A small extent of seawater intrusion significantly enhanced Cd uptake by rice in coastal paddy fields

Paddy fields located around estuaries suffer from seawater intrusion, and how and to what extent salinity levels influence Cd accumulation in rice grains is still unclear. Pot experiments were carried out by cultivating rice under alternating flooding and drainage conditions with different salinity levels (0.2‰, 0.6‰ and 1.8‰). The Cd availability was greatly enhanced at 1.8‰ salinity due to the competition for binding sites by cations and the formation of Cd complexation with anions, which also contributed to Cd uptake by rice roots. The soil Cd fractions were investigated and found that the Cd availability significantly decreased during flooding stage, while it rapidly increased after soil drainage. During drainage stage, Cd availability was greatly enhanced at 1.8‰ salinity mainly attributed to the formation of CdCl_n^2-n. The kinetic model was established to quantitatively evaluate Cd transformation, and it found that the release of Cd from organic matter and Fe-Mn oxides was greatly enhanced at 1.8‰ salinity. The results of pot experiments showed that there was a significant increase in Cd content in rice roots and grains in the treatment of 1.8‰ salinity, because the increasing salinity induced an increase in Cd availability and upregulation of key genes regulating Cd uptake in rice roots. Our findings elucidated the key mechanisms by which high salinity enhanced Cd accumulation in rice grains, and more attention should be given to the food safety of rice cultivated around estuaries.