關渡和平鎮土壤的砷有效性在水稻生長期間變化之機制

Abstract

臺北關渡平原生產稻米，其土壤砷濃度最高可達500 mg kg-1，然而與其他含砷土壤相比，關渡土壤水稻穀粒之砷轉移效率較低。在前人研究中雖針對關渡平原水稻根圈鐵膜對水稻砷吸收的影響進行探討，然而對於砷在土壤與水稻間的傳輸隨浸水時間的變化尚未完全了解，因此本研究目的為藉由分析土壤、土壤溶液及水稻根部砷、鐵濃度與物種，探討在關渡土壤與水稻根部之間鐵和砷的移動隨浸水時間的變化。 本研究以平鎮土壤做為關渡土壤之對照組，並外添加砷於平鎮土壤中，測得關渡與平鎮土壤總砷濃度分別為321與88.0 mg kg-1。盆栽試驗期間維持浸水，並於不同時間測量土壤pH和Eh值，收集土壤溶液測定Fe、Mn、As、TOC、NO3-和SO42-濃度以及溶液中砷物種組成，同時採集土壤與植物樣品，立即以液態氮冷凍並冷凍乾燥後，利用X光近邊緣結構 (X-ray absorption near edge structure, XANES)分析土壤與水稻根部之砷、鐵物種組成，另取部分植體於70 ℃下烘乾、研磨，並微波消化分解後，分別以感應耦合電漿質譜儀 (Inductively coupled plasma mass spectrometry, ICP-MS) 和原子吸收光譜 (Atomic absorption spectrometry, AAS) 測定植體砷和鐵濃度。結果顯示，由於關渡土壤有機質和無定形鐵氧化物含量較高，使得在浸水初期土壤快速還原，並促進鐵氧化物的還原溶解釋出，然而部分的砷由土壤固相釋出後，會受到無定形鐵氧化物的再吸附作用而降低砷的移動性，使得浸水初期砷釋出速率較為緩慢，而隨浸水時間增加逐漸上升，但由於關渡土壤中高達52.0%的總砷吸附於無定形鐵氧化物上，使得隨鐵氧化物還原溶解釋出的砷比例仍然較平鎮土壤高。當溶液中的鐵移動到水稻根表時，會受到水稻根際泌氧的影響而在水稻根表形成鐵膜，由於本研究中土壤處於長期浸水的狀態下，因此水稻鐵膜的鐵物種以纖鐵礦為主，而當砷由溶液往根圈鐵氧化物分布時，由於平鎮土壤As(V) 和DMA比例較關渡土壤高，使得平鎮土壤溶液中的砷較易被鐵膜所吸附，同時由於關渡土壤水稻根表鐵氧化物形成的空間障礙較大，使得As(III) 與含砷硫化物較不易被氧化且不易往水稻根部運輸，進而降低地上部砷的累積。

The soils at Guandu plain have been known for high As concentration since 2008. When comparing to other As contaminated soils, As in Guandu soils (Gd) seems to be less available and relatively less As was transported from soil to rice grain, which was attributed to the accumulation of As by iron plaque on rice root in previous studies. However, the transportation of As from soil to rice roots at different rice growing stages has not been fully understood. To elucidate how As transports from soil solid phase to rice roots, pot experiment was carried out in an environment-controlled greenhouse with nature light source. For the purpose of comparison, the experiment was also conducted for Pinchen soil (Pc) with a high Fe content. The As concentrations of Gd and As-spiked Pc soils were 321 and 88.0 mg kg-1, respectively. The soils were submerged during rice cultivation. Soil pH and Eh were measured every 10-20 days. Soil solutions, soil, core samples and rice plants were also collected at certain time intervals during the experiments. The results showed that the higher contents of soil organic matter and amorphous iron (hydr)oxides in Gd soil led to a rapid decrease in Eh and the release of Fe in the early stage of submergence, while the releasing rate of arsenic was slower due to the re-adsorption by iron (hydr)oxides in the soil. Nevertheless, a higher ratio of arsenic was released into soil solution compared to the counterpart of Pc soil because there was up to 52% of total arsenic content sorbed on amorphous Fe (hydr)oxides in Gd soil. When Fe(II) is released and moved to the surface of rice roots, Fe(II) would be oxidized to Fe(III), which subsequently precipitates in the rhizosphere and rice root surface due to radial oxygen loss (ROL) from rice roots. In this study, lepidocrocite was the main Fe species in iron plaques of both soils, which suggested that ROL from rice roots under flooded condition was a gradual process. Because of higher ratio of As(V) and DMA in Pc soil solution, arsenic was sorbed much readily onto iron plaques in Pc soil than in Gd soil. Spatial barriers in rice rhizosphere may also limit oxidation and transportation of arsenic from soil solution to rice roots, which then lowered the amount of arsenic accumulated in rice shoot.