Humic Acids Properties of Luvisol of 40-Year Fertilizer Experiment

The purpose of this research was to determine the properties of humic acids isolated from soil samples taken from a 40-year static experiment—the experimental factors were fertilization with manure (30 t ha⁻¹; FYM) and nitrogen at rates of 40, 60 and 120 t ha⁻¹. From the soil samples (Luvisol), humic acids (HAs) were extracted and the following were determined: elemental composition, hydrophilic and hydrophobic properties and spectrometric properties in the UV–VIS and IR range. The HAs of the soil fertilized with manure and N compared to the HAs of the soil fertilized with N (without manure) were characterized by a higher degree of aliphaticity and, consequently, a higher share of hydrophilic fractions and lower values of internal oxidation. Based on the spectrometric parameters, it was indicated that the HA particles of the manure-fertilized soil are characterized by a higher share of undecomposed lignin fragments, a lower degree of humification and at the same time, a higher susceptibility to oxidation. The obtained relationships showed that the aromaticity and hydrophobicity of the HA molecules of the manure-fertilized soil can be increased at certain N doses (60 and 120 t ha⁻¹), which is particularly important in terms of the role that humic substances play in carbon sequestration.