1. Introduction
Cadmium (Cd) and arsenic (As) are two of the most common toxic elements in the paddy soils of China, posing a significant threat to the quality and safety of the nation’s agricultural products and human beings via the food chain [1]. According to the “National Soil Pollution Status Survey Bulletin”, published in 2014 by the Ministry of Environmental Protection and the Ministry of Land and Resources of China, the exceedance rates of Cd and As at agricultural soil sampling points were recorded at 7.0 and 2.7%, respectively [2,3]. The two elements often co-occur in soils naturally or due to common inputs from human activities including mining, smelting, industrial discharge, and sewage irrigation in the proximity of agricultural areas, and they have a high risk of transfer from paddy soil to the edible crop products. Rice (Oryza sativa L.) is one of the most important cereal crops in the world [4]. In China, more than 60% of the population relies predominantly on rice as a staple food. Owing to its distinctive agricultural management practices and specific physiological processes, rice exhibits an enhanced capability for the accumulation of Cd and As [5,6]. This has culminated in rice emerging as a principal conduit for the human ingestion of Cd and As, subsequently initiating conditions such as osteoporosis and various forms of malignancy [7]. Notably, the toxicological implications associated with rice-mediated arsenic exposure primarily stem from the presence of As(V) and As(III) in inorganic As compounds, with the latter being distinguished by heightened solubility and toxicity [8,9]. Therefore, there is an urgent need to implement effective methods to control the uptake and translocation of Cd and inorganic As within the soil–rice system.
Water management constitutes a pivotal agronomic measure for the remediation and secure utilization of farmland soils contaminated by heavy metals [10]. This strategic intervention serves to regulate the redox potential of soils, thereby influencing the chemical valence and speciation of Cd and As species, consequently modulating the translocation dynamics of these elements towards rice plants [11,12]. Accumulated research findings predominantly corroborate that soil inundation tends to promote the passivation of Cd, while desiccation facilitates the oxidation and passivation of As [13,14]. Liao et al. [15] documented that prolonged flooding mitigates the bioavailability of soil Cd, yet concurrently augments the bioavailability of As, thereby inducing rice grains to surpass the stipulated national standards for As content. Hu et al. [16] reported that the soil HCl-extractable As content increased significantly, and the HCl-extractable Cd content decreased significantly with increasing irrigation from aerobic to flooded conditions. These trends were consistent with the As and Cd contents in straw, husk, and brown rice. Moreover, the modality of water management engenders intricate ramifications in regards to the environmental comportment of rice Cd and inorganic As within the soil–rice system, a comportment ineluctably influenced by soil texture [17,18]. The granulometric composition of soils decisively dictates pivotal attributes encompassing water-holding capacity, moisture infiltration, and spatiotemporal distribution and retention within the soil profile [19], consequently affecting the dispersion patterns of Cd and inorganic As within the soil–rice system. At the current juncture, the effect of water management on the accumulation of Cd and inorganic As by rice has been extensively investigated and reported; however, information concerning the synergistic influence of water management strategies and the type and inherent attributes of parent material on these strategies is limited.
In the present study, a field experiment was conducted to investigate the effects of water management regimes under two typical local soil conditions in the Sichuan Basin, Western China, on the accumulation of both Cd and inorganic As. The main objectives of this paper are to explore the effects of water management on the bioavailability of soil Cd and As under different agricultural soil conditions, along with their accumulation, assimilation, and translocation in the rice grains.
2. Materials and Methods
2.1. Site Description and Soil Properties
The experimental site denoted as K1 was located in a conventional agricultural field near Chengdu City, Sichuan Province, Western China (30°59′33″ N, 104°5′29″ E). The soil is paddy soil developed from gray-brown alluvium [20]. In a parallel vein, the K2 site was located in Dazhou City, Sichuan Province, Western China (30°35′56″ N, 107°30′1″ E). The trial soil at this locale is paddy soil developed from weathered shale and slate [20]. The basic physicochemical properties of the topsoil (0~20 cm) before the experiment are shown in Table 1. The soil Cd content at the K1 site and the soil As content at the K2 site exceed the national risk screening thresholds for soil contamination of agricultural land (0.3 and 30 mg kg−1 for Cd and As under soil pH < 5.5, 0.8, and 20 mg kg−1 for Cd and As under soil pH > 7.5, respectively) issued by China (GB 15618-2018) [21].
2.2. Field Experiment
The two types of water management were continuous flooding (T) and dry–wet alternation (CK). For T treatment, a water layer with a depth of 3~5 cm was maintained above the soil surface throughout the experiment until rice harvest. For CK treatment, the soil was first flooded (3~5 cm above the soil surface) and then naturally dried (surface water was not observed). It was then alternately flooded and dried until rice harvest. The water used in the study was agricultural irrigation water from nearby ditches. The pH of the water was 6.92 and 6.98 at the K1 site and K2 site, respectively. The Cd and As contents in the water at both sites were below the detection limits. There were three replicate plots for each treatment. The test plots were arranged in random blocks, with an area of 30 m2 (5 × 6 m), and included a one-meter-wide buffer strip. The tested rice variety was Gangyou 828. Chemical fertilizers were applied as a base fertilizer before rice transplant. Nitrogen (N), phosphorus (P), and potassium (K) were applied at rates of 135, 45 (P2O5) and 90 (K2O) kg ha−1, respectively, in all treatments.
2.3. Sample Collection
Soil samples were collected from a 0~20 cm soil layer in the tillering (TS), jointing (JS), grain filling (FS), and maturing (MS) rice growth stages. In each plot, five cores (each 2.5 cm in diameter) were randomly excavated and mixed as a composite sample in each rice growth stage. The composite samples were placed in plastic bags and transported to the laboratory. The samples were air-dried at room temperature and sieved for determining the soil properties.
Plant samples were also collected in the TS, JS, FS, and MS rice growth stages from each plot. Ten individual plants were randomly selected from each plot. The plant samples were washed with deionized water, dried at 105 °C for 15 min, and then oven-dried at 80 °C to ensure a constant weight. After oven-drying, the plant samples in the TS, JS, and FS rice growth stages were separated into root and straw and ground to pass through a 0.3 mm sieve. For the MS rice growth stage, the plant samples were separated into root, straw, and grains. The rice grains were separated into husk and brown rice. The root, straw, husk, and brown rice were pulverized and preserved.
2.4. Sample Determination
The soil pH value was measured using a pH instrument (PHS-3C, Shanghai Yifen Scientific Instrument, Shanghai, China). The soil Eh value was measured in situ using a portable instrument (SX712, Shanghai Yidian, Shanghai, China). Soil Cd was digested using HNO3-HF-HClO4 (Xilong Chemical Co., Ltd., Shangtou, China) (5:4:2) (v/v/v), and then the Cd content was analyzed by ICP-MS (NexION 300D, Perkin Elmer, Waltham, MA, USA). The bioavailable Cd content was extracted using DTPA extraction and quantified by ICP-AES (ICAP RPO XP, Thermo Fisher, Waltham, MA, USA). The inorganic As (As(III) and As(V)) content was determined as described by Liang [22], i.e., a sample was extracted using one drop of mercuric chloride solution and 10.0 mL of a mixture of 0.1 M H3PO4 (Xilong Chemical Co., Ltd., Shangtou, China) and 0.1 M NaH2PO4·2H2O (Komio Chemical Reagents Co., Ltd., Tianjing, China) (1:9) (v/v) solution and subsequently quantified by HPLC-HG-AFS (LC-AFS 9750, Haiguang, Beijing, China). The contents of OM, TN, TP, and TK were determined as described by Lu [23].
The plant samples (root, straw, husk, and brown rice) were digested using HNO3:HClO4 (4:1) (v/v) and analyzed by ICP-MS (NexION 300D, Perkin Elmer, USA). The inorganic As (As(III) and As(V)) levels of the plant samples were determined by HPLC-ICP-MS (NexION 300D, Perkin Elmer, USA) following the extraction of samples with 10 mL of HNO3 (2%) [24]. Rice (GBW100349) obtained from the National Research Center for Standards in China was used as a standard reference material.
2.5. Statistical Analysis
The transfer factor (TF) [25] was determined using the following formulae:
TFi/j = Ci/Cj(1)
where Ci is the heavy metal content in the root, straw, husk, and brown rice (mg kg−1); and Cj is the heavy metal content in the soil, rice root, straw, and husk (mg kg−1).All data are presented as the means ± SD of three independent replicates. The data were subjected to statistical analysis using SPSS version 25.0 (IBM, Inc., Armonk, NY, USA). Independent sample t-tests detected the differences between treatments at p < 0.05. All of the figures were plotted using Origin 9.0 (OriginLab, Northampton, MA, USA).
3. Results
3.1. Effects of Water Management on Soil pH and Eh under Different Soil Conditions
Figure 1a shows that under the flooding treatment, the soil pH at the K1 site gradually increased with the progression of the rice growth stages, reaching a maximum value at maturity, and there was a significant difference between the tillering and mature stages (p < 0.05). The soil pH at the K1 site under the dry–wet alternation treatment increased at first and then decreased, reaching a minimum value at maturity. Conversely, there were no significant differences between the different water management treatments at the K2 site (p > 0.05). The soil pH under the flooding treatment at the K1 site notably increased by 0.45~0.93 units (p < 0.05), whereas the pH differences at the K2 site under different water management treatments were insignificant (p > 0.05). As can be seen from Figure 1b, the soil Eh under the dry–wet alternation treatment showed a trend of “increase–decrease–increase” with the progression of rice growth stages, but showed no obvious difference among all of the rice growth stages under the flooding treatment at the two experimental sites (p > 0.05). Moreover, the soil Eh of the dry–wet alternation treatment was significantly higher than that of the flooding treatment from tillering to maturity at the two experimental sites (p < 0.05).
3.2. Effects of Water Management on Soil-Available Cd and Inorganic As under Different Soil Conditions
As shown in Figure 2, there were no significant changes in the soil Cd and available Cd content among the treatments with the growth of rice at the two experimental sites (p > 0.05). Moreover, compared with the dry–wet alternation treatment, the differences in the soil Cd content under the flooding treatment were insignificant at both experimental sites (p > 0.05). The soil-available Cd contents for the flooding treatment were 46.9~54.0% lower than those of the dry–wet alternation treatment (p < 0.05) at the K1 site. However, the soil-available Cd contents among all treatments showed no obvious difference at the K2 site (p > 0.05). The soil As(Ⅲ) contents for the flooding treatment were 16.0~74.2% higher than those of the dry–wet alternation treatment (p < 0.05), in contrast to the non-significant difference at the K2 site. In addition, the soil As(V) contents among all water management treatments showed no obvious differences (p > 0.05).
3.3. Effects of Water Management on Cd and Inorganic As Content in Rice Plants under Different Soil Conditions
3.3.1. Cd and Inorganic As Content in Rice Root and Straw
As shown in Figure 3, the Cd levels in the rice root and straw at site K1 under all water management treatments increased with rice growth, reaching a maximum value at maturation. The Cd contents of the rice root and straw under the flooding treatment were 11.5 and 14.7 times higher than those in the tillering stage (p < 0.05), while under the dry–wet alternation treatment, they were 1.4 times and 3.6 times higher than those in the tillering stage (p < 0.05), respectively, at the K1 site. In contrast, the Cd contents of the rice root and straw at the K2 site demonstrated negligible variation with rice growth (p > 0.05). The Cd contents of the rice root and straw in the flooding treatment were 36.1~92.0% and 73.3~96.2% lower than those for the dry–wet alternation treatment (p < 0.05) at the K1 site. The Cd contents of the rice root under flooding treatment significantly decreased by 23.1~36.3% (p < 0.05), while the variance in the straw Cd contents remained statistically insignificant at the K2 site (p > 0.05). Moreover, flooding increased the As(III) contents in rice root and straw at the K1 site by 1.94~4.35 and 1.23~6.53 times, while at the K2 site, it increased by 1.18~1.54 and 1.12~2.47 times, respectively. At the K1 site, the As(V) contents of rice root and straw under the flooding treatment were 1.16~3.12 and 1.97~9.36 times higher than those under the dry–wet alternation treatment, while at the K2 site, they were only 1.15~1.60 and 1.30~2.19 times higher than those under the dry–wet alternation.
3.3.2. Cd and Inorganic As Content in Husk and Brown Rice
Under the dry–wet alternation treatment, the Cd contents in the husk and brown rice at the K1 site were 0.29 and 0.51 mg kg−1, respectively, while those at the K2 site were all 0.01 mg kg−1 (Figure 4). Flooding treatment greatly decreased the Cd contents in the husk and brown rice by 90.2 and 92.2% at the K1 site compared to the dry–wet alternation treatment (p < 0.05), while, there were no significant differences between the two water management treatments at the K2 site (p > 0.05). Under the dry–wet alternation treatment, the As(III) contents in the husk and brown rice were 0.01 and 0.05 mg kg−1 at the K1 site, and 0.07 and 0.17 mg kg−1 at the K2 site, respectively. Compared with the dry–wet alternation treatment, the flooding treatment greatly increased the As(III) contents in the husk and brown rice by 263.6 and 153.3%, respectively. There were no significant differences in the As(III) contents in the husk and brown rice between the two water management treatments at the K2 site (p > 0.05). In addition, the As(V) contents in the husk were 0.20 and 0.39 mg kg−1 under the dry–wet alternation treatment at the K1 and K2 sites, respectively. As the water irrigation volume increased from aerobic to flooding, the As(V) contents in the husk increased by 85.9 and 39.3% at the K1 and K2 sites, respectively. The As(V) contents in the brown rice at both sites were below the detection limits.
3.3.3. Cd and Inorganic As Transport in Rice
As shown in Table 2, in comparison to the dry–wet alternation, flooding significantly decreased the Cd of TFsoil/root, TFroot/straw, TFstraw/husk, and TFhusk/rice by 93.0, 19.0, 87.6, and 23.5% at the K1 site, and it decreased the Cd of TFsoil/root and TFroot/straw by 35.5 and 23.5% at the K2 site, respectively. There were no significant differences in the Cd of TFstraw/husk and TFhusk/rice between the two water management treatments at the K2 site (p < 0.05). Compared with the dry–wet alternation treatment, flooding greatly increased the As(Ⅲ) of TFsoil/root and TFstraw/husk by 2.52 and 3.74 times and the As(V) of the TFsoil/root and TFstraw/husk by 3.37 and 2.10 times, respectively, at the K1 site. There were no significant differences in the As (Ⅲ) of TF between the two water management treatments at the K2 site (p > 0.05).
3.4. Relationships between Soil Properties and Cd and As(III) Content in Brown Rice
At the K1 site, the Cd content in the brown rice showed a strong positive relationship with soil Eh and soil-available Cd content, and a negative relationship with soil pH and As(III) in brown rice. Meanwhile, the As(III) in brown rice showed a strong positive relationship with soil pH and As(III) content, and a negative relationship with soil Eh. Flooding treatment reduces the mobility of cadmium and the subsequent Cd content in brown rice, but significantly increases the bioavailability of arsenic in the soil, thus promoting its accumulation in rice compared to that observed in the dry–wet alternation treatment. At the K2 site, no significant correlations were found between soil pH/Eh and Cd/As(III) content in rice (Table 3).
4. Discussion
4.1. Soil pH and Eh
In general, water management can affect the redox status of agricultural soils, thereby leading to alterations in the soil pH and Eh [26,27,28]. In the present study, the flooding treatment significantly increased the soil pH compared with the dry–wet alternation treatment, which suggests that flooding turns soil into reduction condition, and dry–wet alternation results in dynamic redox fluctuations. Under the flooding condition, the variable negative charge in the soil increases, the soil changes from an oxidized state to a reduced state, and a series of reduction reactions occur in the soil, consuming a large amount of H+, causing the soil pH to increase significantly [28,29]. Moreover, the soil pH at the K1 experimental site showed different changing trends under the flooding and dry–wet alternation modes within the passage of the rice growth period, and the variation amplitude was significant. This may be due to the temperature continuing to rise after filling, and the organic matter in the farmland system decomposing rapidly under dry–wet alternation to produce acidic substances, resulting in a further decrease in soil pH compared with the base value [30,31]. On the other hand, the paddy soil at the K1 site derives from gray-brown alluvium, while the paddy soil at the K2 site comes from weathered shale and slate. Usually, the exchangeable acid content of the paddy soil derived from weathered shale and slate is lower than that of the paddy soil derived from gray-brown alluvium, resulting in a smaller degree of exchangeable acid release under different water management techniques.
As an indicator of soil redox status, soil Eh can characterize the relative degree of soil oxidation or reduction. In this study, the soil Eh under flooding treatment in the two test sites was significantly lower than that of the dry–wet alternation, which is similar to the results of Borch et al. [32]. Flooding blocks the oxygen supply in the soil, and the soil’s original oxygen is consumed by microorganisms. Manganese oxide, iron oxide, and SO4− consume numerous electrons in the reduction process, and the high-valence NO3-N, Mn4+, Fe3+, and S6+ ions in the soil as electron acceptors are reduced to low-valence N2O-N, N2, Mn2+, Fe2+, and S2−, thereby resulting in a decrease in soil Eh [33,34].
4.2. Soil-Available Cd and Inorganic As Contents
The water management approaches markedly affected the soil pH and Eh and therefore, the soil-available Cd and inorganic As contents [35]. Xu et al. [36] reported that the As content in the soil solution was 4~13 times higher under flooding than under aerobic conditions during the period of active rice growth. Arao et al. [37] also reported that continuous flooding treatment reduces the mobility of Cd, but significantly increases the bioavailability of As in the soil. In this study, compared with the dry–wet alternation treatment, the soil-available Cd content significantly decreased under flooding, while the As(Ⅲ) content significantly increased. This is because anaerobic conditions decrease the mobilization of Cd in soils due to the increase in pH, the reduction in Eh, and the combination of Cd with sulfur to form cadmium sulfide (CdS) at low redox potential [1,38,39]. Under alternating wetting and drying conditions, redox reactions oscillate, maintaining a lower pH that encourages the release of Cd2+ into the soil. Moreover, under flooding conditions, the As adsorbed on oxides such as iron and manganese will be released into the soil solution, the bioavailability of As in the soil increases, and As(V) is gradually reduced to As(Ⅲ) [40]. After flooding, the soil-available Cd and inorganic As content at the K2 site were not significantly different from those in the dry–wet alternation, which may be related to the soil-forming parent materials of the two paddy soils. The paddy soil derived from gray-brown alluvium, carries a smaller charge and possesses fewer colloidal properties, enabling Cd2+ to be more easily released. Conversely, the paddy soil, derived from weathered shale and slate, bears a larger charge and is conducive to the adsorption of Cd2+ in the soil.
4.3. Cd and Inorganic As in Rice Plants
It is known that water management regimes strongly affect heavy metal speciation in soils, and consequently, Cd and As accumulation and transportation in rice [12]. Previous studies have indicated that under flooding conditions, with the increase in pH and the decrease in Eh, the mobility of Cd decreases [41,42], while that of As increases [34]. In this study, it was observed that compared to the dry–wet alternation treatment, flooding significantly reduced the Cd content in the root, straw, husk, and brown rice of the plants and concurrently increased the inorganic As content at the K1 site. This may be due to the reduced effectiveness of Cd under flooding conditions coupled with an enhanced effectiveness of As in the soil, thereby influencing the accumulation of Cd and inorganic As in different rice organs [43]. Furthermore, the Cd and inorganic As contents in different plant organs may also be closely related to the plant’s transport capabilities for these elements [44]. A larger transfer coefficient indicates a stronger capacity for the movement of heavy metals between plant organs. Compared with the dry–wet alternation treatment, flooding significantly decreased the Cd of TFsoil/root, TFroot/straw, TFstraw/husk, and TFhusk/rice at the K1 site. These results were similar to the findings of Zhang et al. [41], who indicated that the TF of the root-to-shoot under flooding decreased by more than 54% as compared with the alternate wetting and drying method. This decrease suggests that flooding effectively enhances the adsorption of Cd2+ by the soil, reduces Cd2+ uptake by the rice root, diminishes the transport of Cd2+ from the root to above-ground parts through the xylem, and consequently decreases the Cd2+ content in brown rice [45]. Moreover, the iron plaque formed under the flooding treatment can act as a barrier to prevent the transfer of Cd from the iron plaque to the root, thus decreasing the Cd content in rice [46]. For inorganic As, the TFsoil/root and TFstraw/husk of As(Ⅲ) and As(V) under flooding treatment were significantly higher than those under the dry–wet alternation treatment, indicating that flooding promotes the transport of inorganic As from soil to root, straw, and husk. This may be due to the fact that continuous flooding inhibited the biosynthesis of phytochelatins and vacuolar sequestration in the rice root, subsequently promoting the translocation of As from the root to the aboveground parts [38]. At the K2 site, different water management patterns had an insignificant impact on the Cd and inorganic As among all rice organs. This could be due to the differences in the composition of clay minerals and organic matter content between two paddy soil parent materials [47]. These differences affect the adsorption and desorption of Cd and inorganic As on the soil surfaces, thus directly influencing their distribution in the soil–crop system. With higher soil clay content and lower organic matter content, the K2 site exhibited the reduced mobility of available Cd and inorganic As within the soil–crop system. Moreover, Pearson correlation analysis showed that the contents of As and Cd in the brown rice at the K1 site were significantly correlated with the soil properties, while there were no obvious correlations among As and Cd in the brown rice, soil pH, or Eh at the K2 site.
5. Conclusions
In the present field study, we investigated the effects of water management on Cd and inorganic As accumulation in rice under different soil conditions. Compared to dry–wet alternation, flooding reduced the soil-available Cd content by 49.3~55.8% and increased the soil As(Ⅲ) content by 16.0~74.2% at the K1 site. However, flooding had limited effects on the soil-available Cd and inorganic As contents at the K2 site. In addition, flooding resulted in a remarkable reduction of 90.2% in Cd content in rice husk and 92.2% in brown rice, along with significant increases of 263.6% and 153.3%, respectively, in the As(Ⅲ) content at the K1 site. Compared to the K2 site, there were more drastic changes in the Cd and inorganic As in the plant samples as the water management shifted at the K1 site. These results indicate that the changes in the rice Cd and inorganic As contents in response to water management under different soil conditions exhibit varying results, and the change in the paddy soil derived from gray-brown alluvium was significantly greater than that of the soil derived from weathered shale and slate. Future studies should focus on the investigation of the extractability and oxidation/reduction of Cd and inorganic As associated with soil clay minerals.
Conceptualization, methodology, and writing-reviewing, X.L. and Y.Z.; funding acquisition, and investigation, L.L. and P.W.; data curation and writing-original draft preparation, R.Y. All authors have read and agreed to the published version of the manuscript.
The data presented in this study are available on request from the corresponding author.
We appreciate Liang Jing, Qing Jian, and Xue Zhang very much for their kind help in conducting the experiment, as well as in the sampling and pre-treatment of the samples.
The authors declare no conflict of interest.
Footnotes
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Enlarge this image.Figure 1. Soil pH (a) and Eh (b). Different letters indicate the significant differences among treatments at p < 0.05. Values are the means ± standard deviation (n = 3). CK, dry-wet alternation; T, flooding; TS, tillering stage; JS, jointing stage; FS, grain filling stage; MS, maturation stage.
Enlarge this image.Figure 2. Effects of water managements on soil Cd (a); bioavailable Cd (b); As(III) (c); and As(V) (d) contents under different soil conditions. Different letters indicate significant differences among the treatments at p < 0.05. CK, dry–wet alternation; T, flooding; TS, tillering stage; JS, jointing stage; FS, grain filling stage; MS, maturation stage.
Enlarge this image.Figure 3. Effects of water management on Cd content in root (a) and straw (b); As(III) content in root (c) and straw (d); As(V) content in root (e) and straw (f) under different soil conditions. Values are the means ± standard deviation (n = 3). Different letters indicate significant differences among the treatments at p < 0.05. CK, dry–wet alternation; T, flooding; TS, tillering stage; JS, jointing stage; FS, grain filling stage; MS, maturation stage.
Enlarge this image.Figure 4. Effects of water managements on Cd (a), As(III) (b), and As(V) contents in husk and brown rice (c) under different soil conditions. Different letters indicate the significant differences among treatments at p < 0.05. CK, dry–wet alternation; T, flooding.
Physicochemical properties of the soil used in this study.
| Pilot Zone | pH | OM | TN | TP | TK | Cd | As | As(Ⅲ) | As(V) | Particle Size % | ||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| g kg−1 | g kg−1 | g kg−1 | g kg−1 | mg kg−1 | mg kg−1 | mg kg−1 | mg kg−1 | Sand | Silt | Clay | ||
| K1 | 5.02 | 38.8 | 1.73 | 0.84 | 15.4 | 0.33 | 7.3 | 0.33 | 1.82 | 31.8 | 49.6 | 18.6 |
| K2 | 8.03 | 15.6 | 1.05 | 0.68 | 5.51 | 0.55 | 27.5 | 2.72 | 15.38 | 19.2 | 51.7 | 29.1 |
OM, organic matter; TN, total nitrogen; TP, total phosphorus; TK, total potassium.
Effects of water management on Cd and inorganic As transfer coefficients in rice under different soil conditions.
| Elements | Pilot Zones | Treatments | TFsoil/root | TFroot/straw | TFstraw/husk | TFhusk/rice |
|---|---|---|---|---|---|---|
| Cd | K1 | CK | 13.20 ± 0.63a | 0.75 ± 0.04a | 0.38 ± 0.03a | 1.79 ± 0.13a |
| T | 0.93 ± 0.05b | 0.61 ± 0.06b | 0.05 ± 0.01b | 1.37 ± 0.05b | ||
| K2 | CK | 0.56 ± 0.05a | 0.10 ± 0.02a | 0.06 ± 0.02a | 1.49 ± 0.45a | |
| T | 0.36 ± 0.02b | 0.07 ± 0.00a | 0.06 ± 0.01a | 1.18 ± 0.27a | ||
| As(Ⅲ) | K1 | CK | 8.31 ± 1.40b | 0.02 ± 0.01a | 0.86 ± 0.16b | 3.39 ± 0.97a |
| T | 20.93 ± 1.28a | 0.02 ± 0.00a | 3.21 ± 0.15a | 2.34 ± 0.28a | ||
| K2 | CK | 5.16 ± 0.63a | 0.02 ± 0.01a | 4.52 ± 0.74a | 2.24 ± 0.48a | |
| T | 6.08 ± 0.33a | 0.03 ± 0.00a | 3.18 ± 0.61a | 2.10 ± 0.36a | ||
| As(V) | K1 | CK | 4.71 ± 0.95b | 0.07 ± 0.01b | 2.72 ± 0.43a | 0.00 ± 0.00a |
| T | 15.86 ± 1.18a | 0.15 ± 0.02a | 2.14 ± 0.41a | 0.00 ± 0.01a | ||
| K2 | CK | 1.20 ± 0.14b | 0.22 ± 0.02a | 1.97 ± 0.31a | 0.00 ± 0.02a | |
| T | 1.62 ± 0.07a | 0.25 ± 0.04a | 2.23 ± 0.68a | 0.00 ± 0.03a |
Values are the means ± standard deviation (n = 3). Different letters represent significant differences (p < 0.05) among different treatments, according to independent sample t-tests. CK, dry–wet alternation; T, flooding.
Correlation coefficients between soil properties and Cd and As(III) content in brown rice.
| Pilot Zone | Parameter | Soil pH | Soil Eh | Soil Cd | Soil-Available Cd | Soil As(Ⅲ) | Soil As(V) | Rice Cd | Rice As(III) |
|---|---|---|---|---|---|---|---|---|---|
| K1 | Soil pH | 1 | |||||||
| Soil Eh | −0.933 ** | 1 | |||||||
| Soil Cd | ns | ns | 1 | ||||||
| Soil-available Cd | −0.934 ** | 0.961 ** | ns | ||||||
| Soil As(Ⅲ) | 0.874 * | −0.976 ** | ns | −0.917 * | 1 | ||||
| Soil As(V) | ns | ns | −0.816 * | ns | ns | 1 | |||
| Rice Cd | −0.937 ** | 0.999 ** | ns | 0.953 ** | −0.973 ** | ns | 1 | ||
| Rice As(III) | 0.904 * | −0.966 ** | ns | −0.984 ** | 0.923 ** | ns | −0.958 ** | 1 | |
| K2 | Soil pH | 1 | |||||||
| Soil Eh | ns | ||||||||
| Soil Cd | ns | ns | 1 | ||||||
| Soil-available Cd | ns | ns | ns | 1 | |||||
| Soil As(Ⅲ) | ns | ns | ns | ns | 1 | ||||
| Soil As(V) | ns | ns | ns | ns | 0.842 * | 1 | |||
| Rice Cd | ns | ns | ns | 0.867 * | ns | ns | 1 | ||
| Rice As(III) | ns | ns | ns | ns | ns | ns | ns | 1 |
Note: * significant at 0.05; ** significant at 0.01; and ns, not significant.
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