Objective The aim was to explore the methods of using thermal analysis and infrared spectroscopy to determine the chemical structure and thermal stability of soil organic carbonation, so as to provide scientific basis for simultaneous, rapid and reliable characterization of organic carbon composition and content.

Methods Based on the long-term positioning experiment, 7 fertilization treatments were set up. There were no control fertilization (CK), single application of NPK fertilizer (NPK), 25%, 50%, 75% and 100% of fertilizer nitrogen were provided by organic fertilizer nitrogen (25%M, 50%M, 75%M and 100%M, respectively), and the addition of wheat and corn straw returning to field (SNPK) on the basis of NPK treatment. The chemical structure and stability of soil organic carbonation under different fertilization treatments were determined by thermal analysis and infrared spectroscopy.

Result The results showed that the uniformity and representativeness of the sample were ensured, and the stability of the sample curve and the complete combustion of the sample were ensured when the soil sample of 10 mg was measured at the heating rate of 10 K min⁻¹ and the gas flow rate of 20 mL min⁻¹, with the empty crucible as the control and the test background value deducted. Based on this, the thermal analysis curve characteristics of soil under different fertilization treatments were further analyzed. It is found that the weight loss of the sample was mainly divided into the loss of chemical binding water and hygrometric water of salt and organic matter (100℃-200℃), the loss of dehydrogenation and decarboxylation of organic matter (200℃-400℃), the loss of dehydroxylation of clay minerals and the loss of very fine particles (400℃ ~ 400℃). There were 4 stages of volatilization loss (650℃-900℃) of different kinds of carbonates. According to the characteristic peak of infrared spectrum, it was found that the compound components of soil organic carbon were mainly carboxylic acids, alcohols, phenols and polysaccharides under long-term application of organic and inorganic fertilizers. The weight loss in the second stage of thermal analysis was significantly correlated with organic carbonation functional group components, especially with alcohols, phenolic compounds and aromatic compounds (P < 0.001).

Conclusion In summary, both thermal analysis and infrared analysis are effective methods for semi-quantitative analysis of soil organic carbon quality and composition, and infrared spectral characteristic peak and absorption peak area are also effective means for characterizing organic carbonation functional groups. The mutual verification and combination of the two can more effectively analyze the changes of soil organic carbonation chemical structure under different organic fertilizers instead of chemical fertilizers.