Naoya Katsumi

Effects of heating on composition, degree of darkness, and stacking nanostructure of soil humic acids

Wildfires and prescribed burning can affect both the quality and the quantity of organic matter in soils. In this study, we investigated qualitative and quantitative changes of soil humic substances in two different soils (an Entisol from a paddy field and an Inceptisol from a cedar forest) under several controlled heating conditions. Soil samples were heated in a muffle furnace at 200, 250, or 300 °C for 1, 3, 5, or 12h. The humic acid and fulvic acid contents of the soil samples prior to and after heating were determined. The degree of darkness, elemental composition, carbon and nitrogen stable isotope ratios, (13)C nuclear magnetic resonance spectra, and X-ray diffraction patterns of humic acids extracted from the soils before and after heating were measured. The proportion of humic acids in total carbon decreased with increasing heating time at high temperature (300 °C), but increased with increasing heating time at ≤ 250 °C. The degree of darkness of the humic acids increased with increasing heating time and temperature. During darkening, the H/C atomic ratios, the proportion of aromatic C, and the carbon and nitrogen stable isotope ratios increased, whereas the proportions of alkyl C and O-alkyl C decreased. X-ray diffraction analysis verified that a stacking nanostructure developed by heating. Changes in the chemical structure of the humic acids from the heated soils depended on the type of soil. The major structural components of the humic acids from the heated Entisol were aromatic C and carboxylic C, whereas aliphatic C, aromatic C, and carboxylic C structural components were found in the humic acids from the heated Inceptisol. These results suggest that the heat-induced changes in the chemical structure of the humic acids depended on the source plant.

Evaluation of stacking nanostructure in soil humic acids by analysis of the 002 band of their X-ray diffraction profiles

Abstract Humic substances are ubiquitous in the environment. The development of models for their chemical structure can be expected to improve our understanding of their environmental behavior. In this study, we used X-ray diffraction (XRD) to develop models for the stacking nanostructure of 12 humic acids (HAs) extracted from Japanese Andisols, Inceptisols and Entisols. In the XRD profiles of the HAs, the γ band, which was attributed to aliphatic side chains, and the 002 band were separated by means of curve fitting with the Voigt function. The interlayer spacing of the carbon (C) planes (d002) and the mean thickness of the stacking nanostructure along the c axis (Lc) were calculated from the position of the peak for the 002 band and the full width at half maximum by means of Bragg’s and Scherrer’s equations, respectively. The average number of C planes in the stacking nanostructure (Nc) was estimated from d002 and Lc. The Lc and Nc values ranged from 1.04 to 1.84 nm and from 3.01 to 5.40, respectively. The mean Lc and Nc values of the Andisol HAs were larger than those of the Inceptisol and Entisol HAs (P < 0.01). The d002 value of the HAs was approximately 0.34 nm, which was slightly larger than that of graphite (0.335 nm). The Lc value positively correlated with aromatic C content (r = 0.938, P < 0.01) and negatively correlated with the sum of alkyl C and O-alkyl C contents (r = − 0.958, P < 0.01), which were estimated from the liquid-state carbon-13 (13C) nuclear magnetic resonance spectra. Furthermore, the degree of darkness of the HAs positively correlated with Lc (r = 0.961, P < 0.01). Analysis of the 002 band of the XRD profile clarified the number of C planes in the stacking nanostructure and the thickness of the stacking layer unit of the HAs. Our results indicate that the accumulation of polynuclear aromatic C with stacking nanostructures contributed to the darkening of HAs in soil.