Molecular composition of humic acids from coastal wetland soils along a salinity gradient

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Humic acids (HAs) play an important role in regulating the biogeochemistry of soils and sediments. The chemical structural composition of humic substances in wetland soils especially at coastal zone, which is subjected to the influence of seawater intrusion, is not yet fully understood. In this study, HAs from soils of three wetland ecosystems along a salinity gradient namely, a forest swamp (FS), freshwater marsh (FM), and saline marsh (SM), from the Barataria basin of the Louisiana coast were isolated and characterized using elemental analysis, 13C solid state nuclear magnetic resonance (NMR), and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) with or without tetramethylammonium hydroxide (TMAH) treatment. Humic acids from the FS soil, the most inland of the three, showed the most aliphatic nature whereas HAs of the FM soil had the highest percentage of aromatics. There was a general increase in the crystalline domain of alkyl C in these wetland soil HAs along the salinity gradient. The Py-GC/MS TMAH thermochemolysis clearly showed a shift of lignin residues from the dominance of guaiacyl (G-type) and syringyl (S-type) structural units in FS HAs to p-hydroxyphenyl (H-type) structural unit in HAs of both marsh soils. In addition, there was a smaller Syringyl/Guaiacyl (S/G) ratio in the HAs of SM soil than FM soil, and an increasing contribution of sulfur compounds into their HA structural component as moving toward the coast, suggesting the formation of increasingly more recalcitrant nature of HAs along the salinity gradient in these wetlands. The 13C NMR observation was, in general, consistent with the Py-GC/MS characterization. Overall, the result reveals a clear effect of salinity and vegetation on the molecular composition of HAs. © Soil Science Society of America,.

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Soil Science Society of America Journal

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