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.

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.

QTL Pyramiding and Its Use in Breeding for Increasing the Phytoextraction Efficiency of Soil Cd via High-Cd-Accumulating Rice.

Phytoextraction by high-Cd-accumulating rice lacking a functional OsHMA3 allele is promising for Cd removal from paddy soils. To increase rice Cd extraction efficiency, we developed a new high-Cd variety, TJN25-11. For this, we pyramided a nonfunctional OsHMA3 allele from a high-Cd variety, Jarjan, and two QTLs for increased shoot Cd concentrations, which were discovered in a mapping population derived from a high-Cd variety, Nepal 555, and a low-Cd variety, Tachisugata. In two Cd-contaminated paddy fields under drained aerobic soil conditions, TJN25-11 presented significantly higher Cd concentrations in the straw and panicles than the OsHMA3-deficient varieties TJTT8 and Cho-ko-koku. Among the varieties, TJN25-11 had a relatively high shoot biomass, resulting in the highest Cd accumulation in the shoots. The soil Cd decreased by approximately 20% after TJN25-11 growth. The amount of Cd that accumulated in the TJN25-11 aerial parts was much greater than the amount of Cd that decreased in the topsoil, suggesting that Cd was absorbed from deeper soil layers. Thus, we revealed the effects of QTL pyramiding on shoot Cd accumulation and Cd phytoextraction efficiency. Since TJN25-11 has favorable agronomic traits for compatibility with Japanese cultivation systems, this variety could be useful for Cd phytoextraction in Cd-contaminated paddy fields.

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.