ABSTRACT
The structure of humic substances (HSs) and the humification process are critical topics for understanding the dynamics of carbon on the planet. This study aimed to assess the structural patterns of 80 humic acid (HA) samples isolated from different soils, namely, Histosols, Ferralsols, Cambisols, Mollisols, Planosols and vermicompost, by spectroscopic characterization using solid-state 13C nuclear magnetic resonance cross-polarization/magic angle spinning combined with chemometric techniques. All 80 HAs had a similar structural pattern, regardless of their source of origin, but they had different relative quantities of organic C species. The different structural amounts of the various organic C fractions generated different properties in each of the HAs. This explains why there were similarities in the HS functions but why the intensities of these functions varied among the samples from the different soil types and environments, confirming that HSs are a group of compounds with a structural identity distinct from the molecules that give rise to them. There appears to be no single definition for the humification process; therefore, for the soils from each source of origin, a specific humification process occurs that depends on the characteristics of the local environment. Humification can be understood as a process that is similar to a chemical reaction, where the key factor that determines the formation of the products is the structural characteristics of the reactants (organic substrates deposited in the soil). The degree to which the reaction progresses is governed by the reaction conditions (chemical, physical, and biological properties of the soil). The structural patterns for HSs obtained in this study justify the existence of HSs structured as self-assembled, hydrophilic and hydrophobic domains that, under certain conditions, can undergo transformations, altering the balance of organic carbon in the environment.
