施用過氧化鈣對砷汙染土壤中水稻植體砷及穀粒砷物種累積之影響

Abstract

由於水稻生長於厭氧環境下，使水稻與其他穀類作物相比較容易累積砷於穀粒中，因此食用稻米成為人體暴露於砷的主要途徑。許多前人研究以水分管理方法來降低水稻對砷的吸收，但在田間執行上較為困難，例如水稻在特定生長時期會需要大量的水分，且極度乾燥之環境會抑制其生長。而在亞洲，易降雨的氣候會使土壤長時間呈現浸水還原的狀態。因此，添加釋氧劑於浸水土壤中提供氧氣來降低土壤溶液中砷的有效性，可能為一更有效的方法來降低水稻中砷的累積。本研究之目的為以盆栽試驗種植水稻，探討施用過氧化鈣對土壤溶液中砷濃度及物種之變化、水稻吸收砷以及穀粒砷物種的影響。試驗使用四種土壤，分別為未添加砷之淇武蘭低砷濃度土壤 (13 mg kg-1)、人工添加 80 mg As(V) kg-1 之淇武蘭高砷濃度土壤 (78 mg kg-1) 以及兩天然低砷濃度 (16 mg kg-1) 與高砷濃度汙染 (132 mg kg-1) 之關渡平原土壤。過氧化鈣施用量分別為每公斤土壤添加0 g、5 g、10 g、20 g 之四種處理，分兩次施用，施用時機為水稻幼苗移植前三天與移植後第 60 天，種植期間測定土壤 pH、Eh 與土壤溶液中砷、鐵和砷物種濃度，於穀粒成熟期時採收，測定水稻株高、地上部生質量、穀粒產量、根部鐵膜生成量、植體各部位總砷濃度與穀粒砷物種，並另外進行孵育試驗觀察在無水稻生長情況下，施用過氧化鈣對土壤性質及土壤溶液之變化。結果顯示在孵育試驗及盆栽試驗中，土壤溶液總砷與總鐵濃度在大部分的監測時間下皆隨著過氧化鈣施用量的增加而下降。在低砷濃度處理中，水稻根部及地上部總砷濃度隨過氧化鈣施用量增加而下降，推測是由於土壤溶液中總砷濃度下降所導致，地上部生質量與穀粒產量也因而上升；但在高砷濃度處理中，推測由於水稻根部鐵膜生成量隨過氧化鈣施用量增加而減少，導致水稻根部及地上部總砷濃度上升，地上部生質量與穀粒產量也因此下降。施用過氧化鈣於低砷濃度土壤中會使糙米中三價砷的百分比下降，無機砷濃度也顯著下降；高砷濃度土壤中施用過氧化鈣則對糙米中無機砷濃度無顯著影響。

Paddy rice is a staple food worldwide, but it accumulates As more efficiently in comparison with other cereals. Recently, rice has been considered to be a major As exposure pathway to human beings. Due to the high solubility and mobility of As under flooding conditions, and enhancing As uptake and accumulation of As by rice plants. Water management is one of the methods have been investigate to decrease As uptake by rice plants. However, water management is not always achievable, because of the growth of paddy rice is inhibited under water-deficiency conditions and the rainy weather in some area. It predicts the oxygen-releasing compounds application into paddy field may be a more efficient way to reduce As accumulation and toxicity in rice. Therefore, the objective of this study is to investigate the effect of CaO2 application on As accumulation in rice plant and arsenic species in brown rice. Pot experiments of rice growth in the greenhouse were conducted with four soils, including two geogenic As-elevated Guandu soils [GdL and GdH with low (16 mg kg-1) and high (132 mg kg-1) levels of As, respectively] and two Chiwulan soils [CaL and CaH with As-unspiked and spiked (80 mg As(V) kg-1), respectively]. CaO2 was added into soils at the application concentration of 0, 5, 10 and 20 g per kg soils and divided into separate applications. The applications were performed at 3 days before rice transplanting and the 60 days after transplanting respectively. Rice was harvested at the maturity stage. Concentrations of As, Fe and As species in soil solutions and As concentrations in different parts of rice plant were determined. The results indicate that the As and Fe concentrations in the soil solution were decreased significantly by the application of CaO2 most of the time in both incubation and pot experiments. In CaL and GdL soils, As concentration in rice roots and shoots were decreased with the increase of CaO2 application, and gives a positive influence on rice shoot biomass and grain yield. On the other hand, CaO2 application in CaH and GdH soils decreased the formation of iron plaque on the rice root thus contributed to the increase of As concentration in rice root and shoot, and declined rice shoot biomass and grain yield. Effect of CaO2 application on arsenic speciation in rice grain shows that CaO2 application leads to the decline of As(III) percentage and inorganic As concentration in brown rice in CaL and GdL soils; for CaH and GdH soils, DMA percentage decreased and there was no significant difference in inorganic As concentration in brown rice. It suggested that CaO2 might be a potential amendment to decrease As accumulation in rice at low level of As-contaminated paddy soils.