Changes in the soil hydrophobicity and structure of humic substances in sandy soil taken out of cultivation

The aim of the research was to determine how abandoning agricultural land affects soil hydrophobicity and humic substances structure, with a special emphasis on fluorescent properties. Arable and abandoned sandy soils in central Poland were investigated from May to December of a very dry year. The investigations conducted indicated that land use influences the direction of the soil organic matter (SOM) humification process, causing changes in the functional properties of the soil. Taking sandy soil out of cultivation and its spontaneous afforestation resulted in a significant decrease of moisture in the surface layer of the abandoned soil. The change in water regime influenced the direction of SOM transformation, but a period of 30 years was not long enough to cause changes in the elemental and/or fractional composition of humic substances. Nevertheless, the modified conditions of SOM transformation resulted in a marked increase in soil hydrophobicity. Fluorescence investigation proved the predominance of simpler fulvic-like structures in humic acid (HA) extracted from the abandoned soil, which indicated a relatively low molecular weight or a smaller number of chromophores with conjugated double bonds, and a lower humification degree of HA. Obtained results showed that fluorescence spectroscopy can be successfully used as a sensitive technique for the qualitative characterisation of the structures of humic substances transformed under different environmental conditions.