森林新近凋落叶溶出DOM的性质及其对菲增溶作用的影响

凋落叶作为森林凋落物的主要组成部分，其溶出的大量有机质也是森林土壤可溶性有机质（DOM）的主要来源之一。研究森林凋落叶溶出DOM对PAHs增溶作用的影响有利于合理预测及评价森林土壤中PAHs的环境行为和生态风险。本研究采集了南亚热带常绿阔叶人工林的4种常见树种--尾叶桉（Eucalyptus urophylla）、木荷（Schima superba）、大叶相思（Acacia auriculiformis）和湿地松（Pinus elliottii）的新近凋落叶为试验材料，研究其DOM含量、组成与性质，对比分析了不同凋落叶DOM对菲的増溶作用及其与DOM性质的相关关系。结果表明，4种凋落叶的可溶性有机碳（DOC）质量分数在C 11.61~36.25 mg·g-1之间，其中尾叶桉的含量最大，湿地松最小。尾叶桉和木荷DOM的主要组分是可溶性糖（SS）和可溶性酚（SP），两者总C量占DOC的比例超过47%，而大叶相思和湿地松中SS和SP两者总量所占比例均低于30%。另外，4种凋落叶DOM的质量分数（以C计）与其电导率的线性关系图中有明显转折点，说明它们均具有表面活性剂的性质。凋落叶DOM在临界胶束浓度（CMC）之上对菲具有不同程度的増溶作用，其与菲的结合系数（logKDOC）的大小顺序为尾叶桉（3.05 L·kg-1）＞木荷（3.02 L·kg-1）＞大叶相思（2.79 L·kg-1）＞湿地松（2.54 L·kg-1），这表明尾叶桉和木荷DOM的增溶作用明显高于大叶相思和湿地松DOM。经分析表明，logKDOC与各DOM在254、280 nm处的特征紫外吸光度值（SUV-A254、SUV-A280）及其SS、SP的相对含量均呈显著正相关（p＜0.01），与A240/A420、A254/A400比值呈显著负相关（p＜0.01），说明DOM的芳香化程度越高，分子量越大， SS与SP所占比例越高，其对菲的増溶效果越明显。

Characteristics of fresh leaf litter-derived dissolved organic matter and its capacity to enhance the apparent water solubility of phenanthrene

Leaf litter, being a major component of forest litter, can produce substantial amounts of organic matter that contribute largely to dissolved organic matter （DOM） in forest soil. Understanding the effect of leaf litter-derived DOM on the solubility of PAHs helps to reasonably predict their environmental behaviors, and to comprehensively evaluate the ecological risk of PAHs in forest soil. In this study, fresh leaf litters of four common trees （i.e. Eucalyptus urophylla, Schima superba, Acacia auriculiformis and Pinus elliottii） were collected in south subtropical region of China, and the contents, compositions and properties of litter-derived DOM were determined. Besides, the capacity of DOM in increasing water solubility of phenanthrene and its correlation with DOM properties were studied. The results show that the contents of dissolved organic carbon （DOC） in leaf litters ranged from C 11.61 mg·g-1 to 36.25 mg·g-1, with the highest in the water extract of E. urophylla and the lowest in that of P. elliottii. The major components of DOM in leaf litters of E. urophylla and S.superba were soluble sugars （SS） and soluble phenols （SP） which contained over 47%of the total DOC, while in other two plant litters-derived DOM the value was less than 30%. In addition, the conductivities of four DOMs increased upon their increasing aqueous concentrations with a distinct change in the slope observed, indicating that leaf litter-derived DOM could form micelle structure and exhibit surfactant properties. The critical micelle concentration （CMC） values of four DOMs were between 239.1 mg·L-1 to 558.8 mg·L-1. The apparent water solubility of phenanthrene was increased to some degree by the four DOMs when their DOC concentrations were above CMC values. The partition coefficients of phenanthrene to four DOMs （logKDOC） varied in the order of E. urophylla （3.05 L·kg-1）〉S. superba （3.02 L·kg-1）〉A. auriculiformis （2.79 L·kg-1）〉P. Elliottii （2.54 L·kg-1）, indicative of a stronger solubilization of phenanthrene by the water extracted DOM from E. urophylla and S. superba than A. auriculiformis and P. elliottii. Significant positive correlations （p〈0.01） were found between values of logKDOC and specific UV absorption values at 254 nm and 280 nm （SUV-A280, SUV-A254）, and between logKDOC and the relative contents of SS and SP, while significant negative correlations （p〈0.01） were found between logKDOC and the values of A240/A420 and A254/A400. This implies that leaf litter-derived DOM with more aromatic structures, higher molecular weight and higher proportions of SS and SP, tends to hold a greater capacity to enhance phenanthrene solubility.