Optical characterization and distribution of chromophoric dissolved organic matter (CDOM) in soil porewater from a salt marsh ecosystem

To characterize chromophoric dissolved organic matter (CDOM) in marsh porewaters and its contribution as a carbon source, optical properties (absorbance, fluorescence indices, 3-dimensional excitation-emission matrices [EEMs]) of soil porewater and surface water were measured in a southern Californian salt marsh. Absorption coefficients and fluorescence intensities were higher in porewater than in overlying surface waters, consistent with higher CDOM concentration at depth. Humic-type peaks A and C were observed in EEMs in all samples, and peak M was observed in surface waters and shallow porewater to -5 cm depth. Fluorescence: absorbance (flu:abs) ratios and spectral slopes (S) decreased across the surface interface, and emission peak maxima were red-shifted-changes that are consistent with increasing molecular weight (MW) and aromaticity in soil porewater due to humification, and lower-MW, less aromatic material in oxic surface waters from oxidative photochemical and biological processing. At lower depths, bands were observed where intensity, flu: abs ratios and S increased; absorption coefficients decreased; emission maxima for humic-type peaks were blue-shifted; and tryptophan-type protein peaks were observed. These changes are consistent with lower-MW and less aromatic material from enhanced microbial activity. Variations in iron concentrations and sulfate depletion with depth were consistent with these bands having different dominant anaerobic microbial metabolic pathways. Overall, optical property trends suggest that soil porewater is a reservoir of CDOM in the salt marsh, with organic material from terrestrial watershed inputs and in situ production from marsh vegetation stored and processed in sediments.