Electron donating properties of humic acids in saltmarsh soils reflect soil geochemical characteristics

In saltmarsh soils, humic acids (HA) are involved in geochemically important redox processes. The electron donating capacity (EDC) of HA depends on their molecular structure, but also reflects the intensity of biological reduction in tidal environments. We examined twelve soils in three saltmarshes located along a geographical gradient and applied a specific sequential extraction procedure for the isolation of HA fractions bound (BHA) or not (FHA) to the mineral matrix by Ca2+ bridges, and investigated the relationships of their properties, in particular their EDC, with the biogeochemical characteristics of the soils. Spectroscopic assessment was carried out by UV-vis, FTIR and 13C NMR, quantification and characterization of radicals was performed by electron paramagnetic resonance (EPR) spectroscopy. The EDC was determined by using the 2,2 '-azinobis-(3-ethylbenzothiazolinesulfonic acid) (ABTS) decolorization assay and experimental data were fitted to a biphasic model to calculate the contributions of the fast and slow reactions to electron transfer. The results confirmed that the two HA fractions possess different structural characteristics and that BHA present higher EDC values compared to FHA. The EDC of both fractions is strongly related to the geochemical characteristics of soils, and represents an easily measured and highly informative parameter to understand mechanisms affecting redox processes in transitional environments.