Toshimitsu Honma

Optimal Soil Eh, pH, and Water Management for Simultaneously Minimizing Arsenic and Cadmium Concentrations in Rice Grains

Arsenic (As) and cadmium (Cd) concentrations in rice grains are a human health concern. We conducted field experiments to investigate optimal conditions of Eh and pH in soil for simultaneously decreasing As and Cd accumulation in rice. Water managements in the experiments, which included continuous flooding and intermittent irrigation with different intervals after midseason drainage, exerted striking effects on the dissolved As and Cd concentrations in soil through changes in Eh, pH, and dissolved Fe(II) concentrations in the soil. Intermittent irrigation with three-day flooding and five-day drainage was found to be effective for simultaneously decreasing the accumulation of As and Cd in grain. The grain As and Cd concentrations were, respectively, linearly related to the average dissolved As and Cd concentrations during the 3 weeks after heading. We propose a new indicator for expressing the degree to which a decrease in the dissolved As or Cd concentration is compromised by the increase in the other. For minimizing the trade-off relationship between As and Cd in rice grains in the field investigated, water management strategies should target the realization of optimal soil Eh of -73 mV and pH of 6.2 during the 3 weeks after heading.

Rapid quantification of cyanamide by ultra-high-pressure liquid chromatography in fertilizer, soil or plant samples

A rapid and simple method for determination of cyanamide in fertilizer, soil and plants has been developed. In this method, cyanamide is extracted with 2% acetic acid and the extract separated by centrifugation. It is then purified by passing through a membrane filter. The extract was derivatized with 6-aminoquinolyl-N-hydroxysuccinimidyl-carbamate and the derivatized compound separated by ultra-high-pressure liquid chromatography. It is then detected with a UV detector at 260 nm by the same method as is used for amino acid analysis. The proposed method is fast, simple and cheap and also has good selectivity and sensitivity for the determination of cyanamide in a wide range of biotic and abiotic materials.

Simultaneous decrease of arsenic and cadmium in rice (Oryza sativa L.) plants cultivated under submerged field conditions by the application of iron-bearing materials

ABSTRACT The Codex Alimentarius Commission has recently adopted maximum levels for inorganic arsenic (As; in 2014) and total cadmium (Cd; in 2006) in polished rice grains to maintain food safety and to decrease the risk to human health. As rice is a staple crop in Japan and monsoon Asian countries, reducing concentrations of As and Cd in rice is an urgent matter. In flooded conditions, Cd concentration in soil solution decreases whereas As concentration increases. Therefore, we aimed to evaluate the efficiency of iron-bearing materials to decrease As concentration in soil solution and rice (Oryza sativa L.) grain under submerged cultivation, while also considering Cd concentration. In experiments conducted in paddy fields in six regions, As concentrations in the soil solution during the cultivation period decreased in the following order: control (REF) > steel converter furnace slag (SCS) > non-crystalline iron hydroxide (FH) > zero-valent iron (ZVI). The concentrations of As in brown rice were in the same order, with ZVI achieving particularly strong reduction. Cadmium concentrations were low, probably owing to submerged cultivation conditions. Application of iron-bearing materials slightly and insignificantly reduced the yields of brown rice and straw. Application of these materials did not have a significant negative impact on the quality of rice. Our data indicate that the application of iron-bearing materials effectively reduces As concentrations in soil solution and rice grains without negative effects on yield and quality, with a particularly powerful effect of ZVI which is possibly explained by arsenic sulfide formation.

Effects of soil amendments on arsenic and cadmium uptake by rice plants (Oryza sativa L. cv. Koshihikari) under different water management practices

ABSTRACT Concentrations of arsenic (As) and cadmium (Cd) in rice grains are of public concern for human health. We conducted field experiments to investigate the effects of soil amendment applications, combined with different water management practices, on As and Cd uptake by rice plants (Oryza sativa L. cv. Koshihikari). Prolonged flooding, practiced for pre-heading 3 weeks and post-heading 3 weeks, led to elevated As concentrations in the soil solution and rice grain. Rainfed water management, in which no irrigation was practiced after midseason drainage until harvest, led to elevated Cd concentrations in the soil solution and rice grain. Application of short-range-order iron hydroxide (IO) reduced As uptake by rice plants, whereas Cd uptake was reduced by the application of converter furnace slag (CFS). However, it was difficult to simultaneously reduce the As and Cd uptake by a single countermeasure of the water management practice or the soil amendment application. Prolonged flooding combined with the application of IO, or rainfed water management with the application of CFS, were promising measures for the simultaneous reduction of As and Cd uptake by rice plants.

Relationship between Cadmium Fractions Obtained by Sequential Extraction of Soil and the Soil Properties in Contaminated and Uncontaminated Paddy Soils

The method for the sequential extraction of cadmium from soil was adapted to investigate the relationship between different chemical forms of cadmium in soils and the soil properties of Cd-contaminated and uncontaminated paddy soils. Air-dried soil samples from each field site were sequentially fractionated into five forms: exchangeable Cd, inorganically bound Cd, organically bound Cd, oxide-occluded fraction, and residual Cd. The average and range of soil properties such as pH, total C, total N, CEC, exchangeable Ca, Mg, K, base saturation, available phosphate, particle size distribution, free iron oxide, oxalate extractable Al, and Fe were somewhat similar between uncontaminated and contaminated soils. The average total Cd in uncontaminated and contaminated soils was 0.26 and 0.65 mg kg−1, respectively. The proportions of soil Cd fractions did not differ between the uncontaminated and contaminated soils, although the Cd concentration of several fractions in contaminated soils was statistically higher than those in uncontaminated soils except for residual fraction. The proportion of exchangeable Cd was correlated with the CEC and phosphate absorption coefficient in contaminated soil but not in uncontaminated soil. Thus, soil properties appear to affect the proportions of soil Cd fractions in contaminated soil and should be considered when evaluating soil Cd mobility.

Effect of application of molasses to paddy soil on the concentration of cadmium and arsenic in rice grain

Molasses, a liquefied by-product of sugar production from sugarcane, is one of the most easily decomposable organic substances. We investigated the effect of the application of diluted molasses in paddy rice (Oryza sativa L.) fields on cadmium (Cd) and arsenic (As) absorption in rice grains as a measure of the shortage of irrigation water or rainfall. The application of diluted molasses on to the top of the soil surface at one week before or after heading led to a decrease in the redox potential of the soil. The Cd concentration in the rice grains was significantly decreased with the application of molasses (2000 kg ha−1) at heading or one week after heading. However, early application of molasses (one week before heading) did not decrease the Cd concentration in the rice grains with downsizing of grain particle size. Meanwhile, As concentration in the rice grains did not change with early application at one week before heading, while it increased with application of molasses at heading or one week after heading. This method is useful especially for decreasing Cd concentration in rice grains in a region under lack of irrigation due to drought.

Air-Filled Porosity as a Key to Reducing Dissolved Arsenic and Cadmium Concentrations in Paddy Soils

Simultaneous suppression of rice ( L.) uptake of As and Cd is challenging because these toxic elements are immobilized under contrasting redox conditions. Given the notion that oxygen diffusion governs redox conditions in temporarily drained paddy soil, we assume that the key to simultaneous suppression of dissolved As and Cd concentrations is air-filled porosity (AFP) of soil. The objectives of this study were to reveal relationships between AFP and dissolved As and Cd concentrations in paddy soils and to identify the optimum water management, in terms of AFP, for simultaneous reduction of As and Cd. Dissolved As and Cd concentrations were determined in soil cores collected at different depths and times during rice growing seasons. Dissolved As concentrations were appreciable (>3 µg L) only when AFP was below a threshold value of 0.04 to 0.10 m m, suggesting that dissolved As was rapidly immobilized once AFP exceeded the threshold value on drainage. Dissolved Cd concentrations were roughly proportional to AFP, with higher concentrations associated with lower soil pH. Although dissolved As concentrations tended to be low in soil samples with high dissolved Cd concentrations and vice versa, both concentrations were low when AFP was slightly above the threshold value for As immobilization. The results suggest that dissolved As and Cd can be simultaneously kept at low levels by appropriate water management practices that produce AFP slightly above the threshold value for As immobilization.

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.