添加磷肥於砷汙染土壤中對砷釋出至土壤孔隙水中以及水稻幼苗累積砷之影響

Abstract

前人研究指出，水耕環境下，水稻隨著磷濃度的增加，可以降低水稻的砷毒害以及降低植體中砷的含量，其原因為水稻吸收 As(Ⅴ) 的通道即為水稻吸收磷酸根之通道。但土耕試驗中，施用磷肥至土壤中，磷會與砷競爭土壤中的吸附位置，導致砷的有效性提高，反而增加水稻毒害和砷累積。因此，本研究主要目的為在土壤中添加高濃度之磷肥以探討磷對土壤孔隙水中砷的釋出以及水稻幼苗吸收砷之影響。本研究選用四種磷吸附容量不同之土壤，分別為三種低砷土壤將軍系 (Cf)、平鎮系 (Pc)、關渡低砷 (GdL) 以及高砷土壤 (GdH)，其中將軍系、平鎮系土壤人工添加80 mg As(Ⅴ) kg-1 之砷，製備成砷汙染土壤。依據作物施肥手冊以及Bray-1萃取之有效性磷得知磷肥推薦施用量 (一倍)： 6 mg P kg-1 (將軍系和平鎮系土壤) 以及 8mg P kg-1 (關渡低、高砷土壤)。以磷酸氫二鈣作為磷處理，磷肥施用量為：(1) control (2) 一倍 (3) 二倍 (4) 五倍 (5) 十倍之磷推薦施用量，浸水50天期間進行孔隙水測定以及水稻幼苗盆栽試驗。結果顯示，高砷濃度土壤中，孔隙水砷釋出量由高至低分別為：將軍系高砷濃度土壤 > 關渡高砷濃度土壤 > 平鎮系高砷濃度土壤，原因為將軍系土壤的磷吸附容量最低，因此，砷釋出的也高；平鎮系土壤富含結晶型鐵，且平鎮系土壤為低有機質和酸性土壤之特性，具有強烈的砷吸附能力；關渡土壤富含無定形鐵，浸水期間無定形鐵易還原溶解而釋出砷。當磷施用量的增加，將軍系土壤孔隙水中砷濃度也隨之上升，但平鎮系與關渡土壤中孔隙水中砷濃度並沒有因磷施用量增加而有顯著差異，其與磷吸附容量較大有關，因平鎮系和關渡土壤有較高的磷吸附容量，當磷添加進土壤中後，磷隨即被土壤中的鐵 (氫) 氧化物吸附，而減少了與砷酸根競爭的機會。水稻幼苗盆栽試驗結果顯示。種植於低砷濃度土壤中的水稻幼苗生質量並沒有因為施用磷肥而有顯著差異。種植於高砷濃度的將軍系、關渡土壤中水稻幼苗因磷肥的施用而有生質量下降趨勢，且將軍系土壤之水稻幼苗根部砷含量有增加之趨勢，結果顯示，三種高砷土壤中，水稻幼苗生質量並沒有因為高磷肥的施用而降低水稻幼苗砷毒害現象，特別在將軍系高砷濃度土壤中，提高磷肥的施用量，反而造成水稻幼苗毒害程度加劇。

Previous studies have shown the phosphate additions in solution culture studies decrease As uptake and mitigate As-caused phytotoxicity symptoms. Phosphate has an inhibitory effect that competes with arsenate for the same transporter during uptake by the plasma membrane. Summarizing result of pot and field experiment leads to a different conclusion. Plant uptake of As has been shown to increase upon P application in pot experiments, presence of P causes As-P competition for sorption sites resulting in increased As bioavailability, and hence higher As concentration in plant. Therefore, the aim of this study is to evaluate the effect of application of phosphate on growth and arsenic uptake of rice seedling. Three different anion capacity soils, Chengchung (Cf) and Pinchen (Pc) soils which were spiked with 0 and 80 mg As(Ⅴ) kg-1 respectively and two Guandu soils (Gd) with low and high levels of As concentration were used in this study. The P recommended application rate 6 mg P kg-1¬ for Cf and Pc soils, 8 mg P kg-1 for Gd soils. Phosphate were added as Ca(H2PO4)2 • H2O, the P application levels included: (1) control (0 time) (1) 1 time (2) 2 times (3) 5 times (4) 10 times of P recommended. Concentration of As and P in soil solution under flooded soil for 50 days, plant dry weight and As concentration in roots and shoots of rice seedling were determined. The results indicate that the addition of phosphate significantly increased the concentration of As in soil solutions of Cf soil. However, the phenomena were not observed in Pc and Gd soils. The addition of P resulted in increase in As desorption, and the effect was more pronounced in low anion capacity soils. There are two different As sorption site in Pc and Gd soils which were crystalline Fe and amorphous Fe respectively. The properties of Pc soil has amount of crystalline Fe, acidity and low organic matters contents, thus Pc soil has highest As and P retention. On the other hand, although Gd soil has highest anion capacity, amorphous Fe has easily reduced in anaerobic condition results of increasing As released. The results of pot experiments of rice seedlings show that the effect of phosphate on biomass of rice seedling was not significant in non-added As(Ⅴ) soils and low level As concentration soil of Gd. However, the dry weight in shoot and root increased with increasing rates of phosphate in Pc soil. On the contrary, the growth of rice seedling in As-contaminated soils was inhibited with increasing rate of phosphate application, and the effect was less pronounced in Pc soil. The above results suggest, in As-contaminated soils, the amounts of As in soil solution were greatly increased with increasing phosphate, thus resulting in the increase of As toxicity of rice seedlings ,the effect was more pronounced in low anion capacity.