稻殼添加對兩種排水不良砷污染土壤中糙米砷濃度的影響

Abstract

砷可分為有機態的甲基砷與無機態的三價砷與五價砷，存在土壤內。水稻是亞洲地區主要糧食作物，由於水稻生長需長期浸水，厭氣環境下土壤中的砷主要以移動性及毒性較高的三價砷存在，若水稻種植於砷汙染地區，將吸收砷累積於穀粒當中，影響水稻生長及食品安全。前人研究發現水稻生長期間進行不同水分管理方式，在不影響產量的前提下，透過排水曬田可有效降低穀粒砷濃度，但效果卻會受到土壤質地影響。本試驗目的在探討不同土壤水分管理方式配合稻殼的添加對兩種砷污染土糙米砷濃度的影響。試驗選用兩種不同土系表土，分別外添加10與15 mg As kg-1，為二林系土壤(總砷濃度10及20 mg kg-1)及後湖系土壤(總砷濃度9及24 mg kg-1)，水分管理方式為水稻最大分蘗期曬田十天之排水十天(drain)及水稻生長期間維持土表浸水3-5 cm之連續淹水(flooding)，並配合未添加及添加2.5 %稻殼兩種稻殼處理，探討對水稻糙米砷濃度的影響。 研究結果顯示兩種土壤在人工模擬砷污染環境下，相較於連續淹水，排水十天有效地提高土壤的氧化還原電位，降低孔隙水中砷的生物有效性，進提高產量，降低糙米總砷的累積。雖然稻殼的添加可促進排水，但新鮮稻殼的施用仍會微生物快速分解，使土壤呈強還原的狀態，造成水稻生長受砷毒害影響，顯著降低穀粒產量並提高糙米總砷濃度，故判斷稻殼的添加對於水稻生長負面的影響高於其助益排水的效果。糙米中高濃度的砷，經物種分析後發現僅0.2至0.3 mg/kg為三價砷的部分，其餘皆為雙甲基砷物種存在，種植於後湖系砷污染土壤的糙米雙甲基砷濃度甚至高達3至4 mg/kg，此總砷濃度雖高，但大部分為毒性較低的雙甲基砷物種，是否仍具高毒性則需後人試驗評估。

The As speciation of soil can be divided into arsenite (As(III)), arsenate (As(V)), monomethylarsonic acid (MMA) and dimethylarsonic acid (DMA). Rice (Oryza sativa L.) is the major crop for food in the Asian country. The As is shown in arsenite form under long term continuous flooding and this result will affect the rice productivity and food safty of brown rice grown in As-contaminated soils. Previous studies had indicated that As concentration of brown rice can be significantly reduced by different soil water managements, especially in drainage or saturated condition which do not affect the rice productivity. This result is affected by soil texture. Therefore, the objective of this study is to understand the effects of soil water managements and adding husk on the As concentration of brown rice grown on two different soil texture soils. Two surface soils of two series were spiked with 10 or 15 mg As kg-1. The total As concentration of Erlin soil are 10 and 20 mg/kg and the total As concentration of Houhu soil are 9 and 24 mg/kg. Soil water managements are (1) drain management with flooding except keeping drainage for 10 days after maximum tiller stage, and (2) flooding management with the flooding soil surface at 3-5 cm depth. The adding husk treatments are added 2.5 % husk and without husk. The results indicated that the drain water management can significantly reduce As concentration of brown rice by increasing the soil redox potential (Eh), reducing the As concentration of the pore water and also increasing the rice production, compared the flooding treatment. The addition of husk can significantly affect soil drainage, but this treatment also produce the more serious As toxicity by increasing the microbiological activities to produce more reduced condition and high As concentration of brown rice. The As speciation analysis of total As in brown rice of different treatments indicated the concentration of As(III) are almost 0.2-0.3 mg/kg and the other As concentration are almost DMA, which indicated the maximum concentration of total As for Houhu soil. The result indicated that the real health risk assessment should be more studied for high percentage of DMA in high total As concentration of brown rice.