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| Advanced 13C NMR analysis of humic acid after long-term rice straw incorporation into paddy soil |
| Received:August 11, 2016 |
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| KeyWord:humic acid;paddy soil;multiCP/MAS;δ13C;elemental analysis |
| Author Name | Affiliation | E-mail | | XU Ji-sheng | State Key Laboratory of Soil and Sustainable Agriculture(Institute of Soil Science, Chinese Academy of Sciences), State Experimental Station of Agro-Ecosystem in Fengqiu, Nanjing 210008, China
University of Chinese Academy of Sciences, Beijing 100049, China | | | ZHAO Bing-zi | State Key Laboratory of Soil and Sustainable Agriculture(Institute of Soil Science, Chinese Academy of Sciences), State Experimental Station of Agro-Ecosystem in Fengqiu, Nanjing 210008, China | bzhao@issas.ac.cn | | ZHANG Jia-bao | State Key Laboratory of Soil and Sustainable Agriculture(Institute of Soil Science, Chinese Academy of Sciences), State Experimental Station of Agro-Ecosystem in Fengqiu, Nanjing 210008, China | |
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| Abstract: |
| As an essential component of soil organic matter, humic acid may vary in chemical composition after long-term straw incorporation. However, the quantitative studies on structural differentiations of soil humic acid were limited by the development of analytical techniques, especially for samples in the paddy soil. This study investigated the qualitative and quantitative characteristics of humic acids extracted 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 incorporation 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, the δ13C 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 decomposition 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 incorporation while the protonated anomerics and O-alkyls in the samples decreased. The proportion of nonprotonated carbon were independent of the rice straw incorporation. Specially, aromatics between δ113 and δ93 were found in both humic acids while the experimental samples showed higher proportion. Our results indicate that both the quantity and chemical structures of humic acid changed after rice straw incorporation and stress the importance of advanced NMR techniques in elucidating the fertility-improving mechanism through rice straw incorporation on soil. |
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