水分管理對兩種受砷污染土壤種植水稻累積砷之影響

Abstract

於砷污染水田耕作水稻有較高的穀粒砷污染風險，土壤水分管理被認為是降低穀粒砷污染風險的可行方案，在台灣的水稻生產中已存在慣行的水分管理模式，故本試驗中採用三種以降低穀粒砷污染風險所設計的水分管理模式，評估其是否較慣行方法更能降糙米砷濃度。水分管理方法描述如下：(1) 水稻插秧後35日進行10天土壤好氧處理的慣行方法 (CK)，(2) 抽穗前進行3週土壤好氧處理 (A)，(3) 抽穗後進行3週土壤好氧處理 (B)，與(4) 抽穗前進行3週土壤好氧處理後，再持續3週土壤好氧處理 (C)。由於土壤性質亦影響水分管理成效，本試驗選用外添加砷後乾濕交替四個月之二林系(外添加10 及20 mg/kg砷)與平鎮系(外添加20 及40 mg/kg砷)土壤為供試土壤，試驗水稻品種為台南11號(Oryza sativa L. Cv Tainan 11)。監測結果顯示於好氧處理的期間土壤氧化還原電位提升60-400 mV，二林系土壤之土壤溶液中砷、鐵、錳濃度皆顯著下降，然此現象在平鎮系土壤中不明顯，連續浸水情況下土壤溶液中也未監測到大幅度砷濃度提昇。在二林系土壤各水分管理處理間糙米砷濃度未有顯著差異；在平鎮系外添加40 mg/kg砷的土壤中，A處理下糙米砷濃度 (0.29 mg/kg) 顯著低於CK 處理 (0.42 mg/kg) 。於平鎮系土壤種植之水稻總砷吸收量隨土壤砷濃度提昇而增加；但在土壤有效性砷含量高的二林系土壤中，水稻總砷吸收量反而隨土壤砷濃度的增加而降低，除了砷逆境可能影響水稻生長外，也可能存在限制砷傳輸至穀粒之生理調控機制。在有效性砷濃度未達影響水稻生長的等級，但仍會造成糙米砷含量過高的土壤種植水稻，如本試驗中平鎮系添加40 mg/kg砷的土壤，水稻抽穗前3週的土壤好氧處理（策略A）是降低糙米砷污染風險較可行的方法。

Growing rice on arsenic contaminated paddy soils existed higher risk of grain arsenic contamination. The risk was considered to be reduced by water management. In Taiwan, conventional water management strategy will discharge water after the maximum tiller stage which has been practiced in paddy rice cultivation for decades. Therefore, three water management strategies that designed to reduce the risk of rice contaminated by arsenic were selected to evaluate the effects on lowering arsenic accumulation in brown rice with the conventional method. Four water management treatments were listed as followings: (1) Aerobic treatment for 10 days after 35 days of rice seedling, denoted as CK, (2) aerobic treatment for 3 weeks before rice heading, denoted as A, (3) aerobic treatment for 3 weeks after rice heading, denoted as B, and (4) aerobic treatment for 3 weeks before rice heading and continue for another 3 weeks after rice heading, denoted as C. Two different soils (Erlin soil denoted as Eh and Pinchen soil denoted as Pc) with two arsenic spiking dose for each soil series (10 and 20 mg As/kg soil for Erlin soil and 20 and 40 mg As/kg soil for Pinchen soil) were used for this study. The rice cultivar, Oryza sativa L. Cv Tainan 11, was selected for this study. The monitoring results showed that the soil redox potential increased about 60-400 mV during the aerobic treatment period. Simultaneously, the concentration of arsenic, iron and manganese of soil solution decreased significantly in Erlin soil during areobic treatments. However, the decresing of arsenic concentration was not obvious in Pinchen soil for same aerobic treatment. During the flooding time, the arsenic concentration of soil solution was not increased dramatically in Pinchen soil. For the effects of water management on lowering the As concentration of brown rice, there was no significant difference among all treatments for Erlin soil. In Pinchen soil spiked with 40 mg As/kg soil, the As concentration of brown rice of treatment A (0.29 mg.kg) was significantly lower than CK treatment (0.42 mg/kg). The total As uptake of rice plant increased with increasing soil As concentraion in Pinchen soil. However, the Erlin soil spiked with As produced the high available As, the As uptake in the brown rice decreased with increasing the soil As concentration. It is possilbe that a plant physiological mechanism regulate the As transport in the brown rice except the reason of As stress to affect the rice growing. For the Pinchen soil spiked with 40 mg As/kg soil, the soil has enough bioavailable arsenic enhance the As content in brown rice but not induce toxic effect on rice growing, soil aerobic treatment for 3 weeks before the rice heading (treatment A) was the good method to reduce the health risk of brown rice growing at As- contaminated soils.