Soil Amendments and Flooding Cycle Impact Cadmium Bioavailability in Paddy Soil


Chronic cadmium (Cd) exposure is directly associated with osteoporosis, renal dysfunction, and various forms of cancers. In recent years, human activities such as fuel combustion, mining, and industrial manufacturing have caused Cd contamination in soils. Today, consumption of plant-derived foods accounts for approximately 90% of human Cd exposure for the general non-smoking population, with rice being the major dietary source.

Paddy rice is grown under episodic cycles of flooding and draining. Typically, paddy water is drained during the later phase of grain filling, resulting in the release of insoluble or sorbed Cd into the soil solution. An estimated 80% of Cd accumulation in rice grain occurs during the grain filling period when paddy water is drained.

It is important to understand how Cd is released from the soil solid phase during the drainage period. The effect of the flooding-draining cycles on soil Cd chemical speciation has been previously studied, but a direct correlation between Cd chemical speciation and Cd release kinetics has not been developed. The latter is particularly important for assessing the availability of Cd to uptake by rice plants.

Researchers used X-ray absorption spectroscopy on Stanford Synchrotron Radiation Lightsource beamline BL7-3 along with stirred-flow kinetics to investigate the effects of flooding-draining cycles, and CaCO3 and CaSO4 soil amendments, on Cd speciation and release kinetics from a Cd-spiked paddy soil.

Extended X-ray absorption fine structure (EXAFS) analysis showed that Cd was predominantly bound to non-iron clay minerals (e.g., Cd-kaolinite, Cd-illite, and Cd-montmorillonite) in the air-dried soil and 1- or 7-day flooded samples. After prolonged flooding (30 and 120 days), Cd-iron mineral complexes became the predominant species.

Stirred-flow kinetic analysis showed that both prolonged flooding and the amendments with CaCO3 and CaSO4 decreased the maximum amount and the rate coefficient of Cd release. However, the effect of prolonged flooding was reversed after a short period of draining, indicating that although Cd was immobilized during flooding, it became mobile rapidly after the soil was drained, possibly due to pH decrease and rapid oxidation of CdS.

The effects of the amendments on Cd uptake in rice plants were tested in a pot experiment using the same paddy soil without Cd spiking. Data show that amendment with CaCO3, and to a lesser extent CaSO4, decreased the Cd accumulation in two rice cultivars. The combination of CaCO3 amendment and a low-Cd-accumulating cultivar was effective at limiting grain Cd concentration to a low, toxicologically-acceptable level.

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Yan, J., et al. 2020. “Cadmium Speciation and Release Kinetics in a Paddy Soil as Affected by Soil Amendments and Flooding-Draining Cycle,” Environmental Pollution 268B, 115944. [DOI:10.1016/j.envpol.2020.115944]