於水田土壤以早苗蓼作為綠肥對水稻幼苗吸收砷相關效應探討

Abstract

除了以施用矽肥、磷肥、鐵物質、水分管理、植生復育等方法降低水稻砷有效性，藉由於土壤施用有機質提高穀粒有機砷物種比例降低無機砷物種比例也是近來降低攝食稻米帶來的砷健康風險的另一研究方向，因為有機砷物種對人體毒性較低。但過往研究也發現有機質施用會提高水稻砷有效性，以有機質添加提高水稻植體有機砷物種比例時，需同時評估砷有效性提高的風險。本研究以綠肥作物早苗蓼栽種作為有機質施用來源探討裏作早苗蓼對下一期水稻砷有效性與砷物種分布之影響。早苗蓼在水稻田為具自播性的水生植物，施用成本低廉; 此外，過往研究亦證實種植早苗蓼可藉由其根部類似鐵膜之結構固定土壤砷，降低土壤砷有效性。為兼顧砷有效性評估，本研究以關渡系、菜公厝系兩種砷吸附容量不同的土壤與淇武蘭系土壤額外添加砷等土壤進行盆栽試驗。第一階段早苗蓼盆栽試驗先種植、翻耕早苗蓼，並接續種植水稻，進行第二階段水稻盆栽試驗。早苗蓼盆栽試驗顯示，在田間容水量之下栽種早苗蓼，其植體砷總吸收量稀少，早苗蓼植體在本研究無植體吸收並固定土壤砷的功能，但也不會因植體吸收砷再釋出造成水稻砷有效性提高。水稻盆栽試驗期間孔隙水砷、鐵濃度分析顯示，翻耕早苗蓼可促進鐵氧化物還原溶解使更多鐵、砷釋出，但從孔隙水砷、鐵濃度相關性分析得知，有機質與砷競爭非特異性吸附位相可能為砷移動性提高的另一原因。孔隙水砷物種分析顯示，翻耕早苗蓼可提高孔隙水中有機砷物種比例，但同時有更多無機砷釋出。水稻植體分析顯示，在黏粒含量較高、非特異性吸持砷少的土壤如關渡低砷、淇武蘭低砷，翻耕早苗蓼雖促進砷釋出，但因土壤質地使有機質分解較慢而未造成明顯毒害，同時植體有機砷物種比例提高，可作為提高水稻有機砷物種比例的田間操作。在砂粒含量較高或非特異性吸持砷多的土壤如關渡高砷、菜公厝土壤、淇武蘭高砷土壤，因土壤質地使有機質分解較快，翻耕早苗蓼雖提高孔隙水有機砷物種比例同時有大量砷釋出造成毒害。再者，非特異性吸持砷劃分多的土壤，除了因促進鐵氧化物還原溶解使砷釋出，有機質與砷競爭吸附位相也可能是促進砷釋出的重要機制，使翻耕早苗蓼造成砷移動性提高的效應比非特異性吸持砷劃分少的土壤顯著。

Besides various mitigation methods suggested to reduce arsenic(As) bioavailability, decreasing inorganic arsenic accumulation in grain via organic matter addition into soil have been discussed in previous studies since organic As species were less toxic and may furthered leave rice plants through volatilization. However, previous studies also revealed that organic matter addition may mobilize soil-As resulting from promoted reductive dissolution of iron oxide. To evaluate feasibility of increasing organic As species by organic matter application, risk of increasing As availability should be considered at the same time. Our research aimed to investigate the effect of green manuring Black heart (Polygonum lapathifolium) into paddy soils on arsenic uptake by rice seedlings. Green manuring has received more attention recently owing to its low-cost and landscaping. Among of green manure plants, Black heart is a self-broadcasting wetland plant in paddy field and has been identified as the plant with potential on As remediation. In our research, three paddy soils in Taiwan, Guandu series (Gd), Chaikongtso series (Cl) and Chiwulan series (Ca) were used for pot experiments. There were two levels of As concentrations in each soil (i.e. high level(H): 74 -139 mg As kg-1 and low level(L): 21-38 mg As kg-1). In the results, As contents in black heart were mere comparing to that in rice plants, which implied that As releasing from black heart caused little influence on As availability of rice seedlings. Plowing black heart into soil brought slightly difference on pH and Eh measurements while As and iron(Fe) release were promoted based on porewater analysis. Competition of non-specific adsorption site between organic matter and As should also contribute to promoted As releasing after plowing black heart. Based on rice seedlings analysis, in soils that were with fine texture and less non-specifically adsorption-As such as GdL and CaL soil, plowing Black heart mobilized As without severe toxicity. In addition, increased organic-As proportion in plants was observed. In soils that were with coarse texture or more non-specifically adsorption-As such as GdH,ClL,ClH and CaH soil, plowing Black heart mobilized As and moreover, aggravated As toxicity in rice seedlings.