Temporal And Spatial Dynamics Of As In The Soil-Rice System Under Three Water Regimes

Arsenic (As) exposure from consumption of rice can be substantial, particularly for the population on subsistence rice diet in South Asia for whom rice is the staple food. Paddy rice has a much enhanced As accumulation compared with other cereal crops. When drinking water levels of arsenic are at the World Health Organization's 10mg/l limit, 0.05mg/kg arsenic in rice contributes 60 % of dietary arsenic exposure. So practical measures are urgently needed to decrease As transfer from soil to grain. We investigated the tempral and spatial dynamics of As in the soil solution, quartz sand, different part of rice plant under different water regimes using a soil-quartz combined culture system. We also analysed the As speciation in the rice shoot, straw and grain under flooded, flooded-unflooded and unflooded conditions.In general, the concentrations of As in the soil solution, quartz sand, root plaque and any components of the rice plant were all following the order of flooded water regime>flooded-unflooded water regime>unflooded water regime(P<0.001); Fe also had the same trend (P<0.001) as arsenic. Dynamics of As and Fe among the whole growing stages showed a characteristic of higher concentrations in the filling grain stage under both flooded and flooded-unflooded conditions, while the two elements achieved their general max-values in the tillering stage under unflooded condition.As and Fe showed perfect corelationship in all parts of the soil-rice plant system. The details were as follow: The concentrations of total As and Fe in the soil solution showed significantly positive correlation in flooded treatment (r=0.75, P<0.001), while the concentrations of As3+ and Fe2+ got their max-correlative coefficient under flooded-unflooded condition(r=0.76, P<0.001), but there were no correlation between As and Fe under unflooded water regime.When in the quartz sand, the concentrations of As and Fe were positively correlated only in flooded treatment (r=0.45, P<0.05). As-Fe both showed significantly positive correlation in the root plaque and the rice roots under all three water regimes, and reached their max-correlative coefficient of 0.79 under flooded condition.Arsenic concentrations were distributed in different components of mature rice with the ranking of Root> Straw or Shoot>Husk>Grain. In addition, there were significant differences in arsenic translocation factor among water regimes. Concentrations of As in the rice straw or shoot significantly increased with the growing periods ( P<0.001), but there was a reverse trend of As in rice grain.As5+ and Fe3+ were the predominate forms under unflooded and flooded-unflooded water regimes, while As3+ and Fe2+ dominated especially in the middle and bottom layer of the soil under the third water regime, except for Fe2+ in the maturity stage. As species were analyzed in both rice straw (shoot) and grains.The result showed that As (â…¢) were predominate form in all the samples, followed by As (â…¤) in the straw and As(â…¤) , dimethylarsinic acid (DMA) in the rice grain, and no organic As detected in the straw. Monomethylarsonic acid (MMA) was only detected in few grain samples, and its concentration was absolutely low.