Objectives Solid-state ¹³C-NMR techniques can be used to qualitatively and quantitatively characterize organic functional groups. Cross polarization / magic angle spinning / total suppression of sidebands (CP/MAS/TOSS) technique provides limited information on carbon functional groups. Dipolar dephasing (DD) is a spectral editing technique of solid ¹³C-NMR, which is used to distinguish protonated and non-protonated carbon or mobile and rigid components in organic compounds based on ¹³C-¹H heteronuclear dipole coupling with specific dephasing delay time (T_D). The objective of this study is to obtain more information on carbon functional groups in organic fertilizer by DD technique.

Methods The organic fertilizer samples included 3 kinds of straw (wheat straw, corn straw, rice straw), 3 kinds of manure (chicken manure, cow manure, pig manure) and 3 kinds of humic acid fertilizer (ammonium humate, sodium humate, potassium humate) samples. The spectra of organic fertilizer samples from different sources were determined by CP/MAS/TOSS conpined with DD technology, and the carbon distribution ratios were compared.

Results O-alkyl C of cellulose origin are dominant in straw and manure samples, accounting for 55% and 31%−37% of the total organic carbon, respectively, and the proportion of non-protonated O-alkyl carbon did not exceed 3% in either straw or manure. Manure had more non-protonated fatty and aromatic carbon than straws, the proportions of non-protonated aromatic carbon in chicken manure and cow manure were 15% and 11%, respectively, while that in straws and swine manure was as low as 3%−4%. The percentage of methoxycarbon in manures and straws was lower than the percentage of alkyl nitrogen. Aromatic, carboxyl and amide carbon were the highest in humic matter samples, with 48% to 72% of aromatic carbon and no less than 12% of carboxyl or amide carbon, non-protonated aromatic carbons in humic matter samples account for 25% to 37%, indicating that there may be more substitutions of aromatic skeleton in humic matter fertilizers.

Conclusions The DD combined with CP technique with 40 µs delay time (T_D) can be used to identify protonated and non-protonated carbon or mobile and rigid components in organic fertilizers and to study the characteristics of carbon functional groups in straw, manure and humic matter fertilizers. It is suggested that the aromaticity of humic acid fertilizers is much higher than that of manures and straws.