Revisiting molecular characteristics responsive for the aromaticity of soil humic acids

Aromaticity of soil organic matter has often been considered an independent index of biogeochemical maturity, recalcitrance, and persistence of C in soils. The structural characteristics of soil humic acids (HAs) from various origins are studied by bi- and multivariate statistical exploratory analyses to select chemical descriptors surrogated to aromaticity. Structural features of 16 HAs were determined using analytical pyrolysis, wet chemical oxidation, and a variety of spectroscopic analyses. Data management was approached by (a) linear correlations between classical HAs structural descriptors and the concentration of aromatic C as seen by C-13 NMR spectra as independent variable and (b) multivariate statistics to define HA aromaticity as an emergent property defined by the shared contribution of a set of variables. Significant correlations were found between HAs optical density (465 nm; E4) and aromatic C regions (C-13 NMR). These also paralleled the intensity of the Fourier Transform Infrared Spectroscopy (FT-IR) bands ascribed to aromatic structures. Likewise, significant positive correlations were observed between E4 and aromatic compounds released by wet chemical degradation, with negative correlations with yields of aliphatic and lignin-derived methoxyphenols released by pyrolysis. Multivariate classification of HAs' analytical descriptors allows the identification of variables, i.e., E4, H/C atomic ratio, intensity of aromatic C in the C-13 NMR region between 110-140 and 140-160 ppm, and yield of benezenecarboxylic acids after chemical degradation, as surrogates for HAs' aromaticity. This HA characteristic is also responsive to the variability between environmental scenarios, mainly the effect of wildfires and soil management (clearing and cultivation).