添加不同有機質材至砷污染土壤中對土壤溶液砷濃度及水稻幼苗砷吸收量之影響

Abstract

前人研究指出，有機質會影響砷在土壤中的移動性及有效性，而可能提高水稻植體對於砷之吸收，並透過食物鏈使人類健康受到危害，但有機質和砷在土壤中之關係尚未清楚。因此，本研究目的為探討施用不同性質的有機質材於砷污染土壤中，其影響砷在土壤中的有效性，及水稻幼苗中累積砷含量之差異。本試驗以不同性質之平鎮系 (Pc) 與將軍系 (Cf) 兩種農地土壤，添加 0 及 150 mg As (Ⅴ) kg-1，另取兩個不同砷濃度之關渡土壤表土作為供試土壤。分別添加 0、1 及 4 % 之大豆粕 (SB)、蔗渣堆肥 (SC) 及牛糞堆肥 (CD) 三種有機質材於供試土壤中，在田間容水量下平衡 21 天後進行浸水孵育之批次試驗與水稻幼苗盆栽試驗。由盆栽試驗結果顯示，施用有機質材至砷污染土壤中會提高水稻對砷之吸收，雖然鐵膜生成量增加，且鐵膜上的鐵及砷具顯著的正相關 (P < 0.001)，推測鐵膜會阻隔砷進入水稻植體，但整體而言，水稻植體中的砷濃度仍增加。1% SB 施用於砷污染之 Pc 及 Gd 土壤，皆顯著提高水稻地上部之砷濃度，但水稻生長並未受到抑制，反而提高水稻幼苗之生質量。除此之外，施用 4 % SB 處理在 Pc 土壤中造成肥傷；砷污染之 Cf 土壤之水稻幼苗則受砷毒害而死亡。存在溶液相中的砷易被水稻幼苗吸收，由浸水孵育的結果顯示，施用三種有機質材至試驗土壤後，土壤溶液中的砷濃度提高，而影響水稻幼苗對砷之累積含量，但不同土壤及有機質種類結果相異。有機質材的添加可提高土壤溶液中可溶性有機碳 (DOC) 的濃度，且三種土壤皆以 SB 處理最為顯著，土壤溶液中 DOC 及砷濃度具顯著正相關，因此推測 DOC 可與砷競爭土壤吸附位置或和砷形成複合物所致。此外，添加有機質亦會導致土壤溶液 pH 值上升及土壤呈還原境況，這些綜合結果造成土壤溶液中砷濃度增加，其中皆以 CD 處理之影響較不顯著。Cf 土壤 pH 值高、黏粒含量低及無定型鐵鋁氧化物含量低，對於砷的吸持能力較弱，導致 4 % SB 處理在孵育試驗期間的土壤溶液中砷濃度高達 15 mg L-1。由以上結果得知，施用有機質材至砷污染土壤中，須謹慎評估有機質材及土壤特性，應避免施用易分解之有機質材與避免施用在高 pH 值或砷吸附容量低之土壤。

Previous studies have shown that organic matter (OM) can affect the arsenic (As) mobilization and bioavailability in soil as this may lead to the increase of As uptake into rice plants and pose human health risks since the As-exposure pathway via food chain. But the relationship between organic matter and As in the soils are not clear. Therefore, the aim of this study is to evaluate the effect of various organic amendments on As availability in As-contaminated soils and accumulation in rice seedlings. Two different properties of agricultural soils (Pinchen soil, Pc, and Chengchung soil, Cf) which were spiked with 0 and 150 mg As(V) kg-1 respectively and two Guandu soils (Gd) with low and high levels of As concentrations were used in this study. Soybean meal (SB), cattle-dung compost (CD), and sugarcane dregs compost (SC) were added into soils at the application rates of 0, 1, and 4 % (dry weight bases). The soils equilibrated at field water capacity condition for 21 days, and then flooding incubation and pot experiments for growing rice seedlings were performed. The results showed that the formation of iron plaque were increased, and there was a significantly positive correlation (P＜0.001) between the amounts of As and Fe in iron plaque, which might sequestrate As. However, the application of organic amendments increased As uptake by rice seedlings. The use of organic amendments enhanced As accumulation in rice plant whereas shoot biomass of rice seedlings grown in As-contaminated Pc and Gd soils treated with 1% SB was significantly increased. In addition, the rice seedlings in both non-added As(V) and As-contaminated Pc soils died which amended with 4 % SB treatment, and it was probably due to fertilizer burn; and the rice seedlings under additions of 4 % SB and SC in As-contaminated Cf soils were dead but not found in non-added As(V) Cf soils, it was subjected to serious As toxicity. The proportion of As in solution phase is easily absorbed by rice seedlings. The concentrations of As in soil solutions were increased by the applications of organic amendments, and thus As uptake by rice seedlings was enhanced, while the extent of increase varied depending on soils and organic amendments. The application of organic amendments will increase the DOC concentration in soil solutions while the extent of increase was the highest under the SB treatment. The DOC concentrations were significantly correlated to the As concentrations in soil solution, presumably that DOC can compete with As for adsorption sites at soil surface or forming soluble OM-As complexes. On the other hand, the increase of soil pH and the decrease of the soil redox potential (Eh) were also observed. The variation of soil characteristics resulted in difference of elevating As release to the soil solution amended with organic amendments, and the lowest effect were found under the CD treatments. Cf soil which has high pH and low levels of clay content and iron and aluminum oxides content was found to have low affinity for As and thus resulting in As concentration in soil solutions of 4% SB treatment was higher than 15 mg L-1. In conclusion, the addition of organic amendments, especially for more decomposable organic amendments could increase the As release and consequent As uptake by rice. This effect could be more obvious in soils with high pH value and low As sorption ability which was resulting in the increase of As toxicity of rice seedlings. Therefore, it is important to evaluate the properties of organic amendments and soils carefully while adding organic materials into As-enriched soils.