Masaharu Murakami

Heavy metal contamination of agricultural soil and countermeasures in Japan

Many heavy metals exist in minute amounts in natural agricultural soil. However, when their amounts exceed a certain level due to pollutants brought from outside, soil contamination occurs and agricultural products become contaminated. There have been many cases in Japan of heavy metal contamination originating from old mines and smelters, and soil contamination of agricultural land has become a social issue. In particular, cadmium (Cd) is one of the most harmful heavy metals. If agricultural products absorb an excessive amount of Cd, they may adversely affect people’s health, and therefore allowable concentrations are regulated by law. If agricultural land has become contaminated with Cd, measures for minimizing the absorption of Cd by agricultural crops are necessary; these include: (1) soil dressing, (2) water management (paddy field), (3) chemical cleaning of soil, (4) phytoextraction, and (5) use of different varieties and rootstock. Other heavy metals such as arsenic, lead, copper, zinc, and mercury also sometimes cause contamination of agricultural soil.

Genotypic Variation in Shoot Cadmium Concentration in Rice and Soybean in Soils with Different Levels of Cadmium Contamination

A pot experiment was conducted to investigate whether the shoot cadmium (Cd) concentration in 11 rice and 10 soybean cultivars varied among 4 soils with different levels of Cd contamination. Significant differences in shoot Cd concentration were found among rice or soybean cultivars grown in the 4 soils. The ranking of the rice cultivars for the shoot Cd concentration varied considerably among the soils. On the other hand, the soybean cultivars were ranked similarly in terms of shoot Cd concentration in the 4 soils. Significant and positive correlations were found between the Cd and Zn concentrations and between the Cd and Mn concentrations in the shoot of rice cultivars, when they were grown in 2 soils with relatively moderate levels of Cd contamination. The shoot Cd concentration in the soybean cultivars, however, was not correlated with the concentrations determined for any of the metals (Zn, Mn, Cu, and Fe) across the 4 soils. Significant and positive correlations between the concentrations of Cd in younger shoots and mature seeds were detected among the soybean cultivars in 2 soils used, unlike among the rice cultivars, indicating that it may be difficult to evaluate the genotypic variation in seed Cd concentration using relatively younger shoots in the case of rice. These results revealed that genotypic differences in shoot Cd concentration in rice or soybean are variable or invariable among soils, respectively.

Is Brassica juncea a suitable plant for phytoremediation of cadmium in soils with moderately low cadmium contamination? – Possibility of using other plant species for Cd-phytoextraction

Abstract We evaluated the ability of Brassica juncea (L.), which has already been recognized as a plant suitable for metal phytoremediation, and of several other plant species (maize, rice and sugar beet) to extract cadmium (Cd) from soils with moderately low levels of Cd contamination. Two of the 56 cultivars of B. juncea were preliminarily screened as high-Cd accumulators using a hydroponic culture solution containing a high level of external Cd (1 mg L−1). Thereafter, 7 cultivars within 4 plant species (maize, B. juncea[2 cultivars], rice [3 cultivars with different subspecies] and sugar beet) were grown in a hydroponic culture solution containing a low Cd level (0.05 mg Cd L−1) or in pots filled with 2 types of contaminated soils containing moderately low Cd levels under upland conditions. The 2 soils consisted of a Fluvisol and an Andosol and contained 1.82 and 4.01 mg Cd kg−1 on a dry soil weight basis, respectively, determined using 0.1 mol L−1 HCl-extraction. The results indicated that B. juncea was less able to accumulate Cd in shoots compared with hydroponically cultured rice and sugar beet, and was even less effective when grown in soil culture. Rice and sugar beet displayed a higher accumulation not only of Cd but also of other heavy metals (Cu, Fe, Mn and Zn) in their shoots than B. juncea when they were grown in the two Cd-contaminated soils. Maize displayed the lowest metal accumulation among the plant species tested. Growing the rice cultivars in both soil types led to the most significant decrease in soil Cd concentration determined using extraction with 0.1 mol L−1 HCl. In contrast, we did not observe any significant decrease in soil Cd concentration in B. juncea. Sequential Cd extraction of soil revealed that rice was more effective than B. juncea in phytoextracting Cd from less-soluble fractions in soils. Based on the plant and soil analyses, it was suggested that B. juncea does not offer much promise for phytoextraction of Cd from soils with relatively low contamination, and that rice may be an eligible plant for metal phytoremediation of such soils.

Potential for phytoextraction of copper, lead, and zinc by rice (Oryza sativa L.), soybean (Glycine max [L.] Merr.), and maize (Zea mays L.)

Phytoextraction by hyperaccumulators has been proposed for decreasing toxic-metal concentrations of contaminated soils. However, hyperaccumulators have several shortcomings to introduce these species into Asian Monsoons agricultural fields contaminated with low to moderate toxic-metals. To evaluate the phytoextraction potential, maize (Gold Dent), soybean (Enrei and Suzuyutaka), and rice (Nipponbare and Milyang 23) were pot-grown under aerobic soil conditions for 60d on the Andosol or Fluvisol with low to moderate copper (Cu), lead (Pb), and zinc (Zn) contamination. After 2 months cultivation, the Gold Dent maize and Milyang 23 rice shoots took up 20.2-29.5% and 18.5-20.2% of the 0.1molL(-1) HCl-extractable Cu, 10.0-37.3% and 8.5-34.3% of the DTPA-extractable Cu, and 2.4-6.5% and 2.1-5.9% of the total Cu, respectively, in the two soils. Suzuyutaka soybean shoot took up 23.0-29.4% of the 0.1molL(-1) HCl-extractable Zn, 35.1-52.6% of the DTPA-extractable Zn, and 3.8-5.3% of the total Zn in the two soils. Therefore, there is a great potential for Cu phytoextraction by the Gold Dent maize and the Milyang 23 rice and for Zn phytoextraction by the Suzuyutaka soybean from paddy soils with low to moderate contamination under aerobic soil conditions.

Chapter Four - Cadmium Contamination and Its Risk Management in Rice Ecosystems

Cadmium (Cd) has been identified as one of the major heavy metals reaching the food chain through various geogenic and anthropogenic activities. In many East and South Asian countries including Japan, Bangladesh, Indonesia, and Korea, Cd accumulation in rice (Oryza sativa L.) ecosystems and its subsequent transfer to the human food chain is a major environmental issue. Rice soils in these countries have been affected by Cd accumulation derived from fertilizer and manure application, mine tailings, and refining plants. Excessive intake of Cd into the human body is detrimental to human health, causing serious illnesses such as itai-itai disease. To ensure the safety of foods, the concentrations of Cd in staple crops should be below a standard value; this applies particularly to rice because 34–50% of the Cd intake by people in many Asian countries has been derived from rice. Therefore, development of remediation methods for Cd-contaminated rice soils has become an urgent task to ensure food safety. This chapter provides an overview of the various sources of Cd in rice ecosystems and the biogeochemical processes that regulate Cd bioavailability to organisms, including microbes, plants, animals, and humans. Because of the complexity involved in dealing with Cd in rice ecosystems, exacerbated by the Cd source, site characteristics, and the nature of water management strategies, we have attempted to describe an “integrated” approach that employs a combination of remediation technologies, with the aim of securing methods that are economically and technologically viable.

Reduction of cadmium uptake in spinach (Spinacia oleracea L.) by soil amendment with animal waste compost.

A field experiment was conducted to evaluate the efficacy of animal waste compost (AWC) in reducing Cd uptake by spinach (Spinacia oleracea L.). Spinach was grown in a field that had been treated by having cattle, swine, or poultry waste compost incorporated into the soil before each crop throughout 4 years of rotational vegetable production. Cadmium concentration was 34-38% lower in spinach harvested from the AWC-treated soils than in the chemical fertilizer-treated soil. Although the repeated application of swine and poultry compost caused significant P accumulation in the cropped soils, that of cattle compost did not. These results indicate that cattle compost with high affinity for Cd and low P content should be the preferred soil amendment when used to reduce Cd uptake by spinach.

Practical phytoextraction in cadmium-polluted paddy fields using a high cadmium accumulating rice plant cultured by early drainage of irrigation water

Abstract Some indica rice varieties are potential phytoextractors for paddy fields polluted with Cd because of their high biomass and because they can accumulate Cd to moderate levels in their shoots. To establish a practical phytoextraction system, phytoextraction using two indica rice cultivars (MORETSU and IR-8) was carried out in a paddy field polluted with moderate Cd levels (2.91 and 2.52 mg kg−1, respectively). The Cd concentration and Cd uptake of MORETSU increased when irrigation water was drained at the maximum tillering stage, and the paddy soil was under oxidative conditions until harvesting. The Cd uptake of MORETSU and IR-8 increased and reached 516 and 657 g ha−1, respectively, at the beginning of October. After phytoextraction using these high Cd accumulating rice varieties for 2 years, the Cd concentration in the paddy field decreased by 18% compared with the initial Cd concentration. The Cd concentration in the rice grains of a japonica ordinary rice variety (HINOHIKARI) subsequently grown on the field after the phytoextraction was lower than the concentration in rice grown on a non-phytoextracted field. These results suggest that phytoextraction using high Cd accumulating rice varieties with early drainage of irrigation water is a practical remediation system for moderate Cd polluted paddy fields in southwest Japan.

Evaluation of Extraction Methods for Plant‐Available Soil Cadmium to Wheat by Several Extraction Methods in Cadmium‐Polluted Paddy Field

To develop a new method for the extraction of plant-available cadmium (Cd), the correlation between the Cd content of polluted soil extracted with several extractants and the Cd content of wheat plants (variety “A” Triticum aestivum L.) was examined. Among the HCI concentrations of soil extractants tested, the content of 0.025 mol L−1 HCI (HCl0.025)-extractable Cd of soil showed a significant correlation with the Cd content of wheat grain and Cd uptake by shoot. The correlation between the soil Cd content extracted with 1 mol L−1 NH4Cl and the Cd content of wheat grain was nearly the same as that in the case of HCI0.025. In contrast, other reagents such as MgCl2, diethylenetriaminepentaacetate (DTPA) and tetra-sodium pyrophosphate (Na4P2O7) could not give a good correlation between the extractable Cd content of soil and the Cd content of wheat grain. Therefore, it was considered that HCl0.025-extractable Cd is a suitable indicator of the content of plant-available Cd of soil to wheat. This extraction method can be applied to weakly acid Grey Lowland soil. Using the equation for the relation between the content of HCl0.025-extractable Cd of soil and the Cd content of grain of the wheat variety “A”, the Cd content of grain of other 11 wheat varieties was evaluated. The wheat variety “A” showed the lowest absorption of Cd. The Cd content of variety “B” grain was located on the regression line of that of variety “A”, suggesting that both varieties exhibited the lowest ability to absorb Cd among the varieties tested. Grain yields of both “A” and “B” varieties were reasonably high. On the other hand, other varieties showed a substantially high ability to absorb soil Cd compared with the varieties “A” and “B”. The new method proposed for the determination of the content of plantavailable Cd proved to be suitable for the evaluation of plant-available Cd of soil and also for the screening of wheat varieties with a high or low Cd accumulation capacity in grain.

Breeding of a practical rice line ‘TJTT8’ for phytoextraction of cadmium contamination in paddy fields

ABSTRACT Previously, we showed that qCdp7, an allele identified in the high-Cd-accumulating indica rice variety ‘Jarjan,’ is associated with effective phytoextraction of Cd from paddy soils. However, ‘Jarjan’ may not be practical for phytoextraction because it is susceptible to seed shattering and culm lodging, which are unfavorable traits for mechanical rice harvesting. In this study, to develop a practical rice line for phytoextraction, we introduced the qCdp7 allele into ‘Tachisugata,’ a rice variety with a nonshattering habit and lodging resistance, to produce a new high-Cd-accumulating rice line designated ‘TJTT8.’ This line inherited high-Cd accumulation and brown pericarps from ‘Jarjan’ and a nonshattering habit and lodging resistance from ‘Tachisugata;’ all of these traits are necessary for rice intended for Cd phytoextraction in Japan. Backcross inbred lines (BILs) were produced by two backcrosses to ‘Tachisugata’ after a cross between ‘Jarjan’ and ‘Tachisugata.’ ‘TJTT8’ was selected from the BILs by means of marker-assisted selection and phenotypic evaluation. When ‘TJTT8,’ the parents, and ‘Cho-ko-koku’ which is a high-Cd-accumulating indica variety were cultivated in Cd-contaminated paddy fields in four locations in Japan, ‘TJTT8’ exhibited lodging resistance and shattering resistance that were higher than those of ‘Jarjan’ and ‘Cho-ko-koku’ and equivalent to those of ‘Tachisugata.’ ‘TJTT8’ accumulated Cd in the aerial parts of the plants at concentrations ranging from 6.5 to 22.7 mg m−2: it showed significantly higher Cd accumulation than ‘Tachisugata’ and was equivalent to ‘Jarjan’ and slightly superior to ‘Cho-ko-koku.’ Soil Cd concentration was estimated to have been reduced by 8.7–33.6% based on the amount of Cd accumulation in the aerial parts of the plants. Thus, we succeeded in using the qCdp7 allele to produce a practical rice line for Cd phytoextraction by improving several agronomic traits for compatibility with Japanese cultivation systems.