利用土壤水分管理降低兩種砷汙染土中糙米砷濃度

Abstract

砷汙染土壤栽培水稻，因土壤長期浸水使砷溶出，導致水稻栽培稻米受砷汙染而危及食品安全。研究假說為透過土壤水分管理使土壤達到好氧境況以降低土壤孔隙水砷濃度，以降低稻米砷汙染的風險。本研究針對土壤水分管理方式及不同性質砷汙染土壤，探討土壤水分管理對降低水稻糙米砷濃度之可行性評估；同時探討土壤水分管理對糙米砷物種之影響。本研究進行盆栽試驗，以二林(Er)及關渡(Gd)砷汙染土及五種土壤水分管理方式，創造不同生育時期土壤水分境況進行土壤水分管理，包括(1) 田間慣行操作方式(conventional)，維持土壤浸水至水稻之最大分櫱期，曬田一週後復浸水至水稻開花，開花後進行週期性灌溉直到水稻收割前兩週進行曬田；(2) 水稻開花期前維持土壤好氧狀態，開花後保持土壤浸水境況(A/F)；(3) 水稻開花期前維持土壤浸水狀態，開花後保持土壤好氧狀態(F/A)；(4) 水稻全生長期維持土壤孔隙為水所飽和(saturated)及(5) 水稻全生長期維持土壤浸水狀態(flooding)；並以浸水處理組為對照組，比較土壤水分管理在不同水稻生長期間維持土壤好氧狀態以降低糙米砷濃度。田間慣行操作方式可降低Gd土壤糙米砷濃度，在Er土壤中則無法達到效果，為Gd及Er土壤排水性質差異所致(p<0.05)。在A/F及水飽和處理在水稻開花前維持土壤好氧境況，可降低糙米砷濃度及減輕水稻砷毒害徵狀。土壤水分管理可降低土壤砷濃度及水稻吸收，降低水稻根部砷濃度(p<0.05)，但無法降低水稻地上部砷濃度。土壤水分管理顯著改變水稻糙米砷物種濃度及組成，在田間慣行操作方式、F/A及浸水處理，糙米雙甲基砷(DMA)濃度與總砷濃度呈顯著正相關(p<0.05)；在A/F及水飽和處理，糙米砷物種以無機砷為主，約佔總砷濃度60-100%。以國人稻米消費量為基準計算無機砷攝取量，未超出WHO之每週容許攝取量15 μg/每公斤體重。砷汙染土壤種植水稻，A/F及水飽和處理較持續浸水栽培可降低糙米砷濃度，為本試驗推薦之土壤水分管理方法。

Rice (Oryza sativa L.) is efficient to translocate arsenic to the grain. In the arsenic contaminated paddy field, arsenic is reduced and mobilized in the submerged condition. Thus, the potential risk of food safety increased when rice grown in the As-contaminated paddy soil. The solubility of arsenic is decreased in the aerobic soil condition while soil drainage is applied. The aims of the study are (1) to mitigate the arsenic concentration in the brown rice grown in two arsenic contaminated soils with soil water managements, and (2) to investigate the arsenic species in the brown rice under soil water managements. The pot experiment conducted with Er and Gd contaminated soil, and with five soil water managements including, (1) conventional treatment : drainage for one week at the rice maximum-tiller-number stage and keep water head at 3-5 cm depth to the flowering stage, and followed by intermittent irrigation; (2) aerobic before flowering treatment (A/F), keep soil aerobic condition before flowering stage, and then keep water head at 3-5 cm depth in the resting cultivation; (3) aerobic after flowering treatment (F/A), keep water head at 3-5 cm depth before flowering stage and then keep soil aerobic in the resting cultivation; (4) water saturated treatment, maintain the soil pore saturated with water and no water ponded on the soil surface; and (5) flooding treatment, keep the water head at 3-5 cm depth during the whole cultivation period. The flooding treatment serves as control, to investigate the effect of soil drainage at different vegetation period on mitigating the brown rice arsenic. Based on the characteristics of soil drainage in Gd and Er soils, the arsenic concentration in the brown rice was found decreased in conventional treatment in the Gd soils rather than in the Er soil. The arsenic toxicity happened to the paddy rice was reduced in A/F and water saturated treatments (p<0.05), and the arsenic concentration of the brown rice was also decreased. The ratio of arsenic species content in the brown rice depends on soil water managements, the DMA concentration is increased with the arsenic concentration in the brown rice under the conventional, F/A and flooding treatments (p<0.05). While inorganic arsenic was found dominant in the brown rice of the A/F and water saturated treatments, and which contributed 60-100% of the arsenic concentration in the brown rice. Provisional tolerance weekly intake of inorganic arsenic of the treatments in this study was all met the WHO standard of 15 μg/kg (body weight). The A/F and water saturated treatment mitigate the total arsenic concentration of brown rice (p<0.05), which are recommended to the application in the field.