Effects of silica-rich rice husk ash on soil Cd activity and Cd accumulation in different varieties of rice

Abstract: Root-applied silicon(Si) fertilizer can reduce cadmium(Cd) accumulation in rice by elevating soil pH, decreasing available Cd, down-regulating the expression of Cd transporter genes as well as forming Si-hemicellulose matrix Cd complex within the cell wall. However, current Si fertilizers for field application have a generally high price with low availability. Therefore, it is necessary to explore an alternative Si source that should be affordable and readily available to most farmers and can enhance soil available Si efficiently. Rice hull ash (RHA) is a low-cost Si-rich material prepared from rice husk. With significantly low crystallinity relative to the original husk, RHA can release sustainably a high level of Si in soil porewater. It has been employed successfully for inhibiting rice arsenic (As) accumulation, decreasing inorganic As in white rice from 0.36 mg/kg to 0.17 mg/kg. The control of RHA on rice Cd accumulation is yet to be clarified. In this study, the effect of RHA on Cd accumulation in rice grains was investigated with pot experiments. RHA was added at four dosages of 0, 0.2%, 1% and 4% (w/w), respectively. A low Cd- accumulating variety (Xiangwanxian）and a high Cd-accumulating variety (Yuzhenxiang) were employed. To identify chemical changes in the rice rhizosphere through its whole growing period, porewater was extracted weekly for analysis of Cd, Fe and Si concentrations, and soil pH and Eh values were measured in situ. Following harvest, the availability of Si and Cd in rhizosphere soil, and Cd concentrations in different soil fractions and rice tissues were determined, as well as Zn, Si, Fe, and Mn concentrations in white roots and straw. The results showed that the application of RHA increased soil Cd availability, due mainly to the enhanced Fe dissolution. With RHA applied, porewater Fe levels increased by up to 449 fold in Xiangwanxian and 386 fold in Yuzhenxiang, resulting in a substantial transformation of Fe/Mn oxides- bound Cd to an easily exchangeable fraction with much elevated Cd bioavailability. The effect of RHA on Cd accumulation in rice differed between varieties. It inhibited Cd accumulation and translocation in low Cd-accumulating variety (Xiangwanxian), reducing Cd concentration in straw by 25%-44%. Correspondingly, translocation from root to straw, from internode I to node I, and from node I to flag leaf decreased consistently by 42%-48%, 2%-55%, and 30%-70%, respectively. By contrast, RHA had no inhibitory effect on the high Cd-accumulating variety of Yuzhenxiang. The results of this work unraveled the differential effect of RHA on rice Cd accumulation and provided a cost-effective Si-rich material for mitigating Cd uptake by rice.