比較水生與旱作栽培下蕹菜(Ipomoea aquatica Forssk.)的砷累積及砷物種分佈

Abstract

蕹菜是亞洲地區常見的蔬菜，可以利用水生及旱作兩種栽培方式，而砷(Arsenic, As)是環境中危險的致癌物質，在水生栽培(還原條件)下土壤有效砷濃度高，且主要砷物種為毒性較高的亞砷酸鹽(iAsⅢ)，故當蕹菜種植在兩種不同栽培方式下，植體中的總砷濃度及砷物種可能也會有差異，並造成不同的健康風險。因此本研究的目的為比較旱作栽培(U)與水生栽培(F)對蕹菜砷累積及砷物種分佈。本研究所採集的土壤為關渡(Gd)和民雄(Ms)兩地區，土壤質地分別為黏土和壤土，另外關渡土壤的總砷濃度為18.4 mg kg-1 (GdL)和103.3 mg kg-1 (GdH)，而民雄土壤的總砷濃度為22.9 mg kg-1 (MsL)和72.1 mg kg-1 (MsH)，草酸銨萃取砷濃度則依序為：GdH> MsL> MsH> GdL，以上四種土壤會分別進行旱作(田間容水量)或水生(土表以上淹水3公分)兩種栽培方式。結果顯示，水生及旱作栽培下，蕹菜地上部皆只有檢測到無機型態的三價砷(iAsⅢ及)五價砷(iAsV)，且以iAsⅢ為主，同時也發現蕹菜地上部的iAsⅢ濃度與孔隙水有顯著相關(p<0.001)。在水生栽培下，因為土壤中鐵氧化物會溶解並釋放被固定的砷，增加土壤孔隙水中總砷及iAsⅢ濃度，進而導致地上部的總砷及iAsⅢ濃度皆高於旱作栽培，故相較於旱作蕹菜，食用水生蕹菜的風險較高。此外，除了水分管理，土壤的質地及被無定型鐵氧化物固定的砷含量也會影響砷在土壤的溶解度及移動性造成影響。綜合本研究結果，蕹菜利用旱作栽培，並種植於壤土且孔隙水砷濃度低的環境下，可以有效降低食用部位iAsⅢ濃度，進而降低其食用風險。

Water spinach is the popular vegetable in Asia area, and it can be planted in both flooding and upland conditions. Arsenic (As) is considered as one of the most dangerous carcinogens in natural environment. In the flooding (reducing) condition , the available As in the soil is higher and the dominant species is arsenite (iAsⅢ), which is the most toxic As species, thus there were differences in As accumulation and major As species in plants between two water managements, and further leading to the different health risk to human. Therefore, the objective of the present study was to compare the As speciation and As accumulation in water spinach in soils under flooding (F) and upland (U) conditions. In this study, two tested soils collected from Guandu (Gd) and Minsyong (Ms), which were clay and loam, respectively. The concentration of As in Gd soils are 18.4 mg kg−1(GdL) and 103.3 mg kg−1 (GdH), and those in Ms are 22.9 mg kg−1(MsL) and 72.1 mg kg−1 (MsH). Besides, the order of the concentration of ammonium oxalate extractable As is GdH> MsL> MsH> GdL All tested soils were conducted in upland (field capacity) and flooding (3 cm above the soil surface) treatments in this study for planting water spinach. The results showed that the inorganic As including iAsIII and iAsV were detected in the water spinach, and iAsⅢ was the dominant species in root and shoot under upland and flooding conditions. It was also found that there was significant correlation between the concentrations of iAsIII in shoot of water spinach and those in soil pore water (p<0.001). Under flooding treatment, the increase of total As and iAsIII concentrations in pore water through the reduction dissolution of iron oxides, which results in the concentrations of total As and iAsIII in shoot were higher than upland treatment. Therefore, the dietary risk of water spinach planted in flooding soil was higher than upland soil. In addition, the solubility and mobility of As in soils was not only affected by water management but also affected by the texture and the contents of DCB-As of soils. In summary, it suggests that the concentrations of iAsIII in edible part of water spinach and dietary risk can be decreased while it planted in upland loamy soils with low As in soil pore water.