稻田满江红溶解性有机质光谱特征及其对汞甲基化影响

Spectral characteristics of dissolved organic matter derived from Azolla imbricata in rice paddy and its effect on mercury methylation

满江红作为稻田重要的有机质输入来源,研究其溶解性有机质(DOM)对稻田土壤中汞形态及转化机理具有重要意义.以稻田满江红DOM为研究对象,运用紫外吸收光谱(UV-Vis)和三维荧光光谱(3D-EEM)技术对不同腐解时间满江红DOM进行表征;此外,通过树脂串联技术分离得到疏水酸性物(HOA)、疏水碱性物(HOB)、疏水中性物(HON)、亲水酸性物(HIA)、亲水碱性物(HIB)、亲水中性物(HIN)等6种DOM亚组分,将不同腐解时间提取的满江红DOM及其亚组分与Hg^(2+)溶液混合培养,分析其对汞甲基化过程的影响,以期为提高汞污染农田生产安全性提供理论依据.结果表明,腐解初始时满江红DOM的芳香性最大,随腐解进行呈骤降缓升趋势.腐解中期小分子量DOM较多.类蛋白质组分始终占据主导地位,腐解后期含量缓慢减少,类腐殖质组分含量则呈现先低后高的趋势.甲基化实验表明,不同腐解时间满江红DOM对汞的甲基化反应均具有促进作用,当DOM芳香性较高,分子量较小时,其促进汞甲基化能力增强,腐解初始时DOM促进汞甲基化作用最强,随腐解时间呈现先降低后升高逐渐平稳的趋势.疏水性组分对汞甲基化促进作用显著高于亲水性组分,其中HOB促汞甲基化能力最强.腐解后期HIA、HIN对汞甲基化无促进作用.

The dissolved organic matter(DOM)derived from Azolla imbricata,one of major organic matter contributors in the rice field,plays an important role in the morphology and transformation mechanism of mercury in paddy soil.In this study,taking DOM of A.imbricata from paddy field as research object,such advanced instruments as UV Absorption Spectroscopy and 3D Fluorescence Spectroscopy were employed to characterize the DOM derived from A.imbricata which was sampled at different intervals during the decomposition of A.imbricata biomass.In addition,six sub-fractions of the DOM(HOA,HOB,HON,HIA,HIB,HIN)were obtained via a tandem resin setup.The DOM and its subfractions sampled from the different interval during decomposition of A.imbricata were mixed with Hg^(2+)solution respectively,in an attempt to reveal their effects on the mercury methylation process and thereafter provide theoretical basis for improving the production safety of mercury-contaminated farmland.The results showed that the highest SUVA254 value of the DOM was observed at the initial stage(0 day)of the decomposition experiment,followed by a sudden decline and slow rise as the decay of A.imbricata biomass lasted.Small molecular weight DOM increases in the middle stage of decomposition.In accordance with spectral of fluorescence spectroscopy,the protein-like component dominated the whole decomposition process,with a slight decline at the late stage.The humus-like component decreased at the early stage and then increased gradually.The DOM derived form A.imbricata could promote the methylation of mercury at different decomposition intervals.And DOM with high aromaticity and small molecular weight has a stronger ability to promote MeHg production.It was at the initial time of decomposition that DOM exhibited the strongest promoting effect on the production of methylmercury,and followed by a trend of decreasing first and then increasing gradually.The hydrophobic component showed a significantly higher capacity than the hydrophilic ones in term of promoting the methylation reaction of mercury.Among six subfractions,HOB seemed to be the greatest contributor toward the promotion of the methylation of mercury while the contribution from hydrophilic components such as HIA and HIN was negligible,especially in the later stage of decomposition.