The effect of water flooding at grain-filling stage on the uptake of heavy metal(loid)s by rice in slightly alkaline paddy soil

Continuous flooding has been widely used in paddy field to decrease the accumulation of heavy metal(loid)s by rice due to their decreased solubility and bioavailability of most heavy metal(loid)s under water flooding condition. A field experiment with six drainage treatments during grain-filling stage was performed to investigate the influence of different water flooding conditions on availability of cadmium (Cd), lead (Pb), chromium (Cr), arsenic (As), and mercury (Hg) and their accumulation by rice in alkaline paddy soil. The results showed at the end of experiment, the availability of Cd and Pb in soil with continuous flooding for above 25 days after full heading significantly decreased compared with draining immediately after full heading of rice. But some increase of the As availability in soil with different water regimes was observed at 15 days after beginning of experiment where the As availability was below its detection limit. Meanwhile, the concentrations of Cd and Cr both in rice grains and straws were decreased evidently with prolonged flooding at rice filling stage. The concentrations of Pb and Hg in grain were all below the detection limit, but increased in straw with draining 15 days later compared with draining immediately after full heading of rice. However, water regimes during grain-filling stage had little effect on As uptake by rice in alkaline paddy soil. Moreover, this study also discovered that organic matter may played a critical role in controlling availability of heavy metal(loid)s in alkaline soil. This work demonstrated that at alkaline paddy soil, maintaining water flooding until 5 days before harvest in rice grain-filling stage was an effective method to improve rice safety without decreasing yields in paddy field polluted with Cd, Cr, and As, but careful consideration is required for Pb and Hg.