The effects of phosphorous application and soil water management on total arsenic concentration and arsenic species distribution in brown rice grown in two arsenic-contaminated soils

食米已成為人類攝入砷的途徑之一。水稻對磷的吸收與砷的氧化狀態優勢物種(As(V))使用相同的傳輸途徑，兩者會在根部產生競爭吸收，進而降低植體內的砷濃度。然而，水稻長期生長在還原水分境況，在此狀態砷並非以 As(V)為主要物種。本研究利用不同田間水分管理，使水稻種植在相對較氧化的水飽和狀態，嘗試以添加不同磷肥用量抑制水稻吸收砷。研究土壤選用二林系及平鎮系之表土，其砷濃度皆為10 mg kg-1 (Eh10及Pc10)，並以人工添加砷至20及40 mg kg-1(Eh20 及 Pc40)。各處理土壤分別施用三種磷肥處理(150, 525, 900 kg P2O5 ha-1 )以及兩種水分處理(浸水、飽和)，浸水處理為水稻生長週期全程表面浸水 5~6 cm，水飽和處理為稻米移植後 15 天內浸水，之後維持土壤孔隙水飽和狀態(操作型定義為土表產生局部積水之水分狀態，無土壤表層之湛水位)。 研究結果顯示水飽和處理能使土壤氧化還原電位上升，抑制鐵還原溶解，使砷被吸附在鐵氧化物上，降低孔隙水中砷濃度。而兩種水分處理下，施用磷肥對孔隙水砷濃度沒有顯著影響。二林土壤在水分處理及磷肥處理下，稻藳及糙米的砷濃度均無顯著差異。平鎮土壤以水飽和處理可顯著降低稻藳及糙米中砷濃度，磷肥處理結果僅在飽和處理下施用 900 kg P2O5 ha-1，可使平鎮砷污染土壤(Pc40)稻藳砷濃度顯著下降(p<0.05)，糙米砷濃度雖有下降趨勢，但未達統計上顯著差異。糙米總砷濃度與雙甲基砷(DMA)濃度呈良好的正相關(R2=0.97)，不論糙米總砷濃度為何，其三價砷(As(III))濃度大約維持在 0.2 mg kg-1。雖然種植在受污染的平鎮土壤(Pc40)糙米砷濃度超過 1 mg kg-1，但大多以毒性較低的有機砷(DMA)存在，並非致癌性的無機砷(As(III))，在稻米食用之健康風險仍是較低的。Previous studies have reported that human beings would expose to arsenic (As) by consumption of rice (Oryza sativa L.). Rice roots absorb phosphate and arsenate (As(V)) through the same transport pathway. Therefore, P application is assumed to decrease As concentration of rice via competitive absorption by rice roots. However, paddy rice is grown under the reducing condition which As(V) is not the predominant As species. To create more aerobic condition during the rice growing period, we involved a water management which keeps soil saturated in comparison with conventional flooding in this study. Simultaneously, P was applied to evaluate its inhibition effects on As absorption by rice. The topsoils of Erlin soil (Eh) and Pinchen (Pc) were collected. Background As concentration of both soils were 10 mg As kg-1 (Eh10 and Pc10). A portion of each soil was artificially spiked with As to reach the levels of 20 and 40 mg kg-1 for the Eh and Pc soils, respectively (Eh20 and Pc40). Treatment consisted of three P levels (150, 525, 900 kg P2O5 ha-1) and two water management (flooding and saturating). For the flooding treatment, the water head wad kept at 5~6 cm depth above soil surface throughout the rice cultivation. For the saturation treatment, flooding was conducted after transplantation for only 15 days and then the soils were kept saturated (operationally, saturated means that partial waterlogged condition with no water head above the soil surface). The results show that compared to conventional flooding, the soil redox potential increased under with saturation treatment, which inhibited reductive dissolution of iron (hydr-) oxides. Thus As concentrations of pore water were low because the As adsorption capacity remained high. However, As concentration of pore water were no significant difference among P treatments. In the Eh soils, neither water managements nor P treatments, the As concentrations of straw and brown rice were not significantly different. In the Pc soils, As concentrations of straw and brown rice decreased under saturation treatment. Only under the saturation treatment, the significant decreases of As concentrations in straw could be found after P application at 900 kg P2O5 ha-1 in the Pc40 soils (p<0.05). The As concentration of brown rice slightly decreased by P treatment, but there was no statistical significance. The positive correlation between the concentration of total As and DMA in brown rice were found (R2=0.97). As(III) concentration of brown rice approximately maintained 0.2 mg kg-1. Although total As concentration of brown rice exceeded 1 mg kg-1 in the Pc40 soils, arsenic species was predominated by dimethylarsinic acid (DMA) but not the highly toxic inorganic arsenite (As(III)), which indicates that consuming rice may not pose high health risks in this study