FTIR-spectroscopic characterization of humic acids and humin fractions obtained by advanced NaOH, Na4P2O7, and Na2CO3 extraction procedures

Aim of our study was the development of the methodological basis for the characterization of humic fractions of a long-term field experiment. Humic acids (HAs) were extracted from three layers of a nontilled soil using three different extractants (1 M NaOH, 0.1 M Na4P2O7, 1 M Na2CO3), and the humin fraction was enriched. NaOH as extractant for FTIR analysis of humic substances yields higher resolved IR spectra, especially in the important regions of stretching vibrations including aromatic and aliphatic groups and in the fingerprint area including amides, aliphats, and aromats than the other extractants. The NaOH extraction has lower extraction yields as compared to Na4P2O7 and Na2CO3 and represents a different part of the soil organic matter (SOM). This is reflected by lower C : N ratios and higher E-4 : E-6 and fulvic acid-to-humic acid ratios as compared to the other extractants. The FFIR band areas of HA fraction obtained by NaOH showed an increase of the aromatic and carbonyl groups and a decrease of amide groups with increasing soil depth. Aliphatic groups showed contradicting results: The bands of the stretching vibrations increased, and the band of the bending vibrations decreased. We assume that band interactions in the bending vibrations were responsible for that phenomenon under the assumption of an increase of aliphatic groups with increasing soil depth. The IR bands of the enriched humin fraction showed a decreasing trend in case of both aliphatic bands deriving from stretching vibrations and an increase of aromatic characteristics with depth. Our study led to the conclusion that HA fractions obtained by 1 M NaOH represent a small and dynamic fraction indicated by the measured yields in combination with values of N-t, C : N, E-4 : E-6 ratios, and ratios of fulvic acids (FA) to HA. The humin fraction has a high contribution to the total organic C and represents a more stabilized fraction of SOM which still shows changes in its aromatic and aliphatic characteristics with soil depth.