Cadmium (Cd) and arsenic (As) accumulation in rice grains is a great threat to its productivity, grain quality, and thus human health. Pot and field studies were carried out to unravel the effect of different water management practices (aerobic, aerobic-flooded, and flooded) on Cd and As accumulation in rice grains of two different varieties. In pot experiment, Cd or As was also added into the soil as treatment. Pots without Cd or As addition were maintained as control. Results indicated that water management practices significantly influenced the Cd and As concentration in rice grains and aerobic cultivation of rice furnished less As concentration in its grains. Nonetheless, Cd concentration in this treatment was higher than the grains of flooded rice. Likewise, in field study, aerobic and flooded rice cultivation recorded higher Cd and As concentration, respectively. However, growing of rice in aerobic-flooded conditions decreased the Cd concentration by 9.38 times on average basis as compared to aerobic rice. Furthermore, this treatment showed 28% less As concentration than that recorded in flooded rice cultivation. The results suggested that aerobic-flooded cultivation may be a promising strategy to reduce the Cd and As accumulations in rice grains simultaneously.
Adverse effects of heavy metals on crops and increasing health hazards have attracted more and more attention in recent years. In China, nearly 10 billion ha of arable land has been contaminated by heavy metals mainly including cadmium, arsenic, lead, copper, nickel, zinc, mercury, and chromium [
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It is crucial to develop practical and effective strategies for reducing the amount and mobility of As and Cd in soil or to limit their uptake and accumulation in rice grains. Zhao et al. [
Although plenty of data are available on the effect of water management practices on Cd and As accumulation in rice grain, nevertheless, information is lacking regarding influence of aerobic-flooded rice system (dry direct-seeding culture) on Cd and As accumulation in rice grains. Present study was carried out in pots as well as in field aiming at investigating the Cd and As accumulations in rice grain under aerobic and flooded cultivations and to determine the effectiveness of aerobic-flooded treatment in minimizing the Cd and As accumulations in rice grains.
Pot experiment was carried out in greenhouse environment at Huazhong Agricultural University, Wuhan, Hubei Province, China (30°28′N 114°20′E). Five-liter plastic pots were filled with 5.0 kg air-dried, pulverized, and well-mixed soil taken from the top 25 cm layer of a field located at University Experimental Station. Physico-chemical properties of the soil are given in Table
Physico-chemical properties of soils used for the pot and field experiments.
Parameter | Pot experiment | Field experiment | Ave. S.E.* |
---|---|---|---|
pH | 5.8 | 5.0 | 0.08 |
Organic C (g kg−1) | 13.9 | 17.7 | 0.96 |
Total |
1.39 | 1.75 | 0.035 |
Olsen P (mg kg−1) | 4.8 | 30.4 | 1.08 |
Available K (mg kg−1) | 75 | 81 | 2.8 |
Arsenic (mg kg−1) | 9.47 | 4.29 | 0.215 |
Cadmium (mg kg−1) | 0.10 | 0.12 | 0.003 |
*Ave. S.E. is the mean of standard error (SE) of soil parameters in pot and field experiments.
Each pot received 2.0 g urea and 2.0 g potassium dihydrogen phosphate one day prior to sowing followed by topdressing (1 g urea) at 30 days after sowing. Starter dose of fertilizer was promptly mixed into the soil and light irrigation was applied one day before sowing, to keep the soil moist. The treatments were Cd (2.5 mg Cd kg−1 soil as cadmium chloride) and As (5.0 mg As kg−1 soil in the form of arsenate (Na2HAsO4.7H2O, 7H2O)). The pots without Cd or As addition were considered as control.
In order to adapt to the aerobic condition, two aerobic rice varieties, Hanyou-3 and Lvhan-1, which have characteristics of drought tolerance and water saving were used. Hanyou-3 is a three-line hybrid rice variety, while Lvhan-1 is a recently developed inbred aerobic rice variety. To avoid the notion of shading, distance between pots was kept at 25 cm. Five uniform and healthy dry seeds were sown in each pot on 8th May 2012 and pots were kept saturated with water for one week after sowing to promote better stand establishment. One week after sowing, seedlings were thinned to two uniform seedlings per pot. Five pots from each treatment were kept under aerobic conditions, whereas the other five were kept under flooded conditions. The experiment was arranged in a completely randomized design with five replicates maintaining one pot per replication. The pots kept under aerobic condition were watered once after 1–3 days to keep the soil moisture tension from −15 to −25 kpa at 15 cm depth. For the flooded pots, 3–5 cm of standing water was kept in the pots throughout the experiment. Weeds were removed manually. Pesticides were sprayed 3-4 times to control insect damage. At maturity, plants were harvested and grains were threshed from panicles.
A field experiment was conducted at the experimental station located in Dajin County, Hubei Province, China (29°51′N 115°33′E). Dajin County is one of the three main rice producing areas in Hubei Province. The same two rice varieties, Hanyou-3 and Lvhan-1, were used. The soil of the experimental site was gley and its physic-chemical properties are listed in Table
Three water management treatments, namely, aerobic, aerobic-flooded, and flooded, were arranged in a randomized complete block design replicating four times with plot area of 30.0 m2 (6 m × 5 m). Aerobic and aerobic-flooded plots were dry-ploughed and harrowed during land preparation; soil was kept wet for one week after sowing to enhance better crop establishment. After one week, rainfed conditions were maintained in aerobic rice plots, whereas aerobic-flooded rice received flooding conditions from 5-leaf stage up to 2 weeks before harvest. Flooded plots were puddled and kept continuously flooded with 5–10 cm of water depth from transplanting until 2 weeks before harvest. For flooded rice, 25-day-old seedlings from wet bed nurseries were transplanted on 27th May 2012 at the rate of 3 seedlings per hill keeping the distance of 25.0 × 13.3 cm. For both aerobic and aerobic-flooded rice, dry seeds were directly sown in furrows manually with row spacing of 25 cm on 17th May 2012 using seed rate of 60 kg ha−1.
A standard fertilizer dose of 150 : 40 : 100 kg N : P : K ha−1 was applied in the form of urea, calcium superphosphate, and potassium chloride, respectively. All of the phosphorus, potassium, and 1/3rd of the N were applied as a starter basal dose (one day before sowing/transplanting), while residual N was split equally at middle tillering stage and panicle initiation stage. Pests, diseases, and weeds were intensively controlled. At maturity, plants were sampled from 0.5 m2 subplot, then panicles were manually threshed and filled spikelets were separated from unfilled spikelets.
Filled rice grains were ground to fine powder after oven drying at 70°C. Samples (0.5 g) were digested in 5 : 1 (v/v) HNO3/H2O2 (5 mL) in a microwave oven for 30 minutes (MLS 1200, Milestone, FKV, Italy) [
Data were analyzed to confirm its variability following analysis of variance using Statistix 8.0. The differences between treatments were separated using least significance difference (LSD) test at 0.05 probability level.
Results showed that aerobically grown rice had significantly higher Cd and lower As accumulation compared with rice grown under flooded conditions (Figure
The Cd and As concentration in grains of Lvhan-1 and Hanyou-3 grown in the soil with additions of Cd, As, and without Cd or As addition (ck) under aerobic and flooded cultivation in a pot experiment. The rates of Cd and As were applied at 2.5 mg Cd kg−1 of soil as cadmium chloride and 5.0 mg As kg−1 of soil as cacodylic acid sodium salt, respectively. Error bars represent standard error of mean (SE).
When Cd was applied at 2.5 mg Cd kg−1 soil, on average 421.5% of more Cd was accumulated in rice grains under aerobic conditions compared with that under flooded conditions (Figures
More Cd was accumulated in grains of rice cultivated under aerobic conditions than under flooded conditions; however, As concentration was significantly lower in aerobic rice grains than flooded rice (Figure
The Cd and As concentration in grains of Lvhan-1 and Hanyou-3 grown under three water management, aerobic, flooded, and aerobic-flooded cultivation in a field experiment. Error bars represent standard error of mean (SE).
In the pot experiment, Cd concentration in rice grain was significantly higher; however, As concentration in grains was markedly lower under aerobic condition than that under flooded condition regardless of Cd or As addition (Figure
Treatment of aerobic-flooded significantly decreased Cd and As concentrations in rice grains in comparison with the aerobic or flooded treatments, respectively (Figure
Previous research reported that Cd or As concentrations in rice might be mitigated by strategies, such as agronomic management practices, breeding, and genetic engineering [
Present study is of great concern because it justified the previous investigations in different locations under different edaphic and climatic conditions using different set of varieties in pot and field experiments. Although previous researchers have already reported different water management practices for minimizing Cd and As accumulation in rice grains, their rice establishment method was transplanting [
Rice grown aerobically obviously decreased As accumulation in rice grains but markedly increased Cd accumulation. Likewise, flooded rice has a demerit of enhanced As accumulation in rice grains. The present study suggested that a significant reduction of both Cd and As accumulations in rice grains could be achieved by aerobic-flooded cultivation of rice. However, the magnitude of decrease in Cd and As concentration in aerobic-flooded rice grains may be influenced by the duration of aerobic or flooded periods. This emphasizes the need for further research to get deeper insight regarding this area.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work is supported by the National Natural Science Foundation of China (Project no. 31371571), the National Science & Technology Pillar Program (2013BAD20B06), the Fundamental Research Funds for the Central Universities (Project no. 2013PY109), and the Open Project Program of Key Laboratory of Crop Ecophysiology and Farming System, Ministry of Agriculture (Project no. 201301).