Humic substances in Fluvisols of the Lower Vistula floodplain, North Poland.

The present study describes properties of humic substances of the Fluvisols (the Lower Vistula, Poland). Fluvisols under agricultural management (arable soil and grassland) were sampled from the surface horizon 50, 200, 600, and 900 m from the Vistula River. The content of carbon in the fractions of humic acids (C), fulvic acids (C), and humins (C) as well as the content of dissolved organic carbon (DOC) were assayed. The organic matter of the soils that were sampled 200 m from the river demonstrated a lowest share of the humic acids (HAs) and fulvic acids (FAs). The percentage share of the hydrophilic fractions (HIL) in the HAs and, as a result the value of the HIL/ΣHOB ratio, increased with the distance from the riverbed. The HAs of the soils located further from the riverbed had a higher degree of humification compared to the HAs of the soils that were sampled 50 and 200 m away. Based on the research results, it was determined that the properties of HAs can be used to evaluate the effect of flood events, their location, and scope on the transformation of the organic matter in fluvial sediments.