农业土壤中土壤有机质和腐殖酸不同粒级的结构表征

Structural Characterization of Soil Organic Matter and Humic Acids in Particle-Size Fractions of an Agricultural Soil

农业土壤中的有机质主要来源于植物残体的部分分解、微生物生物量以及腐殖物质。13C核磁共振、羟化四甲铵气相色谱/质谱法和热解气相色谱/质谱法等先进的分析技术被用来研究地中海的深褐色土壤中土壤有机质及相应的腐殖酸的化学结构。羟化四甲铵法对大多数土壤和腐植酸样品的热化学分析的主要产物是木质素提取复合物和非木质素芳香化合物、杂环氮、脂肪酸甲酯和二羧酸二酯。在>250μm的羟化四甲铵色谱分馏所得的木质素提取复合物要明显多于<2μm时的。脂肪酸甲酯和二羧酸二酯团长链峰值的相对强度在团聚体大小上有所降低,但是他们却对土壤有机物质的结构表现出明显的贡献。热解气相色谱/质谱法和羟化四甲铵气相色谱/质谱法数据表明在腐植酸的结构中含有大量的木质素和表皮材料。在氧化过程的最后阶段随着颗粒大小的降低腐植酸样品中含有木质素提取物单元,而来源于微生物的活动多种脂肪酸以及高含量的非木质素芳香结构。我们的数据表明:新鲜的有机物质被分解和腐化成腐植酸时存在一个类似于结构的稳定阶段,但这部分产物进一步分解(主要是通过氧芳香碳取代氧烷基)转化为腐化产物。

Abstact：Organic matter （OM） in agricultural soils consists mainly of partially decomposed plant residues, microbial biomass, and humic substances. The advanced analytical techniques of 13C nuclear magnetic resonance （NMR）, tet- ramethylammonium hydroxide （TMAH） therrnochemolysis-gas chromatography （GC）/mass spectroscopy （MS）, and pyrolysis-GC/MS （Py-GC/MS） were employed to study the chemical structure of bulk soil organic matter （SOM） and its corresponding humic acid （HA） extracts from different aggregate-size fractions （〈2 and 〉250 prn） of a dark brown Mediterranean soil. The main products released by TMAH thermochemolysis of the bulk soil and HA samples were：lignin-derived compounds （LG） and nonlignin-derived aromatic compounds, heterocyclic N, fatty acid methyl esters （FAMEs）, and dicarboxylic acid dimethyl esters （DAMEs）. The TMAH chromatogram of the 〉250-gin size fraction revealed more LG than that of the 〈2-μm fraction. The relative intensity of the long-chain FAMEs and DAMEs peaks decreased with aggregate size, but their presence highlights the contribution of aliphatic biopolymers to the structure of the SOM. Both Py-GC/MS and TMAH-thermochemolysis data suggest that the HAs contain large portions of lignin and cuticular materials in their structure. With decreasing particle size, the HA contained more lignin-derived units in the final stages of oxidation, more fatty acids originating from microbial activity, and higher contents of aromatic nonlignin- derived structures. Our data suggest that a steady-state situation exists for the presence of HA, where fresh OM is be- ing decomposed and humified into the HA-like structures, but this fraction is subject to further decomposition （mainly through loss of O-alkyl and transformation of O-substituted aromatic carbons） as humification proceeds.