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Effects of alternating wetting and drying versus continuous flooding on chromium fate in paddy soils
Ecotoxicology and Environmental Safety 113 (2015) 439–445
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Ecotoxicology and Environmental Safety journal homepage: www.elsevier.com/locate/ecoenv
Effects of alternating wetting and drying versus continuous flooding on chromium fate in paddy soils Wendan Xiao a,n, Xuezhu Ye a, Xiaoe Yang b, Tingqiang Li b, Shouping Zhao a, Qi Zhang a a Zhejiang Province Key Lab for Food Safety, Institute of Quality and Standard for Agro-products, Zhejiang Academy of Agricultural Sciences, Hangzhou 310021, China b Ministry of Education Key Laboratory of Environmental Remediation and Ecological Health, College of Environmental and Resource Sciences, Zhejiang University, Hangzhou 310058, China
art ic l e i nf o
a b s t r a c t
Article history: Received 24 September 2014 Received in revised form 15 December 2014 Accepted 16 December 2014
Anthropogenic chromium (Cr) pollution in soils poses a great threat to human health through the food chain. It is imperative to understand Cr fate under the range of conditions suitable for rice growth. In this study, the effects of irrigation managements on dynamics of porewater Cr(VI) concentrations in rice paddies and Cr distribution in rice were investigated with pot experiments under greenhouse conditions. Soil redox potential in continuous flooding (CF) treatments showed that reducing conditions remained for the whole duration of rice growing period, while soil redox potential in alternating wetting and drying (AWD) treatments showed that soil conditions alternately changed between reducing and oxic. As soil redox potential is an important factor affecting Cr(VI) reduction in paddy soils, dynamics of Cr(VI) concentration were clearly different under different irrigation managements. In CF treatments, porewater Cr(VI) concentrations decreased with time after planting, while in AWD treatments porewater Cr(VI) concentrations were increased and decreased alternately response to the irrigation cycles. Chromium(VI) concentrations in the CF treatments were lower than those in AWD treatments for most part of ricegrowing season. Moreover, Cr concentrations in rice tissues were significantly influenced by irrigation with relatively higher values in the AWD treatments, which might be attributed to the higher porewater Cr(VI) concentrations in AWD treatments. Therefore, it would be better to use CF than AWD management in Cr-contaminated paddy soils to reduce Cr accumulation in rice, and thus to reduce the potential risk to human health. & 2014 Elsevier Inc. All rights reserved.
Keywords: Chromium(VI) concentration Soil redox potential Continuous flooding Alternating wetting and drying Oryza sativa L.
1. Introduction As the 21st most abundant element in Earth’s crust (Barnhart, 1997), chromium (Cr) has been extensively used in industrial activities such as ore refining, electroplating industry, tanning, paper making, steel production and automobile manufacturing (Francisco et al., 2002). As a consequence, there is a continual influx of Cr contaminants into the environment. The most stable oxidation states of Cr in the environment are Cr(III) and Cr(VI). Chromium (III), an essential trace element for mammals (Dayan and Paine, 2001), is generally considered immobile and nonbioavailable due to the low solubility of Cr(III) (hydr)oxides at neutral pH (Rai et al., 1987). Conversely, Cr(VI) exists as highly soluble oxyanionic spe− 2− (dicies, i.e., CrO2− 4 (chromate), HCrO4 (bichromate), and Cr2O7 chromate), is a known human carcinogen (Costa and Klein, 2006). The mobility, toxicity and plant uptake of Cr depend strongly on its n
Corresponding author. Fax: þ 86 0571 86419052. E-mail address: [email protected] (W. Xiao).
http://dx.doi.org/10.1016/j.ecoenv.2014.12.030 0147-6513/& 2014 Elsevier Inc. All rights reserved.
oxidation states. Once released to the environment, Cr is susceptible to oxidation–reduction reactions that dramatically alter its physical and chemical properties. The reduction of Cr(VI) by Fe(II) sustained through the activity of Fe-reducing bacteria are able to couple the oxidation of organic compounds to the reduction of Fe (III) (Li et al., 2012; Liu et al., 2011). The lack of appropriate disposal facilities has led to severe Cr pollution in waters and soils (Sethunathan et al., 2005), thus posing a great threat to human health through the food chain (Lavado et al., 2007). As rice (Oryza sativa L.) is the major stable food crop for nearly 40% of the world population and more than 60% of the population in China (Mae, 1997), strategies to reduce rice plant exposure to Cr are therefore urgently needed and require a detailed understanding of Cr behavior under the range of conditions suitable for rice growth. Generally, rice cultivation is carried out in flooded paddy fields because the flooded water supplies micronutrients, washes out substances harmful to rice growth, and weakens the activities of pathogenic bacteria and fungi under the reductive conditions of the soil. However, as water shortage during the dry season of
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irrigated rice farming in Northeast China for climate change and human development (Yang et al., 2007), alternating wetting and drying (AWD) practices are commonly used as a water-saving practice in China (Cabangon et al., 2004). For AWD practices, the fields are managed as irrigated lowland rice but top soil layer is allowed to dry out to some degree before irrigation is applied again (Bouman and Tuong, 2001). The number of days under nonflooded soil conditions vary depending on plant development stages and availability of water. Compared to continuous flooding, the alternating oxic and anoxic top soil conditions imposed by AWD irrigation may lead to the dynamic changes of soil redox potential and Cr redox state, thus the mobility, toxicity and plant uptake of Cr. This makes it crucially relevant to understand the behavior of Cr in intermittently irrigated rice paddies subject to varying soil redox conditions. However, very little research has been done to investigate the impact of irrigation managments on Cr behavior in rice paddies. The influence of AWD practice on Cr behavior as compared to conventional continuous flooding practices are generally unknown. Hence, we conducted a study with the following main objectives: (i) to determine the effects of irrigation managements on dynamic changes of soil redox potential and porewater Cr(VI) concentration; (ii) to examine whether introduced cycles of AWD affect Cr uptake by rice; and (iii) to investigate the practical use of irrigation management to reduce the potential Cr risk to human health.
2. Materials and methods 2.1. Soil sample collection and preparation Two typical soils classified as Oxisols and Phaeozems according to World reference base for soil resources (IUSS-FAO, 2014), were collected at 0–20 cm depth from Guilin City (104°40′-119°45′E, 24°18′-25°41′N) and Ha’erbing City (126°32′-129°55′E, 44°92′46°32′N), China. After removal of large pieces of plant materials, grit, earthworms, etc. soil samples were air-dried, ground, and passed through a 2-mm sieve prior to use. Soil samples were analyzed for total Cr (Shentu et al., 2008), Cr(VI) (James et al., 1995), pH (Chaturvedi and Sankar, 2006), cation exchange capacity (CEC) (Hendershot and Duquette, 1986), organic matter (OM) contents (Rashid et al., 2001), Fe(II) contents (Schnell et al., 1998), easily reducible Mn [Mn(ER)] contents (Bartlett and James, 1996), and particle size distribution (PSD) (Day, 1965). Relevant physicochemical properties of the soils are shown in Table 1. The soil samples (Oxisols and Phaeozems) were spiked with Cr as K2Cr2O7 (with a purity 498% from Aldrich Chemical Co.) to establish three contaminant levels of CK (background values), 200, and 400 mg Cr kg 1 soil. All spiked soil samples were aged for 1 year at a moisture content of 70% of water holding capacity prior to pot experiments. 2.2. Pot experiment The rice (O. sativa L.) variety used was Zhongzheyou 1, which is a single season indica variety with an average plant height of 120 cm. This long duration variety takes about 140 days to mature. The seed of the variety was obtained fromthe Zhejiang Seed Company. Seeds were surface-sterilized by washing with 70% ethanol for 1 min and soaking in 0.01 g mL 1 sodium hypochlorite for 5 min, rinsed thoroughly in deionized water, and then imbibed in deionized water for 48 h at 30 °C (Wu et al., 2011). The seeds were germinated in quartz sand washed with 5% (v/v) HCl. For the first two weeks, only deionized water was supplied. After 14 days when seedlings grew onto two-leaf stage, nutrient solution was
Table 1 Physio-chemical properties of the soils. Soil
Oxisols
Phaeozems
Chromium (mg kg 1) Total Cr (mg kg 1) Cr(VI) (mg kg 1)
68.57 2.54 0.28 7 0.01
65.5 7 2.01 0.45 7 0.01
Chemical characteristicsa pH OM (g kg 1) CEC (cmolc ( þ ) kg 1) Fe(II) (mg kg 1) Mn(ER) (mg kg 1)
5.03 7 0.05 19.17 0.56 17.3 7 1.96 34.4 7 1.48 2.647 0.23
7.23 7 0.03 32.2 7 0.34 34.0 7 2.51 30.5 7 0.56 1097 5.21
Soil texture Sand (%) Silt (%) Clay (%)
10.6 7 0.15 39.8 7 1.26 49.67 1.19
20.6 7 1.54 60.2 7 2.21 19.2 7 1.24
a CEC, cation exchange capacity; Mn(ER), easily reducible Mn; OM, organic matter.
supplied. The composition of nutrient solution was the same as described by Yang et al. (2004). On May 30, 2012, 4 seedlings (thirty days old) were transplanted to individual pots with a diameter of 21 cm and depth of 25 cm. Each pot had 5 kg of soil. Before transplanting, the standard recommended dose of NPK fertilizer was applied to all pots at the rates of 187.5 kg N ha 1 (70% applied as basal dose and 30% as topdressing at panicle initiation stage), 70 kg P2O5 ha 1 and 93 kg K2O ha 1 (Wei et al., 2012). The irrigation management factor had two levels: (1) alternating wetting and drying (AWD), the irrigation method is characterized by a mid-season drainage during the late tillering stage of the rice crop, and periodic soil drying of 2–3 days between irrigation events from panicle initiation to grain filling with water level kept at 0–2 cm above the soil surface, and the soil was not irrigated from 100 day until the harvest (Fig. 1); (2) continuous flooding (CF), in which water depth was maintained at 3 cm above the soil surface for the whole duration of the experiment (Fig. 1). All treatments were conducted in triplicate, and the pots were randomly arranged in a greenhouse under a photo flux density of 400 μmol m 2 s 1, a light/dark period of 16/8 h, day/night temperatures of 30/25 °C, and day/ night relative humidity of 75/85% (Wu et al., 2010). Portions of fresh soil (100 g, oven-dry basis) were sampled for Cr(VI) determination at the intervals of 28, 34, 45, 48, 63, 66, 81, 84, 99, 102, 108, and 114 days after transplanting, and then soil samples were immediately stored in sealed plastic containers at 4 °C (Hopp et al., 2008). Each pot contained three permanently-installed platinum electrodes for redox measurement, placed in three pairs at the depth of 7.5 cm, and horizontally distributed around the perimeter about 1 cm from the walls (Johnson-Beebout et al., 2009). The depth was chosen to represent root-zone depth (Patrick and Delaune, 1972). Soil redox potential was monitored on each day of Cr (VI) measurement with platinum-tipped electrodes and a portable Eh meter (EH-120; Fujiwara Scientific Company Co. Ltd., Tokyo, Japan). Plants were harvested at maturity and air dried. Plant samples were manually threshed to separate grains, then the dry weights of grains and straws were recorded. The brown rice was prepared by removing the husk using a laboratory de-husker (JLGJ4.5, Taizhou Cereal and Oil Instrument Co. Ltd., Zhejiang, China), the polished rice was prepared by polishing the bran by a laboratory polishing machine (JNMJ3, Taizhou Cereal and Oil Instrument Co. Ltd., Zhejiang, China). The husk, brown rice, and polished rice samples were ground using a ball mill (Retsch, MM-301, Germany) and passed through a 60-mesh sieve, then keep at 20 °C prior to Cr analysis.
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Fig. 1. Irrigation management in CF (continuous flooding) and AWD (alternate wetting and drying irrigation).
2.3. Analytical methods 2.3.1. Porewater preparation Fresh soil porewater was prepared by centrifuging 50 g (wet weight) of the sieved soil samples in a 100-ml plastic centrifuge bottle at 10,000 rpm for 30 min (Bondarenko et al., 2007). The supernatant from multiple replicates was carefully removed with a pipette and pooled in a 100-ml glass bottle for use in the following experiments. 2.3.2. Porewater Cr(VI) concentration Porewater Cr(VI) concentration was measured by the 1,5-diphenyl-carbazide spectrometry (DPC) colorimetric method, using a phosphate acid buffer (pH 2) to control pH for color development (Kim et al., 2001). The absorbance was determined in a 1-cm cell at 540 nm on a UV-9100 Spectrophotometer (Beijing Ruili Corp.) (Kim et al., 2001). The method had a detection limit of 0.1 μM. Three replications were conducted for each sample. The reliability of the determination procedure was assessed by doing recovery studies. Recovery studies were carried out by adding known concentrations of Cr(VI) standards (1 and 2 mg L 1) to Cr(VI) free water samples. The analysis was done immediately after “spiking” in order to minimize the reduction of Cr(VI) to Cr(III) under the optimum extraction conditions described earlier. The recovery of spiked Cr(VI) was 91.272.5% and 93.3 73.4%. 2.3.3. Total Cr in plant Rice samples (0.1 g) of each treatment were digested with HNO3–H2O2 (4:1) and the digested solution was transferred to a 50 ml volumetric flask, made up to volume and filtered (Wu et al., 2011). The concentrations of Cr in the filtrate were determined using inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7500a) with a detection limit of 0.3–0.4 μg Cr L 1. The ICPMS was operated at the following conditions: the radio frequency power at the torch 1.2 kW, the plasma gas flow 15 L min 1, the auxiliary gas flow 0.89 L min 1, and the carrier gas flow 0.95 L min 1 (Llorent-Martínez et al., 2011). The validation of the presented procedure was checked by the analysis of certified reference material (rice NCSZC73008) approved by General Administration of Quality Supervision, Inspection and Quarantine of the People’s Republic of China (AQSIQ), with the recovery rate of 103.4%. Three replications were conducted for each sample. 2.4. Analytical methods Means of data were compared by the least significant difference (LSD) tests at the 5% significance level. The one-way analysis of variance (ANOVA) was performed using the statistical package
SPSS 18.0 for Windows (CoHort Software, Berkeley, CA, USA).
3. Results and discussion The data reported here are the first to target the effects of irrigation managements on in situ dynamics of soil redox potential and porewater Cr(VI) concentration in rice paddies, and the Cr distribution in rice, thus to investigate the practical use of irrigation management to reduce the potential Cr risk to human health. 3.1. Effects of irrigation managements on soil redox potential The soil redox potential responded as expected to the irrigation methods, soil Eh values under different irrigation managements was clearly different (Fig. 2). In CF treatments, soil Eh values monitored at continuously flooded paddy soils were decreased significantly with time after planting, and decreased from 342 mV to 370 mV and from 368 mV to 324 mV, respectively, for Oxisols and Phaeozems soils. Soil redox potential in CF treatments showed that reducing conditions remained for the whole duration of rice growing period. Similar trends were observed during continuously flooded rice growth in greenhouse studies (Bogdan and Schenk, 2008; Li et al., 2009; Xu et al., 2008) and in the field (Takahashi et al., 2004), suggesting that O2 loss into the rhizosphere is unlikely to oxidize the bulk soil under flooded conditions. The oxidation zone in rhizosphere soil is typically confined to within 1 mm of rice roots (Revsbech et al., 1999). In AWD treatments, soil Eh values decreased immediately after flooding, and then increasing again as the soil dried out prior to the next irrigation, and ranged from 291 mV to 142 mV and from 254 mV to 189 mV, respectively, for Oxisols and Phaeozems soils, therefore, for these two soils, soil conditions alternately changed between reducing and oxic during rice growing period. Soil Eh values in the AWD treatments were higher than those in the CF treatments for the whole duration of rice growing period. Moreover, porewater Cr(VI) concentrations seem to depend on soil Eh values. 3.2. Effects of irrigation managements on porewater Cr(VI) concentration Variations in porewater Cr(VI) concentration were found dependent on the type of irrigation management during the rice growing season. Dynamic changes of Cr(VI) concentration under different irrigation managements were clearly different (Fig. 3). In CF treatments, soil Cr(VI) concentrations decreased with time after planting, while in AWD treatments soil Cr(VI) concentrations were
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Fig. 2. Effects of irrigation management on soil redox potential with different loading rates of Cr. Oxisols Cr–CK (A), Oxisols Cr-200 (B), Oxisols Cr-400 (C), Phaeozems Cr–CK (D), Phaeozems Cr-200 (E) and Phaeozems Cr-400 (F). CF refers to continuous flooding; AWD refers to alternate wetting and drying irrigation.
Fig. 3. Effects of irrigation management on soil Cr(VI) dynamics with different loading rates of Cr. Oxisols Cr–CK (A), Oxisols Cr-200 (B), Oxisols Cr-400 (C), Phaeozems Cr–CK (D), Phaeozems Cr-200 (E) and Phaeozems Cr-400 (F). CF refers to continuous flooding; AWD refers to alternate wetting and drying irrigation.
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increased and decreased alternately response to the irrigation cycles. For example, in CF treatments, soil Cr(VI) concentrations decreased from 2.67 to 1.43 mg kg 1 and from 5.34 to 3.14 mg kg 1, respectively, at the Cr rate of 200 and 400 mg kg 1 in Oxisols, and soil Cr(VI) concentrations decreased from 5.11 to 4.39 mg kg 1 and from 9.13 to 8.15 mg kg 1, respectively, at the Cr rate of 200 and 400 mg kg 1 in Phaeozems soils. However, in AWD treatments, soil Cr(VI) concentrations were decreased immediately after flooding, and then increasing again as the soil dried out prior to the next irrigation, and ranged from 0.28 to 0.37 mg kg 1, from 2.67 to 2.75 mg kg 1 and from 5.34 to 5.79 mg kg 1, respectively at the Cr rate of CK, 200 and 400 mg kg 1 in Oxisols, while soil Cr(VI) concentrations ranged from 0.29 to 0.45 mg kg 1, from 5.11 to 5.13 mg kg 1 and from 8.98 to 9.13 mg kg 1, respectively at the Cr rate of CK, 200 and 400 mg kg 1 in Phaeozems soils. This trend was consistent with soil redox potential, since the soil oxygen status is altered by varying conditions of soil redox potential, and these transformations consequently induce changes in Cr(VI) concentration. Soil redox potential is an important factor affecting Cr(VI) reduction in paddy soils (Kozuh et al., 2000). In CF treatments, as soil redox potential decreased, more Cr(VI) was reduced. On the other hand, Cr(VI) peaks in AWD treatments were observed during rice maturity stage after long drainage periods at soil Eh values above 100 mV. According to the literature (Rupp et al., 2010), reduced conditions result in the reduction of Cr(VI) into Cr (III) and the immobilization of chromates due to decreasing redox potential (Eh). Du Laing et al. (2009) also reported that soils tend to undergo a series of sequential redox reactions when the redox status of the soil changes from aerobic to anaerobic conditions during flooding. Major reactions include Cr(VI) reduction, manganic manganese [Mn(IV)] reduction, ferric iron [Fe(III)] reduction, sulfate (SO2− 4 ) reduction, and methanogenesis. In terms of Cr(VI) reduction, the CF treatment did significantly better than the AWD treatment (Fig. 3), indicating that the long flooding time allowed the lower-depth redox potential to increase the amount of Cr(VI) reduced. Chromium(VI) can be reduced by biological and chemical processes in soils, and chemical reduction is the major avenue by which chromate is reduced (Kozuh et al., 2000; Xiao et al., 2012). Shaheen et al. (2014a) noted that two mechanisms might be proposed to explain the behavior of Cr under reducing conditions as compared to oxidizing conditions in flood–dry-cycles: (1) direct reduction from the high to the low oxic form, and (2) indirect changes of pH and OM caused by changes of Eh values. Furtherly, Shaheen and Rinklebe (2014b) also noted that Cr in soils can be fractionated sequentially to seven fractions as follows: F1—soluble þexchangeable, F2—easily mobilizable, F3—bound to Mn oxides, F4—bound to soil organic matter (SOM) (might include sulfides), F5—bound to low crystalline (amorphous) Fe oxides, F6—bound by crystalline Fe oxides, and F7 —residual fraction, and the mobile fraction of Cr was correlated with soil pH and OM. Chromium(VI) concentrations in the CF treatments were lower than those in the AWD treatments for the most part of ricegrowing season. It can therefore be expected that rice paddies managed by AWD will contain more Cr(VI) than continuously flooded ones. It would be better to use CF than AWD management in Cr-contaminated paddy soils to decrease soil porewater Cr(VI) concentration, and thus to reduce Cr mobility and potential risk to human health. Further research is required with field-scale experiments. 3.3. Effects of irrigation managements on biomass yield of rice Significant differences in the growth of rice were found between two soil types owing to the differences in Cr
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Table 2 Effects of irrigation management on rice yield with different loading rates of Cr. Soil
Cr loading level (mg kg 1)
Irrigation managementa
Grain weight (g pot 1 DW)
Straw weight (g pot 1 DW)
Oxisols
CKb
CF AWD CF AWD CF AWD
41.3 7 1.12 45.6 7 1.43n 35.17 1.26 37.2 7 2.31 32.4 7 1.54n 27.0 7 1.42
38.2 7 1.43 41.8 7 1.25n 31.3 7 1.49 32.5 7 1.83 28.0 7 1.74n 24.27 1.68
CF AWD CF AWD CF AWD
47.5 7 2.06 53.8 7 1.75n 40.8 7 1.94n 35.87 1.59 33.77 1.64n 28.6 7 1.02
43.3 7 0.95 46.9 7 1.23n 35.37 1.53n 29.3 7 1.41 26.8 7 0.85n 20.17 0.97
200 400
Phaeozems CK 200 400
n P o 0.05 refers to significant difference between CF and AWD treatments at each Cr level within the same columns. a CF refers to continuous flooding, AWD refers to alternate wetting and drying irrigation. b CK refers to soil samples without spiking Cr; 200 and 400 refers to soil samples with Cr contaminant levels of 200 and 400 mg kg 1, respectively.
phytoavailability (Xiao et al., 2013) (Table 2). Rice yield was also influenced by external Cr loading rates and irrigation managements (Table 2). Dry weights of rice decreased gradually as the Cr rates increased, up to 59.3%–78.3% and 53.2%–71.0% of the control at Cr rate of 400 mg kg 1, respectively for Oxisols and Phaeozems soils. The growth inhibition might be attributed to Cr toxicity (Athar and Ahmad, 2002). The stimulating effect of AWD on the growth of rice occurred at Cr rate of CK and 200 mg kg 1 in Oxisols, and CK in Phaeozems, while the growth of rice was inhibited by AWD at Cr rate of 400 mg kg 1 in Oxisols, and 200 and 400 mg kg 1 in Phaeozems (Table 2). The stimulation of AWD treatment on rice growth at low Cr rates may be as a result of higher nitrogen availability and organic matter content in paddy soils for wetting and drying cycles (Dong et al., 2012; Wang et al., 2011). Alternating wetting and drying irrigation may also increase rice roots in deeper soil layers, maintain root activities and promote N uptake at later stages (Horie et al., 2005). Moreover, AWD may increase a greater partitioning of plant accumulated N to the grains (Lin et al., 2006). On the contrast, other literatures reported that the grain yield of rice was reduced by AWD irrigation (Belder et al., 2004; Tabbal et al., 2002). The discrepancies between the studies are probably attributed to the variations in the timing of irrigation method applied and soil hydrological conditions (Belder et al., 2004). In this study, the inhibition of AWD on rice growth at high Cr rates were mainly ascribe to the high toxicity of Cr(VI), because soil porewater Cr(VI) concentrations were higher in AWD treatments than those in continuous flooding. 3.4. Effects of irrigation managements on Cr distribution in rice Chromium concentrations in rice tissues varied with Cr levels and irrigation managements, and increased with increasing Cr loading rates (Fig. 4). Xiao et al. (2013) also noted that Cr concentrations in rice were significantly related to total Cr and Cr(VI) concentrations in soils. Chromium concentrations in rice was in the order of husk4 brown rice 4polished rice, and ranged from 0.53–17.2, 0.39–13.5 and 0.30–10.2 mg kg 1, respectively (Fig. 4). Chromium concentrations in rice tissues varied between two soil types, were 0.30–0.58 and 0.47–1.02 mg kg 1 at the Cr rate of CK, respectively for Oxisols and Phaeozems soils. The corresponding values were 2.48–5.13 and 3.58–7.69 mg kg 1 at the Cr
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Fig. 4. Effects of irrigation management on Cr distribution in rice with different loading rates of Cr. *Po 0.05 refers to significant difference between CF (continuous flooding) and AWD (alternate wetting and drying irrigation). Oxisols Cr–CK (A), Oxisols Cr-200 (B), Oxisols Cr-400 (C), Phaeozems Cr–CK (D), Phaeozems Cr-200 (E) and Phaeozems Cr400 (F).
rate of 200 mg kg 1, and 5.88–11.6 and 8.20–17.2 mg kg 1 at the Cr rate of 400 mg kg 1, respectively. The significant differences between two soil types were mainly due to the differences in Cr (VI) reduction and the Cr phytoavailability in paddy soils (Xiao et al., 2013). Xiao et al. (2012) noted that the reduction of Cr(VI) in Oxisols was more fast than that in Phaeozems soils, which was mainly ascribed to low pH in Oxisols. Moreover, the soil texture and the OM content might also play important roles here. Chromium concentrations in rice tissues were significantly influenced by irrigation with relatively higher values in the AWD treatments. Chromium concentrations in husk, brown rice and polished rice were respectively increased by 9.43%–21.3%, 10.3%– 25.6% and 13.3%–29.8% by AWD irrigation in Oxisols, as compared to continuous flooding. The corresponding values were 15.9%– 21.1%, 14.9%–24.0%, 17.0%–26.5%, respectively in Phaeozems soils. Compared with continuous flooding, soil conditions alternately changed from reducing to oxic during rice growing period significantly increased Cr uptake in paddy soils, which must be attributed to the higher Cr(VI) concentrations in AWD treatments due to slow-down of the Cr(VI) reduction rates (James and Bartlett, 1983; Loyaux-Lawniczak et al., 2001). Therefore, it would be better to use CF than AWD management in Cr-contaminated paddy soils to reduce Cr uptake by rice, and thus to lower the potential risk to human health. Further research is also required with field-scale experiments. By contrast, for some metals with totally different characteristics with Cr, such as As, conflicting results can be obtained. For example, As concentrations in porewater are markedly lower under oxic conditions and generally dominated by low solubility and toxicity species of As(V) rather than As(III) (Roberts et al., 2011). Thus, compared to continuous flooding, aerobic conditions maintained throughout (Xu et al., 2008) or during part of
the rice growing season (Li et al., 2009) significantly reduced As uptake in greenhouse studies.
4. Conclusion The present study examined the effects of irrigation managements on in situ dynamics of soil redox potential and porewater Cr (VI) concentration in rice paddies, and Cr accumulation in rice. Soil redox potential in CF treatments showed that reducing conditions remained for the whole duration of rice growing period, while soil redox potential in AWD treatments showed that soil conditions alternately changed between reducing and oxic. Soil Eh values in the AWD treatments were higher than those in the CF treatments for the whole duration of rice growing period. Porewater Cr(VI) concentrations seem to depend on soil Eh values. In CF treatments, soil Cr(VI) concentrations decreased with time after planting, while in AWD treatments soil Cr(VI) concentrations were increased and decreased alternately response to the irrigation cycles. The trend was consistent with soil redox potential, therefore, it can be expected that soil redox potential is an important factor affecting Cr(VI) reduction in paddy soils. Chromium concentrations in rice tissues were significantly influenced by irrigation with relatively higher values in the AWD treatments. Compared with continuous flooding, soil conditions alternately changed from reducing to oxic significantly increased Cr uptake, which might be attributed to the higher Cr(VI) concentrations in AWD treatments. Therefore, it would be better to use CF than AWD management in Cr-contaminated paddy soils to reduce Cr accumulation in rice, and thus to lower the potential risk to human health. Further research is required with field-scale experiments.
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Acknowledgment This study was financially supported by the Ministry of Environmental Protection of China (Grant no. 2011467057), Zhejiang Provincial Natural Science Foundation of China (Grant no. LY12E09008), and Science and Technology Innovation Ability Promotion Project of the Zhejiang Academy of Agricultural Sciences.
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Contents lists available at ScienceDirect
Ecotoxicology and Environmental Safety journal homepage: www.elsevier.com/locate/ecoenv
Effects of alternating wetting and drying versus continuous flooding on chromium fate in paddy soils Wendan Xiao a,n, Xuezhu Ye a, Xiaoe Yang b, Tingqiang Li b, Shouping Zhao a, Qi Zhang a a Zhejiang Province Key Lab for Food Safety, Institute of Quality and Standard for Agro-products, Zhejiang Academy of Agricultural Sciences, Hangzhou 310021, China b Ministry of Education Key Laboratory of Environmental Remediation and Ecological Health, College of Environmental and Resource Sciences, Zhejiang University, Hangzhou 310058, China
art ic l e i nf o
a b s t r a c t
Article history: Received 24 September 2014 Received in revised form 15 December 2014 Accepted 16 December 2014
Anthropogenic chromium (Cr) pollution in soils poses a great threat to human health through the food chain. It is imperative to understand Cr fate under the range of conditions suitable for rice growth. In this study, the effects of irrigation managements on dynamics of porewater Cr(VI) concentrations in rice paddies and Cr distribution in rice were investigated with pot experiments under greenhouse conditions. Soil redox potential in continuous flooding (CF) treatments showed that reducing conditions remained for the whole duration of rice growing period, while soil redox potential in alternating wetting and drying (AWD) treatments showed that soil conditions alternately changed between reducing and oxic. As soil redox potential is an important factor affecting Cr(VI) reduction in paddy soils, dynamics of Cr(VI) concentration were clearly different under different irrigation managements. In CF treatments, porewater Cr(VI) concentrations decreased with time after planting, while in AWD treatments porewater Cr(VI) concentrations were increased and decreased alternately response to the irrigation cycles. Chromium(VI) concentrations in the CF treatments were lower than those in AWD treatments for most part of ricegrowing season. Moreover, Cr concentrations in rice tissues were significantly influenced by irrigation with relatively higher values in the AWD treatments, which might be attributed to the higher porewater Cr(VI) concentrations in AWD treatments. Therefore, it would be better to use CF than AWD management in Cr-contaminated paddy soils to reduce Cr accumulation in rice, and thus to reduce the potential risk to human health. & 2014 Elsevier Inc. All rights reserved.
Keywords: Chromium(VI) concentration Soil redox potential Continuous flooding Alternating wetting and drying Oryza sativa L.
1. Introduction As the 21st most abundant element in Earth’s crust (Barnhart, 1997), chromium (Cr) has been extensively used in industrial activities such as ore refining, electroplating industry, tanning, paper making, steel production and automobile manufacturing (Francisco et al., 2002). As a consequence, there is a continual influx of Cr contaminants into the environment. The most stable oxidation states of Cr in the environment are Cr(III) and Cr(VI). Chromium (III), an essential trace element for mammals (Dayan and Paine, 2001), is generally considered immobile and nonbioavailable due to the low solubility of Cr(III) (hydr)oxides at neutral pH (Rai et al., 1987). Conversely, Cr(VI) exists as highly soluble oxyanionic spe− 2− (dicies, i.e., CrO2− 4 (chromate), HCrO4 (bichromate), and Cr2O7 chromate), is a known human carcinogen (Costa and Klein, 2006). The mobility, toxicity and plant uptake of Cr depend strongly on its n
Corresponding author. Fax: þ 86 0571 86419052. E-mail address: [email protected] (W. Xiao).
http://dx.doi.org/10.1016/j.ecoenv.2014.12.030 0147-6513/& 2014 Elsevier Inc. All rights reserved.
oxidation states. Once released to the environment, Cr is susceptible to oxidation–reduction reactions that dramatically alter its physical and chemical properties. The reduction of Cr(VI) by Fe(II) sustained through the activity of Fe-reducing bacteria are able to couple the oxidation of organic compounds to the reduction of Fe (III) (Li et al., 2012; Liu et al., 2011). The lack of appropriate disposal facilities has led to severe Cr pollution in waters and soils (Sethunathan et al., 2005), thus posing a great threat to human health through the food chain (Lavado et al., 2007). As rice (Oryza sativa L.) is the major stable food crop for nearly 40% of the world population and more than 60% of the population in China (Mae, 1997), strategies to reduce rice plant exposure to Cr are therefore urgently needed and require a detailed understanding of Cr behavior under the range of conditions suitable for rice growth. Generally, rice cultivation is carried out in flooded paddy fields because the flooded water supplies micronutrients, washes out substances harmful to rice growth, and weakens the activities of pathogenic bacteria and fungi under the reductive conditions of the soil. However, as water shortage during the dry season of
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irrigated rice farming in Northeast China for climate change and human development (Yang et al., 2007), alternating wetting and drying (AWD) practices are commonly used as a water-saving practice in China (Cabangon et al., 2004). For AWD practices, the fields are managed as irrigated lowland rice but top soil layer is allowed to dry out to some degree before irrigation is applied again (Bouman and Tuong, 2001). The number of days under nonflooded soil conditions vary depending on plant development stages and availability of water. Compared to continuous flooding, the alternating oxic and anoxic top soil conditions imposed by AWD irrigation may lead to the dynamic changes of soil redox potential and Cr redox state, thus the mobility, toxicity and plant uptake of Cr. This makes it crucially relevant to understand the behavior of Cr in intermittently irrigated rice paddies subject to varying soil redox conditions. However, very little research has been done to investigate the impact of irrigation managments on Cr behavior in rice paddies. The influence of AWD practice on Cr behavior as compared to conventional continuous flooding practices are generally unknown. Hence, we conducted a study with the following main objectives: (i) to determine the effects of irrigation managements on dynamic changes of soil redox potential and porewater Cr(VI) concentration; (ii) to examine whether introduced cycles of AWD affect Cr uptake by rice; and (iii) to investigate the practical use of irrigation management to reduce the potential Cr risk to human health.
2. Materials and methods 2.1. Soil sample collection and preparation Two typical soils classified as Oxisols and Phaeozems according to World reference base for soil resources (IUSS-FAO, 2014), were collected at 0–20 cm depth from Guilin City (104°40′-119°45′E, 24°18′-25°41′N) and Ha’erbing City (126°32′-129°55′E, 44°92′46°32′N), China. After removal of large pieces of plant materials, grit, earthworms, etc. soil samples were air-dried, ground, and passed through a 2-mm sieve prior to use. Soil samples were analyzed for total Cr (Shentu et al., 2008), Cr(VI) (James et al., 1995), pH (Chaturvedi and Sankar, 2006), cation exchange capacity (CEC) (Hendershot and Duquette, 1986), organic matter (OM) contents (Rashid et al., 2001), Fe(II) contents (Schnell et al., 1998), easily reducible Mn [Mn(ER)] contents (Bartlett and James, 1996), and particle size distribution (PSD) (Day, 1965). Relevant physicochemical properties of the soils are shown in Table 1. The soil samples (Oxisols and Phaeozems) were spiked with Cr as K2Cr2O7 (with a purity 498% from Aldrich Chemical Co.) to establish three contaminant levels of CK (background values), 200, and 400 mg Cr kg 1 soil. All spiked soil samples were aged for 1 year at a moisture content of 70% of water holding capacity prior to pot experiments. 2.2. Pot experiment The rice (O. sativa L.) variety used was Zhongzheyou 1, which is a single season indica variety with an average plant height of 120 cm. This long duration variety takes about 140 days to mature. The seed of the variety was obtained fromthe Zhejiang Seed Company. Seeds were surface-sterilized by washing with 70% ethanol for 1 min and soaking in 0.01 g mL 1 sodium hypochlorite for 5 min, rinsed thoroughly in deionized water, and then imbibed in deionized water for 48 h at 30 °C (Wu et al., 2011). The seeds were germinated in quartz sand washed with 5% (v/v) HCl. For the first two weeks, only deionized water was supplied. After 14 days when seedlings grew onto two-leaf stage, nutrient solution was
Table 1 Physio-chemical properties of the soils. Soil
Oxisols
Phaeozems
Chromium (mg kg 1) Total Cr (mg kg 1) Cr(VI) (mg kg 1)
68.57 2.54 0.28 7 0.01
65.5 7 2.01 0.45 7 0.01
Chemical characteristicsa pH OM (g kg 1) CEC (cmolc ( þ ) kg 1) Fe(II) (mg kg 1) Mn(ER) (mg kg 1)
5.03 7 0.05 19.17 0.56 17.3 7 1.96 34.4 7 1.48 2.647 0.23
7.23 7 0.03 32.2 7 0.34 34.0 7 2.51 30.5 7 0.56 1097 5.21
Soil texture Sand (%) Silt (%) Clay (%)
10.6 7 0.15 39.8 7 1.26 49.67 1.19
20.6 7 1.54 60.2 7 2.21 19.2 7 1.24
a CEC, cation exchange capacity; Mn(ER), easily reducible Mn; OM, organic matter.
supplied. The composition of nutrient solution was the same as described by Yang et al. (2004). On May 30, 2012, 4 seedlings (thirty days old) were transplanted to individual pots with a diameter of 21 cm and depth of 25 cm. Each pot had 5 kg of soil. Before transplanting, the standard recommended dose of NPK fertilizer was applied to all pots at the rates of 187.5 kg N ha 1 (70% applied as basal dose and 30% as topdressing at panicle initiation stage), 70 kg P2O5 ha 1 and 93 kg K2O ha 1 (Wei et al., 2012). The irrigation management factor had two levels: (1) alternating wetting and drying (AWD), the irrigation method is characterized by a mid-season drainage during the late tillering stage of the rice crop, and periodic soil drying of 2–3 days between irrigation events from panicle initiation to grain filling with water level kept at 0–2 cm above the soil surface, and the soil was not irrigated from 100 day until the harvest (Fig. 1); (2) continuous flooding (CF), in which water depth was maintained at 3 cm above the soil surface for the whole duration of the experiment (Fig. 1). All treatments were conducted in triplicate, and the pots were randomly arranged in a greenhouse under a photo flux density of 400 μmol m 2 s 1, a light/dark period of 16/8 h, day/night temperatures of 30/25 °C, and day/ night relative humidity of 75/85% (Wu et al., 2010). Portions of fresh soil (100 g, oven-dry basis) were sampled for Cr(VI) determination at the intervals of 28, 34, 45, 48, 63, 66, 81, 84, 99, 102, 108, and 114 days after transplanting, and then soil samples were immediately stored in sealed plastic containers at 4 °C (Hopp et al., 2008). Each pot contained three permanently-installed platinum electrodes for redox measurement, placed in three pairs at the depth of 7.5 cm, and horizontally distributed around the perimeter about 1 cm from the walls (Johnson-Beebout et al., 2009). The depth was chosen to represent root-zone depth (Patrick and Delaune, 1972). Soil redox potential was monitored on each day of Cr (VI) measurement with platinum-tipped electrodes and a portable Eh meter (EH-120; Fujiwara Scientific Company Co. Ltd., Tokyo, Japan). Plants were harvested at maturity and air dried. Plant samples were manually threshed to separate grains, then the dry weights of grains and straws were recorded. The brown rice was prepared by removing the husk using a laboratory de-husker (JLGJ4.5, Taizhou Cereal and Oil Instrument Co. Ltd., Zhejiang, China), the polished rice was prepared by polishing the bran by a laboratory polishing machine (JNMJ3, Taizhou Cereal and Oil Instrument Co. Ltd., Zhejiang, China). The husk, brown rice, and polished rice samples were ground using a ball mill (Retsch, MM-301, Germany) and passed through a 60-mesh sieve, then keep at 20 °C prior to Cr analysis.
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Fig. 1. Irrigation management in CF (continuous flooding) and AWD (alternate wetting and drying irrigation).
2.3. Analytical methods 2.3.1. Porewater preparation Fresh soil porewater was prepared by centrifuging 50 g (wet weight) of the sieved soil samples in a 100-ml plastic centrifuge bottle at 10,000 rpm for 30 min (Bondarenko et al., 2007). The supernatant from multiple replicates was carefully removed with a pipette and pooled in a 100-ml glass bottle for use in the following experiments. 2.3.2. Porewater Cr(VI) concentration Porewater Cr(VI) concentration was measured by the 1,5-diphenyl-carbazide spectrometry (DPC) colorimetric method, using a phosphate acid buffer (pH 2) to control pH for color development (Kim et al., 2001). The absorbance was determined in a 1-cm cell at 540 nm on a UV-9100 Spectrophotometer (Beijing Ruili Corp.) (Kim et al., 2001). The method had a detection limit of 0.1 μM. Three replications were conducted for each sample. The reliability of the determination procedure was assessed by doing recovery studies. Recovery studies were carried out by adding known concentrations of Cr(VI) standards (1 and 2 mg L 1) to Cr(VI) free water samples. The analysis was done immediately after “spiking” in order to minimize the reduction of Cr(VI) to Cr(III) under the optimum extraction conditions described earlier. The recovery of spiked Cr(VI) was 91.272.5% and 93.3 73.4%. 2.3.3. Total Cr in plant Rice samples (0.1 g) of each treatment were digested with HNO3–H2O2 (4:1) and the digested solution was transferred to a 50 ml volumetric flask, made up to volume and filtered (Wu et al., 2011). The concentrations of Cr in the filtrate were determined using inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7500a) with a detection limit of 0.3–0.4 μg Cr L 1. The ICPMS was operated at the following conditions: the radio frequency power at the torch 1.2 kW, the plasma gas flow 15 L min 1, the auxiliary gas flow 0.89 L min 1, and the carrier gas flow 0.95 L min 1 (Llorent-Martínez et al., 2011). The validation of the presented procedure was checked by the analysis of certified reference material (rice NCSZC73008) approved by General Administration of Quality Supervision, Inspection and Quarantine of the People’s Republic of China (AQSIQ), with the recovery rate of 103.4%. Three replications were conducted for each sample. 2.4. Analytical methods Means of data were compared by the least significant difference (LSD) tests at the 5% significance level. The one-way analysis of variance (ANOVA) was performed using the statistical package
SPSS 18.0 for Windows (CoHort Software, Berkeley, CA, USA).
3. Results and discussion The data reported here are the first to target the effects of irrigation managements on in situ dynamics of soil redox potential and porewater Cr(VI) concentration in rice paddies, and the Cr distribution in rice, thus to investigate the practical use of irrigation management to reduce the potential Cr risk to human health. 3.1. Effects of irrigation managements on soil redox potential The soil redox potential responded as expected to the irrigation methods, soil Eh values under different irrigation managements was clearly different (Fig. 2). In CF treatments, soil Eh values monitored at continuously flooded paddy soils were decreased significantly with time after planting, and decreased from 342 mV to 370 mV and from 368 mV to 324 mV, respectively, for Oxisols and Phaeozems soils. Soil redox potential in CF treatments showed that reducing conditions remained for the whole duration of rice growing period. Similar trends were observed during continuously flooded rice growth in greenhouse studies (Bogdan and Schenk, 2008; Li et al., 2009; Xu et al., 2008) and in the field (Takahashi et al., 2004), suggesting that O2 loss into the rhizosphere is unlikely to oxidize the bulk soil under flooded conditions. The oxidation zone in rhizosphere soil is typically confined to within 1 mm of rice roots (Revsbech et al., 1999). In AWD treatments, soil Eh values decreased immediately after flooding, and then increasing again as the soil dried out prior to the next irrigation, and ranged from 291 mV to 142 mV and from 254 mV to 189 mV, respectively, for Oxisols and Phaeozems soils, therefore, for these two soils, soil conditions alternately changed between reducing and oxic during rice growing period. Soil Eh values in the AWD treatments were higher than those in the CF treatments for the whole duration of rice growing period. Moreover, porewater Cr(VI) concentrations seem to depend on soil Eh values. 3.2. Effects of irrigation managements on porewater Cr(VI) concentration Variations in porewater Cr(VI) concentration were found dependent on the type of irrigation management during the rice growing season. Dynamic changes of Cr(VI) concentration under different irrigation managements were clearly different (Fig. 3). In CF treatments, soil Cr(VI) concentrations decreased with time after planting, while in AWD treatments soil Cr(VI) concentrations were
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Fig. 2. Effects of irrigation management on soil redox potential with different loading rates of Cr. Oxisols Cr–CK (A), Oxisols Cr-200 (B), Oxisols Cr-400 (C), Phaeozems Cr–CK (D), Phaeozems Cr-200 (E) and Phaeozems Cr-400 (F). CF refers to continuous flooding; AWD refers to alternate wetting and drying irrigation.
Fig. 3. Effects of irrigation management on soil Cr(VI) dynamics with different loading rates of Cr. Oxisols Cr–CK (A), Oxisols Cr-200 (B), Oxisols Cr-400 (C), Phaeozems Cr–CK (D), Phaeozems Cr-200 (E) and Phaeozems Cr-400 (F). CF refers to continuous flooding; AWD refers to alternate wetting and drying irrigation.
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increased and decreased alternately response to the irrigation cycles. For example, in CF treatments, soil Cr(VI) concentrations decreased from 2.67 to 1.43 mg kg 1 and from 5.34 to 3.14 mg kg 1, respectively, at the Cr rate of 200 and 400 mg kg 1 in Oxisols, and soil Cr(VI) concentrations decreased from 5.11 to 4.39 mg kg 1 and from 9.13 to 8.15 mg kg 1, respectively, at the Cr rate of 200 and 400 mg kg 1 in Phaeozems soils. However, in AWD treatments, soil Cr(VI) concentrations were decreased immediately after flooding, and then increasing again as the soil dried out prior to the next irrigation, and ranged from 0.28 to 0.37 mg kg 1, from 2.67 to 2.75 mg kg 1 and from 5.34 to 5.79 mg kg 1, respectively at the Cr rate of CK, 200 and 400 mg kg 1 in Oxisols, while soil Cr(VI) concentrations ranged from 0.29 to 0.45 mg kg 1, from 5.11 to 5.13 mg kg 1 and from 8.98 to 9.13 mg kg 1, respectively at the Cr rate of CK, 200 and 400 mg kg 1 in Phaeozems soils. This trend was consistent with soil redox potential, since the soil oxygen status is altered by varying conditions of soil redox potential, and these transformations consequently induce changes in Cr(VI) concentration. Soil redox potential is an important factor affecting Cr(VI) reduction in paddy soils (Kozuh et al., 2000). In CF treatments, as soil redox potential decreased, more Cr(VI) was reduced. On the other hand, Cr(VI) peaks in AWD treatments were observed during rice maturity stage after long drainage periods at soil Eh values above 100 mV. According to the literature (Rupp et al., 2010), reduced conditions result in the reduction of Cr(VI) into Cr (III) and the immobilization of chromates due to decreasing redox potential (Eh). Du Laing et al. (2009) also reported that soils tend to undergo a series of sequential redox reactions when the redox status of the soil changes from aerobic to anaerobic conditions during flooding. Major reactions include Cr(VI) reduction, manganic manganese [Mn(IV)] reduction, ferric iron [Fe(III)] reduction, sulfate (SO2− 4 ) reduction, and methanogenesis. In terms of Cr(VI) reduction, the CF treatment did significantly better than the AWD treatment (Fig. 3), indicating that the long flooding time allowed the lower-depth redox potential to increase the amount of Cr(VI) reduced. Chromium(VI) can be reduced by biological and chemical processes in soils, and chemical reduction is the major avenue by which chromate is reduced (Kozuh et al., 2000; Xiao et al., 2012). Shaheen et al. (2014a) noted that two mechanisms might be proposed to explain the behavior of Cr under reducing conditions as compared to oxidizing conditions in flood–dry-cycles: (1) direct reduction from the high to the low oxic form, and (2) indirect changes of pH and OM caused by changes of Eh values. Furtherly, Shaheen and Rinklebe (2014b) also noted that Cr in soils can be fractionated sequentially to seven fractions as follows: F1—soluble þexchangeable, F2—easily mobilizable, F3—bound to Mn oxides, F4—bound to soil organic matter (SOM) (might include sulfides), F5—bound to low crystalline (amorphous) Fe oxides, F6—bound by crystalline Fe oxides, and F7 —residual fraction, and the mobile fraction of Cr was correlated with soil pH and OM. Chromium(VI) concentrations in the CF treatments were lower than those in the AWD treatments for the most part of ricegrowing season. It can therefore be expected that rice paddies managed by AWD will contain more Cr(VI) than continuously flooded ones. It would be better to use CF than AWD management in Cr-contaminated paddy soils to decrease soil porewater Cr(VI) concentration, and thus to reduce Cr mobility and potential risk to human health. Further research is required with field-scale experiments. 3.3. Effects of irrigation managements on biomass yield of rice Significant differences in the growth of rice were found between two soil types owing to the differences in Cr
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Table 2 Effects of irrigation management on rice yield with different loading rates of Cr. Soil
Cr loading level (mg kg 1)
Irrigation managementa
Grain weight (g pot 1 DW)
Straw weight (g pot 1 DW)
Oxisols
CKb
CF AWD CF AWD CF AWD
41.3 7 1.12 45.6 7 1.43n 35.17 1.26 37.2 7 2.31 32.4 7 1.54n 27.0 7 1.42
38.2 7 1.43 41.8 7 1.25n 31.3 7 1.49 32.5 7 1.83 28.0 7 1.74n 24.27 1.68
CF AWD CF AWD CF AWD
47.5 7 2.06 53.8 7 1.75n 40.8 7 1.94n 35.87 1.59 33.77 1.64n 28.6 7 1.02
43.3 7 0.95 46.9 7 1.23n 35.37 1.53n 29.3 7 1.41 26.8 7 0.85n 20.17 0.97
200 400
Phaeozems CK 200 400
n P o 0.05 refers to significant difference between CF and AWD treatments at each Cr level within the same columns. a CF refers to continuous flooding, AWD refers to alternate wetting and drying irrigation. b CK refers to soil samples without spiking Cr; 200 and 400 refers to soil samples with Cr contaminant levels of 200 and 400 mg kg 1, respectively.
phytoavailability (Xiao et al., 2013) (Table 2). Rice yield was also influenced by external Cr loading rates and irrigation managements (Table 2). Dry weights of rice decreased gradually as the Cr rates increased, up to 59.3%–78.3% and 53.2%–71.0% of the control at Cr rate of 400 mg kg 1, respectively for Oxisols and Phaeozems soils. The growth inhibition might be attributed to Cr toxicity (Athar and Ahmad, 2002). The stimulating effect of AWD on the growth of rice occurred at Cr rate of CK and 200 mg kg 1 in Oxisols, and CK in Phaeozems, while the growth of rice was inhibited by AWD at Cr rate of 400 mg kg 1 in Oxisols, and 200 and 400 mg kg 1 in Phaeozems (Table 2). The stimulation of AWD treatment on rice growth at low Cr rates may be as a result of higher nitrogen availability and organic matter content in paddy soils for wetting and drying cycles (Dong et al., 2012; Wang et al., 2011). Alternating wetting and drying irrigation may also increase rice roots in deeper soil layers, maintain root activities and promote N uptake at later stages (Horie et al., 2005). Moreover, AWD may increase a greater partitioning of plant accumulated N to the grains (Lin et al., 2006). On the contrast, other literatures reported that the grain yield of rice was reduced by AWD irrigation (Belder et al., 2004; Tabbal et al., 2002). The discrepancies between the studies are probably attributed to the variations in the timing of irrigation method applied and soil hydrological conditions (Belder et al., 2004). In this study, the inhibition of AWD on rice growth at high Cr rates were mainly ascribe to the high toxicity of Cr(VI), because soil porewater Cr(VI) concentrations were higher in AWD treatments than those in continuous flooding. 3.4. Effects of irrigation managements on Cr distribution in rice Chromium concentrations in rice tissues varied with Cr levels and irrigation managements, and increased with increasing Cr loading rates (Fig. 4). Xiao et al. (2013) also noted that Cr concentrations in rice were significantly related to total Cr and Cr(VI) concentrations in soils. Chromium concentrations in rice was in the order of husk4 brown rice 4polished rice, and ranged from 0.53–17.2, 0.39–13.5 and 0.30–10.2 mg kg 1, respectively (Fig. 4). Chromium concentrations in rice tissues varied between two soil types, were 0.30–0.58 and 0.47–1.02 mg kg 1 at the Cr rate of CK, respectively for Oxisols and Phaeozems soils. The corresponding values were 2.48–5.13 and 3.58–7.69 mg kg 1 at the Cr
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Fig. 4. Effects of irrigation management on Cr distribution in rice with different loading rates of Cr. *Po 0.05 refers to significant difference between CF (continuous flooding) and AWD (alternate wetting and drying irrigation). Oxisols Cr–CK (A), Oxisols Cr-200 (B), Oxisols Cr-400 (C), Phaeozems Cr–CK (D), Phaeozems Cr-200 (E) and Phaeozems Cr400 (F).
rate of 200 mg kg 1, and 5.88–11.6 and 8.20–17.2 mg kg 1 at the Cr rate of 400 mg kg 1, respectively. The significant differences between two soil types were mainly due to the differences in Cr (VI) reduction and the Cr phytoavailability in paddy soils (Xiao et al., 2013). Xiao et al. (2012) noted that the reduction of Cr(VI) in Oxisols was more fast than that in Phaeozems soils, which was mainly ascribed to low pH in Oxisols. Moreover, the soil texture and the OM content might also play important roles here. Chromium concentrations in rice tissues were significantly influenced by irrigation with relatively higher values in the AWD treatments. Chromium concentrations in husk, brown rice and polished rice were respectively increased by 9.43%–21.3%, 10.3%– 25.6% and 13.3%–29.8% by AWD irrigation in Oxisols, as compared to continuous flooding. The corresponding values were 15.9%– 21.1%, 14.9%–24.0%, 17.0%–26.5%, respectively in Phaeozems soils. Compared with continuous flooding, soil conditions alternately changed from reducing to oxic during rice growing period significantly increased Cr uptake in paddy soils, which must be attributed to the higher Cr(VI) concentrations in AWD treatments due to slow-down of the Cr(VI) reduction rates (James and Bartlett, 1983; Loyaux-Lawniczak et al., 2001). Therefore, it would be better to use CF than AWD management in Cr-contaminated paddy soils to reduce Cr uptake by rice, and thus to lower the potential risk to human health. Further research is also required with field-scale experiments. By contrast, for some metals with totally different characteristics with Cr, such as As, conflicting results can be obtained. For example, As concentrations in porewater are markedly lower under oxic conditions and generally dominated by low solubility and toxicity species of As(V) rather than As(III) (Roberts et al., 2011). Thus, compared to continuous flooding, aerobic conditions maintained throughout (Xu et al., 2008) or during part of
the rice growing season (Li et al., 2009) significantly reduced As uptake in greenhouse studies.
4. Conclusion The present study examined the effects of irrigation managements on in situ dynamics of soil redox potential and porewater Cr (VI) concentration in rice paddies, and Cr accumulation in rice. Soil redox potential in CF treatments showed that reducing conditions remained for the whole duration of rice growing period, while soil redox potential in AWD treatments showed that soil conditions alternately changed between reducing and oxic. Soil Eh values in the AWD treatments were higher than those in the CF treatments for the whole duration of rice growing period. Porewater Cr(VI) concentrations seem to depend on soil Eh values. In CF treatments, soil Cr(VI) concentrations decreased with time after planting, while in AWD treatments soil Cr(VI) concentrations were increased and decreased alternately response to the irrigation cycles. The trend was consistent with soil redox potential, therefore, it can be expected that soil redox potential is an important factor affecting Cr(VI) reduction in paddy soils. Chromium concentrations in rice tissues were significantly influenced by irrigation with relatively higher values in the AWD treatments. Compared with continuous flooding, soil conditions alternately changed from reducing to oxic significantly increased Cr uptake, which might be attributed to the higher Cr(VI) concentrations in AWD treatments. Therefore, it would be better to use CF than AWD management in Cr-contaminated paddy soils to reduce Cr accumulation in rice, and thus to lower the potential risk to human health. Further research is required with field-scale experiments.
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Acknowledgment This study was financially supported by the Ministry of Environmental Protection of China (Grant no. 2011467057), Zhejiang Provincial Natural Science Foundation of China (Grant no. LY12E09008), and Science and Technology Innovation Ability Promotion Project of the Zhejiang Academy of Agricultural Sciences.
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