Disappearance and alteration process of charcoal fragments in cumulative soils studied using Raman spectroscopy

•Raman spectra of charcoal fragments in cumulative soils were obtained.•These spectra were compared to those of fresh charcoal fragments from a modern fire.•There are spectrum differences among fresh charcoals and old and young soil charcoals.•The differences imply the alteration or disappearance of charcoals in the soils.•The charcoal residue rate in soils also depends on charcoal physical properties.

Raman spectra of charcoal fragments in cumulative soils in central Japan, where grasslands have been sustained using intentional burning for ~ 1000 years, were obtained and compared to those of fresh charcoal fragments after modern grass burning to clarify their disappearance and alteration process in the soils. Although the values of each Raman-spectrum parameter of the soil charcoal and fresh charcoal fragments are partially similar, certain differences indicating their alteration or disappearance were observed. Charcoal fragments with lower graphitization in soils altered chemically with age at decadal to century scale, suggested by changes in distance between defects or defect type of their chemical structures. Charcoal fragments with higher graphitization were found in fresh charcoal samples, whereas very few charcoal fragments were found in both young and old soils, indicating that these charcoal fragments disappeared instantly after they were formed. This fact implies that charcoal fragments with higher graphitization tend to not remain in soils, possibly owing to their physical properties such as fragility, density, and hydrodynamic behavior. Our findings suggest that charcoal's physical properties have a vital influence on charcoal residues in soils, as do charcoal's chemical properties.