Transmission electron microscopy indicates supramolecular transformations over time of soil humic acids and their electrophoretic fractions

The influence of various environmental factors on the formation of different morphological forms of soil organic matter has been extensively studied, but there is still uncertainty about how time spent in solution affects the evolution and possible transformation of the morphological structures of soil humic substances. The transmission electron microscopy (TEM) with negative contrast staining by uranyl acetate was used to study morphological differences between soil humic acids (HAs) and their individual electrophoretic fractions before and after 150 days of incubation in the same solution under constant environmental conditions. For the first time, it was found that during 150 days of incubation in constant environmental conditions, soil humic samples can aggregate by two fundamentally different ways, forming vesicle-like or ribbon-flat/sheet-like supramolecular morphological structures. The types of morphological supramolecular structures observed using TEM were found to be related to different chemical compositions and physical–chemical properties of soil humic substances. These data could be relevant to better understanding the overall makeup of soil HAs, their structural organization, and various ecological and biogeochemical functions.