Nuclear magnetic resonance revealed the structural unit difference and polymerization process of pre-humic acid from different organic waste sources.

Pre-humic acid (HA) is brown humus that is only soluble in dilute alkali in natural soil. However, the mechanism underlying HA structural heterogeneity caused by material differences remains unknown. In this study, nuclear magnetic resonance (1H NMR) and fourier transform infrared spectroscopy were used to analyze the structure of HA with varying molecular weights. 1H NMR revealed that HA structures from the same source exhibit similar chemical shifts at various molecular weights, indicating that macromolecular and micromolecular HA had the same structural unit. Principal coordinate analysis demonstrated that the nitrogen-rich source of HA displayed higher structural similarity, whereas the lignin source of HA exhibited remarkable structural differences. This difference was attributed to the different contents of the 11 core structures of HA from different sources. The accuracy of the structural units from different sources is further verified by the predicted chemical shift and the root mean square error. Moreover, the interaction results indicated that HA derived from nitrogen-rich sources contains several hydrogen bonds, and the pine branch demonstrated the highest π-π interaction leading to a tightly packed three-dimensional conformation. The study on the heterogeneity of HA provides a theoretical basis for its evolution in biogeochemical cycle.