhibitory effect of indigenous silicic acid in the soil solution on As uptake was clearly shown. This implied that soils with high plant available Si contents resulted in low plant As contents and that Si application to soils may decrease the As content of rice.
Introduction Paddy rice may contain large quantities of As and contribute significantly to the As intake of humans (1). Food surveys have revealed that paddy rice generally has a higher As content compared to other cereals (2) and a large proportion is the toxic inorganic form of As (3). The reason for the relatively high As content of paddy rice is that anaerobic soil conditions lead to increased availability of soil arsenic (4). Arsenic is widely distributed in the environment from both natural and anthropogenic sources. It is a component of about 200 minerals occurring in the lithosphere. The most common As mineral is arsenopyrite (FeAsS). Arsenic can exist in four valency states: -3, 0, +3, and +5. Elemental arsenic and arsine (-3) can exist only in strongly reducing environments. Under moderate reducing conditions, like it is present in paddy rice cultivation, arsenite (+3) might be the dominant form. In aerobic soils arsenate (+5) is generally the stable form (5). Both arsenite and arsenate have a strong sorption affinity for iron-(hydr)oxide (6, 7). The main source of As in soils is the parent material from which they originated (8). The total As content of soils is in the range of 1-20 mg/kg (9). In soil solution of aerobic soils, As concentration is very low due to its strong binding to iron-, aluminum-, and manganese-(hydr)oxides (10). However, flooding leads to fundamental changes in the soil. The overlying floodwater inhibits oxygen movement into the soil * Corresponding author phone: +49 511 7624762; e-mail:
[email protected]. 10.1021/es801194q CCC: $40.75
Published on Web 10/08/2008
2008 American Chemical Society
and the remaining oxygen is depleted within a short time. Consequently, facultative and obligate anaerobes use oxidized forms in soil for respiration, e.g., iron-(hydr)oxide is reduced to Fe2+ and simultaneously As dissolves into soil solution (10). It is widely described in the literature that in paddy soils the reductive dissolution of iron-(hydr)oxides represents an important source of dissolved As in soils (11-13). Additionally, the reduction of arsenate to the mobile arsenite may contribute to an increase of dissolved As in submerged soils (14). Arsenite is considered to be more mobile in soil solution because it is present as a neutral species (H3AsO3). In submerged paddy soil, arsenite was found as the dominant As species (14) followed by arsenate and smaller amounts of methylated As species (4, 15). The formation of the latter may be enhanced by a high organic matter content resulting in a greater microbial activity (16). With further decreasing redox potential, soluble As may decline due to the formation of arsenic sulfide (17). Other reasons for high As content in paddy rice may be the use of contaminated irrigation water, e.g., in Bangladesh (1, 18) or the cultivation of paddy rice on fields that were contaminated by the use of As containing pesticides or desiccants (19). Until now, studies relating As dynamics in paddy rice soil to As tissue concentration in rice were rare. During 8 weeks of rice cultivation in soil suspension, Marin et al. (20) found that water soluble As concentration increased with decreasing redox potential (up to -200 mV) and there was a high correlation between the concentrations of water soluble Fe and dissolved total As. However, no correlation was found between As concentration in soil solution and As uptake by the plant. This suggests that besides the As concentration in soil solution, other factors must be relevant for As uptake. Phosphate is chemically analogous to arsenate and thus influences arsenate availability in the soil due to competition for binding sites (21). Similarly, phosphate may reduce plant arsenate uptake since both ions compete for the same transporter in the plasma membrane (22). Arsenic uptake may also be affected by silicic acid as shown by Guo et al. (23, 24). Ma et al. (25) recently revealed that two Si transporters mediate the arsenite uptake in rice. Besides that, Si is known to be a beneficial element for rice enhancing plant growth by providing rigidity to plant tissue and promoting photosynthesis (26). The aim of the present study was to elucidate the dynamics of As in soil solution of six soils during paddy rice cultivation and its relationship to the As content of rice straw and grain.
Materials and Methods Soil. For conducting pot experiments in the greenhouse in 2005 and 2006, four Italian and two Spanish paddy soils known to produce widely differing As concentrations in polished rice were selected from a survey of 33 fields in Italy and 11 fields in Spain. The Italian and Spanish paddy soils originated from the Po-area of northern Italy and from the region southwest of Sevilla close to the Atlantic coast in Spain, respectively. The Spanish soils (M and N) differed greatly from the Italian soils (D, L, B, and G) with respect to particle size, sulfur content, and pH. All soils were thoroughly mixed and 16 L of soil used in each 20 L pot. After the cultivation period in 2005, the soils were dried, stored, and used again for the experiment in 2006. The soil characteristics are given in Supporting Information (SI) Table S1. Cultivation. Seeds of rice (Oryza sativa L. cv. Selenio) were germinated in a beaker with tap water covered with tissue for 1 week. Sixty germinated seeds were laid on the soil in water and later reduced to 40 plants per pot. VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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The experiment was conducted under natural radiation in the greenhouse with temperatures adjusted to the Italian climate: in May, June, September, October, 22 and 15 °C during day and night, respectively, and in July and August, 26 and 18 °C during day and night, respectively. Water depth was increased with the growth of plants to 6 cm within 2 weeks. It was then kept constant with tap water during cultivation. Pots were fertilized with urea (Sigma-Aldrich) 3 times (1 g/pot). For this purpose the water table was lowered to 1 cm for 2 days. Harvest. Plants were harvested at maturity about 5 months after planting. Rice grains were separated from the straw which was cut 2 cm above the soil. Rice straw was dried for three days at 60 °C and milled. Rice grains were dried at 40 °C overnight to reduce moisture content and prevent fungal infestation. Afterward grains were processed to polished rice. Soil Solution. A system was constructed which allowed soil solution to be sampled without penetration of oxygen into the soil (SI Figure S1). A perforated PE-bottle, 11 cm high, covered with a 2 µm mesh nylon filter (KMF Laborbedarf, Germany), was placed in the middle of the pot 5 cm above the bottom. Two tubes were inserted into the bottle through a rubber stopper: one was used to extract solution by means of a syringe (30 mL). At the same time, N2 gas was introduced into the system via the second tube in order to equilibrate the pressure. Both tubes were controlled by threeway valves and were immediately closed after use. Soil solution was sampled on two consecutive days each week. On the first day the pH in the sampled soil solution was measured. On the second day, 23 mL of the sampled soil solution was placed in a vessel which contained 400 µL HNO3 (65%). Soil solution was passed through a filter (quantitative, 2.5 µm, Carl Roth, Karlsruhe, Germany). Soil solution samples for analysis of As species were taken as described above but acidified with H3PO4. Furthermore, samples were transferred by syringe into containers which were flushed thoroughly with N2 before and after sample taking. Samples were analyzed for As species the following day. Redox Potential. The oxidation-reduction status of the soils was monitored by means of a Pt-electrode which was permanently immersed in the soil. For reference, a calomel electrode (B 2810 Schott) was used. The redox potential readings were adjusted to the standard hydrogen electrode reading by adding 245 mV. Chemical Analysis. Soils were characterized for (i) particle-size distribution by fractionation using sieving and sedimentation (DIN ISO 11 277) (27), (ii) pH in 0.01 M CaCl2 measured with a pH-Electrode (Sen Tix 41, WTW) (28) (iii) total As by aqua regia extraction according to DIN 38414,S7 (27), (iv) total carbon analyzed by a CNS-Autoanalyser (VarioEL, Elemetar), (v) organic carbon calculated by subtraction of carbonate carbon from total carbon content. The carbonate content of the soil was determined with the aid of the Scheibler-equipment (DIN 19684 T5) (28), (vi) oxalate soluble Fe (29), (vii) sulfur in a soil saturation extract was determined by ICP-OES (Spectro flame-EOP). Total As in the soil solution was measured with ICP-MS 7500c (Agilent Technologies) and Fe and P with ICP-OES (Spectro flame-EOP). Arsenic species in the soil solution were analyzed with HPLC-ICP-MS (Perkin-Elmer) described in Mattusch et al. (30). Silicic acid in soil solution was determined by the colorimetric molybdenum blue method at 811 nm with a spectrophotometer (31). Dried and milled straw (350 µg) and polished rice (480 µg) were digested with 4 mL HNO3 (65%) and 1.5 mL H2O2 (30%) in a microwave (ETHOSplus, MLS GmbH, Germany) for 20 and 15 min at 190 °C, respectively, and analyzed for total As with ICP-MS 7500c (Agilent Technologies). Dried and milled straw was extracted for Si with a mixture of 1 M HCl and 2.3 M HF 7886
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adapted from Novozamsky et al. (32) and determined by a colorimetric method (31). Experimental Design and Statistical Analysis. There were four replicates in 2005 and 2006 and all pots were arranged in a randomized block design. Analysis of variance, correlation analysis, and multiple linear correlations were conducted by using SAS (SAS Institute INC, Cary, U.S.). Mean comparisons were carried out according to the Tukey test.
Results Dynamics in Soil Solution during the Cultivation Period. At the onset of flooding, the redox potential of all the soils reflected aerobic conditions which decreased during the cultivation period as expected (SI Figure S2A). In the Spanish soils, strongly reducing conditions occurred within a few days, whereas in the Italian soils, the decrease was much slower. As usual in flooded soils, soil solution pH increased during cultivation (SI Figure S2B) due to the consumption of protons in the course of the reduction of oxides (33). The magnitude of the pH increase was more pronounced in soils having a low initial pH. Iron concentration in soil solution started to rise about three weeks after flooding (Figure 1A). In general, Italian soil solutions contained more Fe than Spanish soil solutions. The Fe concentration was in the order D, G > B, L > M > N. Arsenic concentration in soil solution, like Fe, also generally increased during the cultivation period (Figure 1B). For soil D, the As concentration was nearly twice as high as for the other five soils. However, in solution of soil N, As concentration decreased from about 40 µg/L at the start to approximately 20 µg/L at the end of the cultivation period. Similar to Fe, As concentration started to increase about three weeks after flooding, indicating a relationship between the two. Indeed, for all soils, except soil N, As concentration was positively related to Fe concentration in soil solution (SI Figure S3). This suggests that As was released from dissolved iron-(hydr)oxides into the soil solution but not from manganese-oxides since no correlation was observed between As and Mn concentrations in solution (data not shown). Speciation of As in soil solution was similar for all soils (SI Table S2). Most of the As was present in the form of arsenite while the organic forms monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) were not detected. Phosphate, like arsenate, is bound to Fe-(hydr)oxides in the soil. Thus, P concentration increased after flooding except for soil L (SI Figure S4). The reason for the low P concentration in soil L was that this soil did not receive any P fertilization for many years. Change of P concentration in the Spanish soil M was similar to that in the Italian soils; however, in the Spanish soil N, P concentration was about 7 times higher. During flooding, silicic acid may also be mobilized. Figure 1C shows that silicic acid increased in soils N, M, and B, but remained nearly constant in the other soils during May and June. During July and August, silicic acid concentration in solution decreased slightly in all soils. This might be due to plant uptake. Arsenic and Si Concentration in Plants. In 2005 the As concentration in rice straw was in the order soil N < M, B, G < D, L differing by a factor of more than 10 (Table 1). The same pattern was observed for the As concentration in polished rice. However, soil G had high As concentration in polished rice, whereas As in rice straw was intermediate. Furthermore, As concentration in polished rice did not vary by more than a factor of 4. The As concentration in rice straw was 12-56 times higher than in the grain. Similar As concentrations in plant matter were observed in 2006, although the As concentrations in soil solution in 2005 were much lower than in 2006. In addition to the concentration of As, the concentration of silicic acid in soil solution might also have an influence on the As content of plant material.
FIGURE 1. Fe (A), As (B), and silicic acid (C) concentration in soil solution of six paddy soils during cultivation (Flooding: 4th of May 2005). **, **** indicate significance at p > 0.01 and 0.0001, respectively.
TABLE 1. Arsenic Concentration in Soil Solution, Rice Straw, And Polished Rice in Rice Cultivation in Six soils in 2005 and 2006 As concentration soil(aqua regia) (mg/kg) soil D L G B M N
6.5 15.1 5.0 6.2 6.9 9.8
soil solutiona (µg/L)
straw (mg/kg d.m.)
polished rice (µg/kg d.m.)
2005
2006
2005
2006
2005
2006
19.4 12.2 9.8 13.7 28.9 33.1
76.4 56.0 53.5 64.7 46.3 28.4
11.7 10.2 3.4 6.5 3.8 0.7
10.2 13.4 4.9 6.3 4.2 0.7
230 228 140 201 140 73
268 302 142 257 141 68
and straw decreased in both years. Without soil N, the coefficient of determination for the relationship between silicic acid concentration in soil solution and As concentration in straw was r2 ) 0.89 and 0.80 in 2005 and 2006, respectively. For polished rice, the data were r2 ) 0.90 and 0.87 in 2005 and 2006, respectively. Silicon concentration in straw reflected the variation in silicic acid in soil solution between soils and was in the order D, L, G < B, M < N (SI Figure S5, Figure 2). Silicon concentration in straw was higher in 2006 than in 2005. Multiple linear regression analysis indicated a significant influence of silicic acid concentration in soil solution on the As content in rice straw and grain in both years (SI Table S3 and Table S4). In contrast, As concentration in soil solution had no significant influence on plant As content.
a Average of weekly measured concentrations until the end of grain filling in the 3rd week of August.
Discussion
Figures 2A and B show that with increasing concentration of silicic acid in soil solution, the As content in polished rice
Iron Concentration in Soil Solution. Iron concentration increased the most in soils D and G although the oxalate soluble Fe content of these soils was comparatively low (SI VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Relationship between mean silicic acid concentration in soil solution and As content in polished rice (A) and straw (B) in 2005 and 2006. *, **, ***, **** indicate significance at p > 0.05, 0.01, 0.001, and 0.0001, respectively. Table S1). This suggests that the amount of reducible Fe is not important, which is represented by the oxalate soluble Fe, but rather the processes in soil affecting reduction on the one hand and precipitation on the other hand. Iron reduction is, among other factors, affected by pH (9). The pH of soil solution was consistently lower than 7 during the cultivation period for all soils except soil N (SI Figure S2B). According to the Eh-pH-diagram under standard conditions (10 (5) Pa, 25 °C) and a redox potential of -200 mV, a pH higher than 7 should result in the formation of R-FeOOH and Fe(OH)3 (9) which might explain the low Fe concentration in solution of soil N (Figure 1A). As well as reduction, precipitation of Fe might occur and influence the Fe concentration in soil solution. In the Spanish soils, FeS may have precipitated which is common in flooded soils with high sulfur content (see Table S1) (9). Furthermore, in anoxic environments, siderite (FeCO3) precipitation can occur (34). King (35) showed that FeCO3 complexes dominate the speciation of Fe(II) in seawater. Therefore, the formation of FeCO3 could also have resulted in a decrease of dissolved Fe in the Spanish soils which were influenced by irrigation with brackish water. Following the formation of siderite, formation of iron phosphate (vivianite) is possible. The pair siderite-vivianite is meta-stable under conditions prevailing in this investigation (Eh -300 - 0 mV and pH 5-7.5) (36). Enhanced formation of vivianite could have contributed to the low Fe concentration in solution of soil N compared to soil M since soil N had a much higher P concentration in soil solution (Figure S4). However, high sulfide concentration alleviates the formation of vivianite as a result of iron sulfide precipitation (36, 37). Arsenic Dynamics in Soil Solution. Arsenic concentration generally increased after flooding (Figure 1B). An initial 7888
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increase of As concentration after flooding due to reductive dissolution is widely described in the literature (4, 11-13), whereas prolonged flooding may decrease soluble As due to the formation of arsenic sulfide (17). With the exception of soil N, dissolved Fe and As concentrations were well related indicating that the major part of dissolved As originated from reduced iron-(hydr)oxides (SI Figure S3). In contrast, the dissolution of manganese-(hydr)oxides apparently played only a marginal role compared to the dissolution of iron(hydr)oxides because no significant relationship was found between dissolved Mn and As (data not shown). The high As concentration in solution of soil N at the beginning of the cultivation period might be explained by competition for binding sites in soil resulting from high phosphate concentration (SI Figure S4). Smith et al. (21) confirmed that phosphate is competing with arsenate as well as arsenite for sorption sites leading to a higher As concentration in equilibrium solution in batch sorption studies. Phosphate concentrations used in the study of Smith et al. (21) were similar to those determined in soil solution of soil N. As well as phosphate, the formation of siderite and vivianite (discussed above) might have influenced the As concentration in soil solution as it was shown that both removed arsenate from solution (38). Decreasing As concentration in soil N suggests that arsenic sulfide precipitated due to the strong affinity of arsenite for S. Bostick et al. (34) showed that after an initial formation of a FeAsS-like precipitate, a conversion to As2S3 took place and that the highest arsenite sorption in sediments of salt marshes occurs at pH > 7. High pH and sulfur concentration in soil solution were also present in the Spanish soils giving the preconditions for As2S3 formation. Besides this reaction, no further precipitation of elements with arsenite is known (10). However, inner-sphere adsorption of arsenite at the iron-(hydr)oxide surface has been shown (39). Arsenic concentration in soil solution was considerably higher in 2006 than in 2005, although the same soils were used (Table 1). After the harvest in 2005, the soil including the whole root mass of the plants was dried and stored in containers for the experiment in 2006. At the beginning of the cultivation period in 2006, the dried root mass would provide microorganisms with a large amount of carbon sources accelerating their activity. Consequently, the redox potential decreased faster and higher concentrations of ironand manganese-(hydr)oxides were found in 2006 (data not shown) than in 2005. As the ratio of dissolved Fe:As was similar in both years (data not shown), the As concentration was also higher in 2006. Silicic Acid Concentration in Soil Solution and Si Concentration of Straw. Silicon in soil solution is present as silicic acid (H4SiO4) and its concentration was within the range that is generally reported (1-60 mg Si/L) (Figure 1C) (9). Silicic acid in soil solution is mainly affected by processes of weathering, adsorption, and desorption (9). Weathering of minerals like sesquioxides, kaolinites, and smectites increases silicic acid in soil solution, whereas silica as quartz (SiO2) is hardly soluble. It is speculated that the soil clay content contributed to high silicic acid concentration in soil solution of soils N, M, and B (SI Table S1, Figure 1C). Additionally, silicic acid has a high affinity for iron(hydr)oxides (9) which dissolve and simultaneously release silicic acid into solution after submergence, as observed in this study. This is in agreement with the literature (40). However, silicic acid concentration in soil solution hardly increased after flooding in soils D, L, and G which may indicate that they were depleted. According to the author’s knowledge, no Si slag or other Si sources were applied to the soils in Italy and Spain. This may have resulted in Si depletion since rice is a Si accumulator plant taking up twice as much Si as N (41). Thus, plant uptake might also have caused the
decrease of silicic acid concentration in soil solution in July and August. Depletion of plant available Si might also be the reason for the considerably lower concentration of silicic acid in soil solution in 2006 compared to 2005 (Figure 2). Despite the lower silicic acid concentration in soil solution in 2006, Si concentration in straw was higher than in 2005 (SI Figure S5, Figure 2). This might be explained by a higher transpiration rate due to higher temperatures of 2 °C on average during the first 4 months of cultivation in 2006 than in 2005 (data not shown), since Si translocation is driven by the transpiration stream. Arsenic Content in Rice Straw and Grain. The As concentration in rice straw was on average about 20 times higher than in polished rice which is consistent with results of Xie and Huang (42). Arsenic concentrations in straw and polished rice were well related (r2 ) 0.95 and 0.88 in 2005 and 2006, respectively) and followed a saturation curve. The range of As concentration in plant matter was similar to that observed in the survey which provided the basis for the selection of soils for the greenhouse trials (data not shown). In the greenhouse trials, like in the field survey, the Spanish soil N showed the lowest As content of the plant followed by the Spanish soil M and Italian soil B indicating that factors affecting the plant As uptake were inherent soil characteristics because cultivation practice as well as climate conditions did not differ in the greenhouse. The As content in straw and grain was only slightly higher in 2006 than in 2005 despite considerably higher As concentration in the soil solution (Table 1). Furthermore, the As concentration in soil solution was positively related to the As concentration in straw and grain (r2 ) 0.5 and r2 ) 0.7, respectively) in 2006, but negatively related in 2005 (r2 ) 0.2 and r2 ) 0.4). The reason for this inconsistency is not known. However, in both years, the silicic acid concentration in soil solution and the As content in straw and grain was strongly negatively correlated (Figure 2). A mean silicic acid concentration of about 10 mg Si/L in soil solution resulted in an As content of less than 150 µg/kg d.m. and 4 mg/kg d.m. in polished rice and rice straw, respectively (Figure 2). A strong inhibiting effect of silicic acid on the plant As uptake was first shown by Guo. et al. (23). They found that silicate reduced the arsenate uptake by about 40%. Our own experiments conducted in nutrient solution showed that arsenite uptake was also strongly reduced in nutrient solution (data not shown). The mechanisms involved in the inhibiting effect of silicic acid (see above) on As uptake are not known so far. Ma et al. (2008) (25) revealed that the Si transporters Lsi1 and Lsi2 mediate arsenite uptake as well. Thus, the reduction of arsenite uptake may be related to the reduced expression of Lsi1 and Lsi2 caused by Si supply (43, 44). Soils differ both in concentration of silicic acid and As in soil solution which confounds estimation of the decisive factor. However, a multiple linear regression analysis considered both concentrations in soil solution and revealed that the As content in rice straw and grain was significantly affected by silicic acid but not by As concentration in soil solution (Table 1). Besides silicic acid, phosphate might also have influenced the arsenate uptake of rice plants since the uptake system of rice plants is not able to distinguish between arsenate and phosphate (22). Thus, the influence of phosphate on the uptake of arsenate and vice versa was often reported (45, 46). Although the As species determined in solution of submerged soils was predominantly arsenite (SI Table S2), it is not known if arsenite may be oxidized in the root rhizosphere to arsenate, as the rice root is known to develop an oxidized root zone (47). Therefore, arsenate may be competing with phosphate for plant uptake. In this regard, the high phosphate concentration in soil solution of soil N may have interfered with arsenate uptake, whereas in soil L low phosphate availability may have enhanced arsenate absorption (SI Figure S4).
Our results suggest that soils with high plant available Si content resulted in low As content in the rice plant. Therefore, Si application to the soil may decrease the As content in rice.
Acknowledgments We gratefully acknowledge the support and funding by Nestle´ Research Center, Lausanne, Switzerland. We are also grateful to Dr. J. Mattusch, UFZ Leipzig, Germany, for the analysis of soil solution samples on As species. We thank Dr. P. Barraclough, Plant Science Department, Rothamsted Research, Harpenden, Hertfordshire, UK, for helpful comments and improvement of the English text.
Supporting Information Available Additional tables and figures. This material is available free of charge via the Internet at http://pubs.acs.org.
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