Up and Down the Paddy

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Science  11 Dec 2009:
Vol. 326, Issue 5959, pp. 1461
DOI: 10.1126/science.326.5959.1461-a

When staple crops such as rice are grown in soils naturally rich in toxic metals, containing the risk of sickness or even death from exposure becomes a top priority. In Asia, with large and often growing populations to feed, simply abandoning land with contaminated soils is not a viable option; water management strategies are instead employed to decrease the uptake of problematic elements while still promoting abundant rice yields. In rice uptake experiments using Japanese soils, Arao et al. found that two common water management strategies, flooding and aerobic treatment, have opposing effects on arsenic (As) and cadmium (Cd) levels in rice. In flooding conditions, significant amounts of As were incorporated into the rice grain and straw, whereas Cd primarily remained in the soil. Conversely, aerobic treatment increased Cd content in rice but caused very little As uptake. The variable effect is explained by changes in mineral solubility with changing soil redox potential: The oxidizing conditions of aerobic treatment convert insoluble Cd sulfide into the mobile sulfate, whereas reducing conditions from flooding increase arsenite mineral dissolution and hence As mobility. However, redox potential alone does not explain why the organic fraction of As in the soil is translocated more easily. Biological factors such as methyl-transferase activity in rice may be responsible for that observation.

Environ. Sci. Technol. 10.1021/es9022738 (2009).

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