Direct Salinity Effect on Absorbance and Fluorescence of Chernozem Water-Extractable Organic Matter

Soil-derived dissolved organic matter (DOM) has a significant impact on aquatic ecosystems. Identifying the fluorescence signatures of DOM from different soils in river and sea waters can provide valuable insights into its migration patterns. This makes crucial assessing the contributions of pH, salinity, and other milieu parameters to the variability of DOM optical properties. Present study investigates the changes in DOM of Chernozems under varying salinity using UV–visible absorbance spectroscopy and 3D-fluorescence spectroscopy coupled with parallel factor analysis (EEMs-PARAFAC). Water-extractable organic matter (WEOM) extracted from soils of two field experiments of contrasting land use: long-term bare fallow (LTBF) and annually mown steppe (Steppe), was used as a proxy for DOM. Diluted extracts were incubated with varying NaCl concentrations in the dark and then examined. Steppe WEOM exhibited fair constancy of optical parameters under increasing salinity, while significant changes of the optical indices and of PARAFAC components’s loadings were observed for LTBF WEOM. The remarkable stability of the Steppe WEOM can be attributed to its chemical diversity. Two distinct and sufficiently stable humic-like PARAFAC components have the potential to serve as markers of Chernozem DOM. The findings clearly demonstrate that salinity itself slightly reduces absorption and fluorescence and changes some optical indices of WEOM of Chernozems.