ABSTRACT Nitrogen (N) fertilization changes both the degradation rate and chemical composition of plant litter and soil organic matter (SOM). We sought to document how N application rates and incorporation of crop straw residues in wheat-rice rotation systems are related to the chemical composition of SOM. Using Fourier transform infrared spectroscopy and 13C multiple cross-polarization/magic angle spinning nuclear magnetic resonance spectroscopy techniques, we investigated the chemical composition of SOM with incorporation of crop straw residues in a 6-year wheat-rice double-cropping system. Nitrogen fertilizer was applied at the rates of 0, 290, 380, and 470 kg N ha−1 per year along with the incorporation of crop residues into the soil. We found that greater additions of N increased the concentrations of both C and N in the soil. The Fourier transform infrared spectra of SOM indicated that the abundance of aromatic functional groups (~1,628 and ~1,515 cm−1) and the amide groups (~1,660 and ~1,550 cm−1) increased in the N-amended treatments compared with the unamended treatment. 13C multiple cross-polarization/magic angle spinning nuclear magnetic resonance spectroscopy results revealed that the abundance of recalcitrant compounds, including alkyl and aromatic C, increased, whereas labile SOM components, dominated by O-alkyl and anomeric C, decreased with the elevated N application rates. These observations were also reflected in calculated indices of A/O-A aromaticity and hydrophobicity. Our results support the hypothesis that recalcitrant compounds of SOM appeared to be enhanced with the increase of the N application rates to promote crop straw decomposition in the rice-wheat rotation system.

中文翻译：

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施氮增加了土壤有机质顽固化合物的丰度

摘要 施氮 (N) 会改变植物凋落物和土壤有机质 (SOM) 的降解速率和化学成分。我们试图记录小麦-水稻轮作系统中氮肥施用率和作物秸秆残留与 SOM 的化学成分之间的关​​系。使用傅里叶变换红外光谱和 13C 多重交叉极化/魔角旋转核磁共振光谱技术，我们研究了 SOM 的化学成分，并在 6 年的小麦 - 水稻双作系统中掺入了农作物秸秆残留物。以每年 0、290、380 和 470 kg N ha-1 的比例施用氮肥，同时将作物残留物掺入土壤中。我们发现添加更多的 N 会增加土壤中 C 和 N 的浓度。SOM 的傅立叶变换红外光谱表明，与未改性处理相比，N 改性处理中芳香族官能团（~1,628 和~1,515 cm-1）和酰胺基团（~1,660 和~1,550 cm-1）的丰度增加治疗。13C 多重交叉极化/魔角自旋核磁共振波谱结果显示，顽固化合物的丰度，包括烷基和芳族 C，增加，而不稳定的 SOM 组分，以 O-烷基和异头 C 为主，随着 N 应用的增加而减少率。这些观察结果也反映在 A/OA 芳香性和疏水性的计算指数中。