葉面與土壤施用矽酸鈉對種植於砷污染土壤水稻幼苗砷累積之影響

Abstract

稻米是人類攝入砷的主要途徑之一，受砷污染之稻米會對於全球人類健康及糧食安全造成威脅。矽和三價砷藉由相同轉運途徑進入水稻根部，前人研究中發現，在水耕試驗中施用矽，可降低水稻吸收砷和砷對水稻之毒性，然而，矽施用於土壤中，對於砷的生物有效性同時有促進和抑制的作用，藉由矽和砷競爭固體吸附位置，土壤溶液中砷濃度有上升現象，矽又可與砷競爭植體之吸收，因而降低植體中砷濃度。由於矽和砷競爭土壤吸附位置可能對水稻生長產生不利之影響，葉面施矽可能為促進水稻生長，減輕水稻砷累積和砷毒性更有效的另一種方式。因此，本篇之研究目的在於評估土壤及葉面施用矽酸鈉對水稻矽和砷累積的影響，探討不同矽施用比例對於水稻累積砷的影響。盆栽試驗於溫室中進行，試驗使用四種土壤，包含兩種天然之砷汙染關渡土壤，關渡低砷土壤砷濃度為17 mg kg-1 ；高砷土壤砷濃度為 128 mg kg-1，以及另外兩種未添加/添加砷之淇武蘭土壤，分為未添加砷之淇武蘭低砷土壤 (16 mg kg-1) 及額外添加80 mg As(V) kg-1之淇武蘭高砷土壤 (114 mg kg-1)。矽的處理則是選用矽酸鈉溶液，土壤施用量分為 0、1 和 3 倍 (S0x, S1x, S3x) 推薦施用量(0.175 g Si kg -1 soil)；葉面施用量則為分別噴灑每盆 50 毫升之 0、0.1% 和 0.3% 的矽酸鈉溶液 (F0x, F1x, F3x)。施用時機分別在水稻幼苗移植後之第 15 和 30 天，種植期間測定土壤孔隙水之 pH、Eh、矽、砷和砷物種濃度，在水稻種植滿 50 天後採收，測定植體株高、地上部生質量、矽和砷濃度。試驗結果顯示，土壤孔隙水中的矽濃度會隨著土壤施矽量增加而顯著上升，且由於淇武蘭土壤無定型鐵鋁及有機質含量較低，對於矽的吸附力較低，因此，淇武蘭土壤孔隙水矽濃度高於關渡土壤十倍，在三倍土壤矽施用量之下，淇武蘭低砷、高砷和關渡高砷土壤，孔隙水中之砷濃度上升。土壤施矽於高砷土壤中，隨著施用率增加，水稻植體生質量顯著下降，地上部矽濃度有顯著增加，而砷濃度則顯著下降。雖然土壤施用矽會使孔隙水中砷濃度提高，但和對照組比較，植體地上部砷濃度降低，低砷土壤中主要是因孔隙水中之矽砷莫耳比值增加，導致矽和砷競爭植體吸收，在高砷土壤中則可能是因為砷對水稻幼苗產生毒害，使水稻吸收砷降低。然而，葉面施矽對於土壤孔隙水之 pH、Eh、矽和砷濃度，以及植體生質量、地上部之矽和砷濃度無顯著影響，推測較有可能的原因為施用之矽未進入植體內，無法被水稻幼苗吸收利用。

Intake of arsenic (As) from rice consumption poses a threat to food safety and human health globally. Silicon (Si) and As(III) share the pathway of uptake and translocation in rice. Si application into hydroponic culture solutions can decrease As uptake and toxicity to rice plant shown in previous studies. However, Si application into soils, both enhancing and inhibition effects on As bioavailability could occur because of increasing As release into soil solutions resulted from competitive sorption, and decreasing As uptake caused by competing pathways into rice plants. Since the adverse effects may come with the competition adsorption onto soils between As and Si, foliar application of Si fertilizer might be an alternative and more efficient way to enhance rice growth and to reduce As accumulation and toxicity than soil applications. Therefore, this study evaluated the effect of foliar and soil application of Si on rice seedlings grown in As contaminated soils and investigated the influence of various application rates of Si on arsenic accumulation in rice seedlings. Pot experiments of rice seedling growth in the greenhouse were conducted with four soils, including two geogenic As-elevated Guandu soils [GdL and GdH with low (17 mg kg-1) and high (128 mg kg-1) levels of As, respectively] and two Chiwulan soils [CaL and CaH with As-unspiked and spiked (80 mg As(V) kg-1), respectively]. In the Si treatments, sodium silicate was added into soils at the application rates of 0, 1, and 3 times (S0x, S1x, S3x) of Si fertilizer recommendation rate (0.175 g Si kg -1 soil) for soil application, and was sprayed on rice at the application rates of 50 mL of 0, 0.1%, and 0.3% Si (F0x, F1x, F3x) solutions per pot for foliar application. The applications were performed at the 15 and 30 days after rice transplanting respectively. After 50 days of growth, the rice seedlings were harvested. The concentrations of Si, As and As species in soil solutions and As concentrations in roots and shoots of rice seedlings were determined. The results show that the Si concentrations in the soil solution were increased significantly by soil application of Si. Due to the lower capacity of Si retention in Ca soils, the concentrations of Si in Ca soil solutions were one order of magnitude higher than those in Gd soils. The concentrations of As in soil solutions were increased significantly treated by S3x in CaL, CaH, and GdH. For high As contaminated/spiked soils with soil application of Si, the biomass of rice plants and shoot-As concentration decreased with the Si application rates, whereas the shoot-Si concentrations were increased. Although soil application of Si led to higher As concentrations in soil solutions, it decreased shoot-As concentrations compared with the control, resulting from the increase of Si/As molar ratio in soil solutions and thus causing competition between Si and As for plant uptake. The decrease of As uptake by rice seedlings may result from As phytotoxicity of rice plants grown in high-As soils. However, foliar application of Si had no significant effect on pH, Eh, Si and As concentrations in soil solution, plant biomass and shoot-Si and shoot-As concentrations of rice seedlings. It suggested that the foliar application of Si in this study were not available for plant uptake.