Strategies to decrease arsenic and cadmium in rice grains by rice residue management and flooding

Abstract

Rice (Oryza sativa L.), a staple food for half of the world’s human population, is a potential source of arsenic (As) and cadmium (Cd). Elevated As and Cd levels in soil and/or irrigation water can affect growth and development of plants, decreasing rice yield and quality. Moreover, contaminated rice grain may threaten human health worldwide. It is therefore imperative to investigate the effects of different strategies to develop new techniques for minimizing the risk for As and Cd transfer into rice grains. Water management has been introduced to decrease As and Cd accumulation in rice grains. However, As and Cd have opposite responses to water management and may still accumulate in rice grains. In contrast, Si management techniques may prove to be highly effective in combating the problem of excessive accumulation of As and Cd in rice grains. Therefore, additional investigations are needed to develop simple and cost-effective methods to decrease As and Cd in rice grains. In this study two experiments were conducted where we (1) developed a technique to sample and concentrate intact Fe plaque minerals to understand the impact of increasing solution-phase Si on the quantity and mineral composition of Fe plaque and controls on shoot transfer of As, and (2) investigated the effects of different water management strategies on As and Cd uptake by rice in well-weathered soil amended with Si-rich rice residues compared to non-amended controls. In the first study, the technique reported was effective at concentrating As-bearing Fe plaque from root systems. Our data indicate that strategies that promote more Fe and As in plaque may still transfer As to shoots but strategies that promote more Si in plaque decrease both shoot As and grain As. In the second experiment, our data indicate that utilizing Si-rich rice amendments is a promising strategy to decrease heavy metals contamination in rice under different water managements without negatively affecting rice yield, and grain Zn or Fe. We suggest that soilincorporation of fresh husk (FH) and rice husk ash (RHA) may be a promising strategy depending on the choice of amendment in view of the fact that the amendments had different behavior over different conditions. Further research is needed to evaluate the residual effects of the amendments as well to confirm both risks and benefits for different management strategies.