施用矽酸鈣於砷污染稻田對不同品種水稻植體中砷累積及物種之影響

Abstract

砷被國際癌症研究中心列為一級致癌物，其可對人體健康造成嚴重的危害。一般而言，食用稻米為亞洲人暴露無機砷的主要途徑之一，因砷在厭氣環境下以移動性高的三價砷為主，導致稻米的砷累積高於其他穀類作物。由於三價砷與矽酸根的化學特性相似，近年來已有許多研究發現矽施用可減緩砷對水稻的毒害並且降低其對砷的吸收。相反地，亦有研究指出土壤中添加矽會使土壤溶液中砷濃度提升，進而促進砷於植體中累積。然而，至今有關矽酸根對水稻吸收砷的田間試驗研究仍顯不足，為評估該方法降低稻穀累積的可行性，本篇研究於臺灣兩處 (民雄及關渡) 土壤含砷之稻田種植兩個稉稻品種 (臺梗9號、臺南11號) 及兩個秈稻品種 (臺中秈10號、臺中秈糯2號)，並進行四種矽酸鈣處理 (0、750、1500、3000 kg SiO2 ha-1)，以探討其對水稻生長及稻穀無機砷累積的影響。結果顯示，施用矽酸鈣於砷 (<60 mg As kg-1) 及有效性矽含量 (<100 mg SiO2 kg-1) 較低的民雄土壤可降低稻稈及穀粒中砷濃度，同時對植株生長及產量無產生負面影響；而在砷 (>60 mg As kg-1) 及有效性矽含量 (>100 mg SiO2 kg-1) 較高的關渡土壤，植體及穀粒中砷濃度雖然可因矽酸鈣處理降低，但可見其植株生長及產量受明顯抑制。另外，糙米砷物種結果顯示，三價砷為主要之砷物種 (75-92 %)，並於矽酸鈣施用後所有試驗品種糙米無機砷濃度皆低於於衛福部訂定之標準0.35 mg kg-1。綜上所述，於砷污染稻田施用矽酸鈣可降低水稻砷累積，其效果隨矽添加量、土壤性質、水稻品種不同而異，因此，於砷污染稻田施用矽肥仍需謹慎評估。

The accumulation of arsenic (As) in rice grain had posed a threat to food safety and human health via the daily intake of rice by Asian people, because of the high bioavailability of arsenite (iAsIII) in paddy soils, thus caused higher accumulation of As in rice grain rather than other cereal crops. Previous studies indicated that increasing concentration of silicic acid (Si) in soil solutions may alleviate the negative impacts of As on rice plants. In contrast, some studies also found that Si application can lead to an increase in As uptake by rice via competitive adsorption between iAsIII and Si on the soil surface. However, field experiment for investigating the dynamic of Si and As in soil-paddy rice system is still scarce. To evaluate the feasibility of Si application on reducing As accumulation in rice grains, four rice (Oryza sativa L.) cultivars including TK9, TN11 (japonica) and TCS10, TCSW2 (indica) commonly planted in Taiwan were used in this study, which were grown in two As-contaminated paddy fields located at Minsyong (Ms) and Guandu Plain (Gd), Taiwan, respectively. Based on recommended application rate of Si (750 kg SiO2 ha-1), there are four calcium silicate application rates (0, 750, 1500, and 3000 kg SiO2 ha-1) used in this study. The results show that Si application could decrease the concentrations of As in straw and brown rice in Ms fields, and no negative effects on plant growth was observed. For Gd soils, the As concentrations in rice plants could be reduced by Si application, but the growth inhibition were also observed. In addition, it found that iAsIII is the predominant As species in brown rice (75-92 %), and the concentrations of inorganic As in brown rice were lower than the permission level (0.35 mg kg-1) under Si treatments. Based on the above results, it suggests the Si application can reduce inorganic As accumulation in brown rice, and the extents of decrease were controlled by the Si application rate, soil properties, and rice cultivars. Therefore, it needs to consider these factors when Si fertilizer is applied to As-contaminated paddy soils.