探討浸水土壤中水稻根系對鐵和鎘分佈的影響

Abstract

水稻生長於浸水土壤中，在土壤後土壤通氣受阻下，土壤逐漸以厭氧呼吸的微生物為主，並導致一序列的還原反應發生。水稻根圈因根系的徑向泌氧(radial oxygen loss, ROL)與分泌物等，導致根圈與本體土壤間氧化還原的空間變異。本研究的目的希冀探討根圈與本體土壤間氧化還原條件的空間變異，是否將會驅使鐵與鎘在不同的位置累積而移動，進而影響水稻根的吸收。透過土壤浸水孵育實驗，觀察浸水土壤的鐵、鎘釋出隨時間的變化；接著利用水稻根盤與根圈箱試驗觀察根表不同距離之土壤溶液中鐵與鎘的濃度變化，藉此探討根圈與本體土壤間鐵與鎘的移動及累積情形。研究結果顯示，土壤隨著浸水時間的增加，鐵會大量的溶解釋出，而鎘在浸水後期則逐漸降低，因此鐵與鎘在土壤中的有效性會受氧化還原程度的影響。根圈箱試驗的結果中根圈與本體土壤間鐵皆產生一濃度梯度，顯示根圈與本體土壤氧化還原電位的差異下，造成鐵的移動並累積於根圈。將土壤XANES圖譜以線性擬合之結果顯示土壤與水稻根盤接觸後，根圈處纖鐵礦(lepidocrocite)的組成增加(45.7至63.1 %)。過去研究顯示此類鐵氫氧化物對鎘有很好的蓄積能力，因此就土壤溶液中鎘的濃度變化，推測鎘在根圈被吸附聚集而降低。此外根圈處有明顯的硫酸根的釋出，因此判斷在還原環境所生成之硫化物物種(CdS)在根圈處受根系再氧化生成易溶解之硫酸鹽物種(CdSO4)導致水稻根對於鎘的吸收量增加。

When a soil is under submerging condition, the water layer above the soil cuts off the oxygen supplying from the air. As a consequence, anaerobic respiration of soil microbial drives the occurrence of a series of reduction reactions. The excretion of oxygen from rice roots result in the oxidized condition in the rice rhizosphere, which may result in a redox gradient from rice rhizosphere to bulk soil. Therefore, it is hypothesized that this redox gradient may lead to the spatial distribution of iron and cadmium between rice rhizosphere and bulk soils, and subsequently affect their uptakes by rice roots. Soil incubation and rhizobox experiments were applied to the study the concentration of iron and cadmium in soil solution as a function of time, and the spatial distributions of iron and cadmium in soil solution, respectively. The results of soil incubation experiment showed that the concentration of ferrous ion in soil solution increased with the time of submergence, while the concentration of cadmium decreased gradually. The difference between the redox conditions in the rhizosphere and bulk soils induced the accumulation of iron in the rhizosphere of rice. The linear combination fitting (LCF) results of Fe k-edge XANES spectra of the soil samples shows that the formation of lepidocrocite in the rhizosphere (63.1 %) was higher than that in the corresponding bulk soil (45.7 %). Accordingly, ferrous ions were oxidized in the rice rhizosphere, leading to precipitation of Fe in the rice rhizosphere, On the other hand, the decrease of Cd concentration in soil solutions from the bulk to rhizosphere soils indicated the absorption of cadmium by ferric hydroxides. In addition, the concentration of sulfate in the rice rhizosphere was higher than the counterpart in the bulk. This indicates that the oxidation of reduced S species (e.g., CdS) in rice rhizosphere may be responsible for the release of Cd, which was then absorbed by rice roots.