Photodegradation of humic acid from the surrounding soils of a sub-alpine Lake Tiancai

Abstract

溶解性有机质（DOM）是湖泊生态系统的物质循环和能量流动的基础.典型亚高山湖泊DOM的主要来源为流域土壤腐殖酸（外源DOM），其具有较强的光吸收特性，光降解是其重要的转化过程，从而改变碳、氮、磷等物质的存在形式.在气候变暖的背景下，亚高山湖泊中DOM的浓度、pH、溶解氧及太阳辐射都可能发生变化；但这些环境变化对亚高山湖泊DOM光降解过程的影响尚不够清楚.由此本文研究了亚高山湖泊——天才湖（树线下200 m）周边土壤提取的腐殖酸（THA）的光降解过程：首先考察在光源为500 W汞灯、不通气以及pH=8.0条件下THA的降解过程中DOC浓度、有色溶解性有机质（CDOM，以a₂₈₀表征）和荧光溶解性有机质（FDOM）的变化，其后定性比较、定量分析不同光降解条件（pH、载气和光照波段）对THA的光降解过程的影响.结果表明，THA含有4种荧光组分，其中C1~C3均为类腐殖质，占总荧光强度83.64%，C4占比16.36%，为类蛋白质物质.在500 W汞灯的照射下，THA分子量逐渐降低，10 h后DOC浓度降低44.5%，a₂₈₀随光照时间的增加逐渐减少，CDOM含量较初始大幅度下降；而FDOM （以荧光组分强度表示）总体趋势表现为先增加后下降.相对于弱碱性的环境（pH=8.0），弱酸性环境（pH=6.0）更有利于THA的光化学过程.氧气的存在很大程度上促进了DOM光化学转化过程，通入空气的条件下a₂₈₀的降解速率是氮气条件下的3.25倍.在不同光波段的实验下表明，可见光和紫外光对THA降解的贡献差异较大，紫外光波段的表观光子降解率（η）约为可见光的5~11倍.结果表明在气候变化背景下，亚高山湖泊中DOM的光化学降解将可能加快，亚高山湖泊在碳循环中的作用可能增强.;Dissolved organic matter (DOM) in lakes is ubiquitous and plays an important role in the aqueous ecosystem. DOM in sub-alpine lakes is dominated by the terrestrially-derived humic acid in the soil. Photodegradation is an important transformation process of DOM because of its strong light absorption, changing the form of carbon, nitrogen, phosphorus. Under the scenario of climate warming, DOM concentration, pH, dissolved oxygen and solar radiation in subalpine lakes may change. However, the effects of these environmental changes on DOM photodegradation in sub-alpine lakes are still unclear. In this paper, the photodegradation of allochthonous DOM in a sub-alpine lake-Lake Tiancai (located 200 m below the tree line) were studied. Firstly, the humic acid (THA) was extracted from the soil around Lake Tiancai. Then the photodegradation of THA was investigated at different light bands (ultraviolet or visible light) and different pH conditions. The results showed that four components were identified by excitation-emission matrix fluorescence coupled with parallel factor analysis (EEM-PARAFAC), including C1-C3 were humic-like fluorescence, accounting for 83.64% of the total fluorescence intensity, and C4 was protein-like substance, accounting for 16.36%. Under the irradiation of 500 W mercury lamp, the molecular weight of THA decreased gradually and DOC concentration decreased by 44.5% after 10 h. The value of a₂₈₀ (a proxy for colored dissolved organic matter (CDOM)) decreased gradually with the irradiation time; while the fluorescent dissolved organic matter (FDOM) showed an overall trend of increasing at first and then decreasing. Compared with the weakly alkaline media (pH=8.0), the weakly acidic media (pH=6.0) facilitated for the photochemical process of THA and acidification may further promote DOM degradation. Oxygen played a crucial role in DOM photochemical reaction as the pseudo-first-order kinetic constant of a₂₈₀ in the presence of oxygen was 3.25 times that in the absence of oxygen. The ultraviolet light was much more efficient for THA degradation than visible light. The apparent quantum degradation ratios (η) of ultraviolet light were about 5-11 times that of visible light. This result of this study suggested that under the scenario of climate change, the photodegradation of DOM in sub-alpine lakes might accelerate, enhancing the role of sub-alpine lakes in the carbon cycle.