生物炭溶解性有机质中不同分子量组分的分布及光谱特性

生物炭（BC）施加至土壤后会释放出溶解性有机质（DOM），能够改变土壤DOM的含量和化学性质，进而对土壤DOM的环境行为产生重要影响。BC DOM的分子组成和结构决定了其复杂的环境行为，然而目前针对其分子量组分的研究几乎为空白。本研究以稻秆和猪粪为原料，在300，400和500 ℃分别制备生物炭，利用纯水萃取-过滤得到DOM，采用超滤方法将其分离为<1，1～5和>5 kDa(千道尔顿)组分。通过溶解性有机碳（DOC）、紫外-可见光光谱（UV-Vis）、三维荧光光光谱结合区域体积积分（EEM-FRI）系统解析了BC DOM中不同分子量级组分的含量和光谱特征。结果显示，不同裂解温度下稻秆和猪粪BC DOM的DOC在<1，1～5和>5 kDa组分中的分布分别为42%～60%，16%～23%和23～29%，α₂₅₄的分布范围分别为4%～27%，8%～49%和26%～81%。表明BC DOM的DOC主要分布在<1 kDa组分，而发色物质主要分布在>5 kDa组分。400和500 ℃下BC DOM中>5和1～5 kDa组分的分子量和芳香度明显高于300 ℃下。相比而言，稻秆BC DOM中>5 kDa组分比猪粪的含有更多的芳香族结构，而猪粪BC DOM中<1 kDa组分的芳香度却高于稻秆。稻秆和猪粪BC DOM中各级分子量组分均具有相似的EEM光谱特征，表明BC DOM是一种连续有机体系。稻秆和猪粪BC DOM的分子量级组分分别以类富里酸和低激发色氨酸荧光物质组成为主。BC DOM中<1，1～5和>5 kDa组分的FI和BIX基本呈现依次降低趋势，而HIX值则呈现出依次升高的趋势，结果表明BC DOM中高分子量组分富集了较多的具有高芳香性和高腐殖化程度的有机组分。研究结果将进一步提升对BC DOM的分子量组成和结构特征的认识，同时可以为准确评估BC DOM的环境行为提供重要的基础数据。

Abundance and Spectral Characteristics of Molecular Weight Separated Dissolved Organic Matter Released From Biochar at Different Pyrolysis Temperatures

Biochar (BC) returned to soil will release amounts of dissolved organic matters (DOM), which can change soil-DOM’s content and chemical properties and then have important impacts on their environmental behaviors. The molecular composition and structures of BC DOM would determine their complex environmental behaviors. However, the studies on chemical composition, especial on molecular weight (MW) separated fractions, are still limited. In this study, two types of biochar were firstly produced by pyrolyzing rice straw, and pig manure at different temperatures (300, 400 and 500 ℃), and then DOM therein was fractionated into three MW fractions, including <1, 1~5 and >5 kDa, using ultrafiltration method. Subsequently, the content and optical properties of MW fractions were investigated based on dissolved organic carbon (DOC), ultraviolet-visible (UV-vis) spectra, and excitation-emission matrix spectra combined with the regional integral protocol (EEM-FRI). The results showed that the proportional DOC distributions of <1, 1~5 and >5 kDa into bulk BC DOM were 42%~60%, 16%~23% and 23%~29%, respectively. The corresponding distributions on α₂₅₄ were 4%~27%, 8%~49% and 26%~81%. These results suggested that the major OC species in bulk BC DOM were portioned into <1 kDa fractions, whereas the major chromophores were partitioned into >5 kDa fractions. The >5 and 1~5 kDa fractions in BC DOM produced at 400, and 500 ℃ were generally contained higher MW and aromaticity than those at 300 ℃. The >5 kDa fractions within rice straw-derived BC DOM contained more aromatic structures than pig manure-derived ones, while the <1 kDa fractions within latter ones contained more aromatic ones. It was worth noting that MW fractions in rice straw and pig manure-derived BC DOM almost exhibited similar EEM spectra characteristics, indicating that BC DOM were complex organic compounds with a chemical continuum. Additionally, EEM spectra showed that fulvic-like and short-wavelength tryptophan-like fluorophores were predominant in MW fractions within rice straw- and pig manure- derived BC DOM, respectively. With increasing MW fractions, fluorescence index (FI) and autochthonous index (BIX) of DOM decreased, while humification index (HIX) increased, implying that high MW fractions enriched organic matters with high aromatic and humification degree. The study enhanced our understanding of the molecular composition and structures of BC DOM, which could also provide beneficial references for accurately evaluating environmental behaviors of BC DOM.