磷肥施用方式對於砷汙染土壤中水稻幼苗吸收砷之影響

Abstract

砷是一種致癌物質，並且在世界各地有許多受砷汙染的土壤，稻米是亞洲地區的居民主要糧食作物之一，然而相較於其他穀類作物，水稻較易累積高濃度的砷於穀粒中。因為水稻生長在還原的環境，在缺氧的情況下使得三價砷由鐵氧化物上溶解出來。先前許多前人研究指出，於養液中添加磷酸鹽會使得水稻抑制對五價砷的吸收。然而研究顯示於土壤中添加磷酸鹽反而會置換吸附於土壤中的五價砷，導致砷的移動性增加，反而造成水稻的毒害。由於土壤施用磷肥可能會促使磷和砷的競爭作用因而導致砷的釋出，因此以葉面施用磷肥可能為一更好的方法來提供水稻所需的養分。本試驗研究目的為評估土壤及葉面施用磷肥以及不同磷施用量對水稻磷和砷累積的影響。 試驗土壤採集自關渡地區以及將軍系，其中關渡地區母質為富含無定型鐵/鋁氧化物的火山灰土壤，其磷吸附容量高，而將軍系土壤的無定型鐵/鋁氧化物較少，因此對磷的吸持能力較低。關渡地區為天然砷汙染土壤， 其低砷汙染地區之砷濃度為17 mg kg-1，高濃度砷汙染土壤的砷濃度為 116 mg kg-1，並另外以人工添加 80 mg kg-1五價砷至將軍系土壤中，做為將軍系之高濃度砷土壤，未添加砷的則為低濃度砷土壤。磷肥的施用分為土壤及葉面施用，土壤施磷的部分是選用磷酸二氫鈣，處理濃度分為不添加磷肥的控制組，以及1倍、5倍、10倍的施用量，施用時間為水稻移植的前兩天，與基肥共同施用。以前人經驗為利，盆栽試驗施用肥料量需為田間推薦量的兩倍左右，故將田間推薦施肥量之兩倍作為1x磷的施用量。葉面施磷的部分是選用磷酸二氫鉀，處理濃度為不噴灑磷肥的控制組，以及噴灑1倍 5倍 10倍的處理組。其中1倍的施用量為 每盆噴灑50毫升含有0.1% P的溶液。噴灑時間為水稻移植後第 15 及 30 天。試驗選用的水稻品種為台梗九號，種植滿50天後採收。在水稻種植期間，會監測土壤之pH及Eh變化，並以孔隙水採樣器收集土壤溶液，測定砷和磷的濃度及砷物種的含量。 試驗結果顯示在將軍系低濃度砷及高濃度砷土壤中，以土壤施用磷肥會導致孔隙水中砷濃度的增加。在浸水初期，孔隙水中的 As(V) 濃度隨著磷施用濃度增加而上升，然而隨著浸水時間增加，As(V) 大多轉變為 As(III) 的型態。而在關渡低濃度砷土壤中，磷的施用對孔隙水砷濃度並無顯著影響。結果顯示土壤磷肥的施用會使種植於將軍系及關渡土壤之水稻生質量下降。推測原因為在水稻移植初期土壤磷肥的施用，會導致孔隙水中的 As(V) 濃度上升，然而隨著時間增加，As(V) 大多轉變為 As(III)，因此磷無法與孔隙水中的 As(III) 競爭水稻根部上的吸收通道。在本試驗中，葉面施磷的處理對水稻幼苗的生質量皆無顯著影響。葉面施磷雖然不能減少水稻對砷的吸收，然而仍然可藉由葉面施磷的方式避免於砷污染土壤中施用磷肥造成土壤中砷移動性的增加以及對水稻的毒害。

Arsenic (As) is a human carcinogen and poses human health risks as a contaminant of ground water and soils. Phosphorous (P) shares the same pathway of uptake and translocation in rice with Arsenate (As(V)). Previous studies have shown that the presence of phosphate inhibited the uptake of As(V) in rice during hydroponic experiments. However, there were studies indicated that phosphate added in soil displaces the sorbed arsenate from exchange sites and therefore increases ability of As(V). Since soil application of phosphate would lead to the competition between P and As for the sorbed sites in soil solids, foliar application of P may be a better choice to supply P to rice. Therefore, this study evaluate the effect of different application rate of P with foliar and soil application on P and As accumulation in rice seedling. Pot experiments grown in the greenhouse were conducted with four soils, including two geogenically As-enriched Guandu soils [GdL and GdH with low (16 mg kg-1) and high (116 mg kg-1) levels of As, respectively] and two Chengchung soils [CfL and CfH with As-unspiked and spiked (80 mg As (V) kg-1), respectively]. In the soil application treatments, Ca(H2PO4)2· H2O was added into soils 2 days before rice transplanting at the application rates of 0, 1, 5, and 10 times (S0x, S1x, S5x, S10x), while S1x was 2 times of recommended amount. Foliar application of P was sprayed on rice at application rates of 50 mL of 0, 0.1%, 0.5%, 1% P (F0x, F1x, F5x, F10x) solutions per pot at the 15 and 30 days after rice transplanting respectively. After 50 days of growth, the rice seedlings were harvested. The concentration of P, As and As species in soil solutions and As concentrations in roots and shoots of rice seedlings were determined. The results showed that soil application of P did not reduce As uptake by rice, but rather aggravate the availability of As in soil. It would lead to the decrease of biomass of rice seedlings, but the total uptake of As in rice seedlings did not raise with the increasing rate of soil P application. The results showed that soil application of P caused an increase in As(V) in soil solution at the early days during the cultivation period. But after the time, most of As(V) was transformed into As(III) under the flooding condition. Although the P/As (M) ratio in soil solution is high, but the As(V) concentration is very low, and therefore could not compete with phosphorus for uptake by rice roots and to reduce the absorption of arsenic into rice plants. In addition, the result showed that there was no significant effect on the growth and As accumulation in rice seedlings by P foliar applications. Although foliar application of P did not reduce As uptake by rice plants, it can avoid the phytotoxic effect to rice plants caused by soil P application to supply P into As-contaminated paddy soil.