Relationship between stable carbon and nitrogen isotope ratios of humic acids extracted from Andisols and non-Andisols

• Stable C and N isotope ratios of humic acids were determined from 26 virgin soils.
• Significant correlation was observed between δ15N and aromatic C contents.
• 13C and 15N were enriched during organic matter decomposition and humification.
• Correlation between δ15N and δ13C of non-Andisol humic acids was significant.
• Contribution ratios of C4 plant-derived C to Andisol humic acid C were estimated.

To relate the isotopic composition of soil humic acids (HAs) to their source plants and their degree of humification (as indicated by the degree of darkening), as well as to soil type, we determined the stable isotope ratios for carbon (δ13C) and nitrogen (δ15N) of HAs extracted from 26 virgin soil samples (14 Andisols and 12 non-Andisols) and from leaf samples from ten C3 plants and seven C4 plants. The high absorption coefficients (estimated at 600 nm for 1% solutions), low hydrogen and nitrogen contents, low H/C ratios, low alkyl C and O-alkyl C contents, high O/H and C/N ratios, high aryl C contents, and high aromaticity of the Andisol HAs indicated a high degree of humification. The δ13C and δ15N values of the non-Andisol HAs increased along with the progression of humification of C3 plant-derived HAs. A significant correlation was observed between the δ15N and δ13C values of the non-Andisol HAs, but not for the Andisol HAs. Most of the Andisol HA δ13C values were higher than the highest non-Andisol HA δ13C value, and δ13C values were distributed between the δ13C values for the C3 and C4 plants. The contributions of C4 plant-derived carbon to total Andisol HA carbon (CRC4) are usually calculated on the assumption that the δ13C values are not substantially altered during the decomposition of plant materials and the formation of soil organic matter. However, if 13C enrichment occurs during organic matter decomposition, CRC4 will be overestimated. Therefore, we suggest a new method to calculate CRC4 values of Andisol HAs with a correction for isotopic fractionation associated with microbial degradation; the corrected CRC4 values ranged from − 5% to 58% and were approximately 22% lower than the uncorrected values.