长期稻草还田对胡敏酸化学结构的影响——高级13C NMR研究

为了从分析技术上对胡敏酸的结构分异进行定量化研究,以湖南桃源地区的长期(1990—2011年)田间试验为平台,采用元素分析、稳定碳同位素分析和高速多重倾斜幅度交叉极化/魔角自旋(Multiple cross-polarization/magic angle spinning,multi CP/MAS)定量测定技术结合选择性官能团测定的固态13C核磁共振(Nuclear magnetic resonance,NMR)方法,旨在利用系列技术阐明连续22年稻草还田对土壤胡敏酸化学结构的影响。结果表明,稻草还田后胡敏酸含量增加了78%,但元素组成没有发生显著变化,其分子饱和程度、氧化度和极性都与不施肥对照很接近。稻草还田土壤胡敏酸的δ~(13)C值低于对照,表明长期稻草还田提高了非极性物质来源的碳。定量multi CP/MAS NMR方法则证实稻草还田后主要提高了土壤胡敏酸的脂类、芳香族物质和木质素比例,而蛋白质、多肽和糖类物质的比例则有所降低,从而导致烷基/烷氧基比值及疏水性指数增大,表明其分解程度增加,疏水性特征更加明显。高级NMR技术进一步发现胡敏酸中有大量的质子化碳,稻草还田后质子化芳香碳比例增加,但质子化异头碳和烷氧基比例降低,而它们的非质子化碳相对稳定;δ113~93化学位移内除异头碳外还含有芳香碳,且稻草还田的芳香碳在此化学位移内的比例更高。可见稻草还田后土壤胡敏酸的量和结构均有所变化,高级测定技术对结构的刻画有助于了解长期稻草还田肥力的提升机制。

Advanced 13C NMR analysis of humic acid after long-term rice straw incorporation into paddy soil

As an essential component of soil organic matter, humic acid may vary in chemical composition after long-term straw incorpora-tion. However, the quantitative studies on structural differentiations of soil humic acid were limited by the development of analytical tech-niques, especially for samples in the paddy soil. This study investigated the qualitative and quantitative characteristics of humic acids ex-tracted from a paddy soil from Taoyuan County in China after 22 years of rice-rice(Oryza sativa L.)rotations, with two fertilizations i.e. no fertilizer application as a control and rice straw incorporation. Several analytical techniques including elemental analysis, isotopic analysis (δ13C)and advanced 13C NMR[i.e. multiple cross-polarization/magic angle spinning(multiCP/MAS)and spectral-editing techniques] were used. Results indicated that the yield of humic acid increased from 2.25 g·kg-1(as in the control)to 4.01 g·kg-1 in the rice straw incorpo-ration treatment, whereas elemental compositions of the two humic acids were similar. The average atomic ratios of H/C, O/C and (N+O)/C were 1.35, 0.51 and 0.60, respectively, indicating the two humic acids experienced similar degree of saturation, oxidation, and polarity. In comparison with the control, theδ13C value of humic acid decrease from-2.783%to-2.849%after straw incorporation, which represents the contents of apolar components increased. The increase was verified through multiCP/MAS results and was ascribe to the higher proportion of lipids, aromatics and lignin. The contents of proteins, peptides, and carbohydrates decreased after rice straw incorporation. The conversion of constituents resulted in higher ratio of the alkyl C to O-alkyl C in humic acids and improved their hydrophobicity, indicating the higher de-composition degree and more obvious hydrophobic characteristics of humic acids after straw incorporation. As revealed by advanced NMR results, substantial protonated carbon existed in two humic acids. More protonated aromatic carbon were found in humic acids after straw in-corporation while the protonated anomerics and O-alkyls in the samples decreased. The proportion of nonprotonated carbon were indepen-dent of the rice straw incorporation. Specially, aromatics betweenδ113 andδ93 were found in both humic acids while the experimental sam-ples showed higher proportion. Our results indicate that both the quantity and chemical structures of humic acid changed after rice straw in-corporation and stress the importance of advanced NMR techniques in elucidating the fertility-improving mechanism through rice straw in-corporation on soil.