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

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 600nm 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 (CR_C4) 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, CR_C4 will be overestimated. Therefore, we suggest a new method to calculate CR_C4 values of Andisol HAs with a correction for isotopic fractionation associated with microbial degradation; the corrected CR_C4 values ranged from -5% to 58% and were approximately 22% lower than the uncorrected values.