TECHNICAL ARTICLENitrogen Application Increases Abundance of Recalcitrant Compounds of Soil Organic Matter A 6-Year Case StudyChen, Xi1; Jin, Mengcan1; Zhang, Yajie1; Hu, Jingwei1; Gao, Hongjian1; Chu, Wenying2; Mao, Jingdong2; Thompson, Michael L.3Author Information 1Anhui Province Key Laboratory of Farmland Ecological Conservation and Pollution Prevention, School of Resources and Environment, Anhui Agricultural University, Hefei, China. 2Department of Chemistry and Biochemistry, Old Dominion University, Norfolk, Virginia, USA. 3Agronomy Department, Iowa State University, Ames, Iowa, USA. Address for correspondence: Dr Hongjian Gao, Anhui Province Key Laboratory of Farmland Conservation and Pollution Prevention, School of Resources and Environment, Anhui Agricultural University, 130 Changjiangxi Road, Hefei, Anhui 230036, China. E-mail: [email protected] Financial Disclosures/Conflicts of Interest: None reported. Received August 29, 2018. Accepted for publication February 4, 2019. Soil Science: September/October 2018 - Volume 183 - Issue 5 - p 169-178 doi: 10.1097/SS.0000000000000243 Buy Metrics Abstract 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. Copyright © 2018 Wolters Kluwer Health, Inc. All rights reserved.