The influence of topography on the nature of humic substances in soil organic matter at a site in the Atlantic Coastal Plain of South Carolina

Abstract

The effect of topography on the nature of humic substances, isolated as water soluble organic carbon (WSOC), fulvic acid (FA), and humic acid (HA) was evaluated by comparing relative proportion and chemical characteristics of these fractions in upland and bottomland Coastal Plain soils in South Carolina. The fractions were characterized by elemental analysis and13C cross-polarization magic angle spinning nuclear magnetic resonance (CPMAS NMR) spectroscopy. The majority of humic substances occur as humic acids, with bottomland soils having higher HA/FA ratios when compared to upland soils. We found no significant differences between upland and bottomland humic substances with respect to yields of WSOC and fulvic acids, and in the C and N content of humic and fulvic acids. Carbon-13 CPMAS NMR spectroscopy revealed that the WSOC and fulvic acid fractions were composed largely of O-alkyl-C structures with bottomland soils having higher amounts of these groups. Humic acid C distribution was similar between upland and bottomland soils and was largely composed of aromatic groups. Our results demonstrate that topography influences the formation of humic acid and the structural and chemical properties of the various humic fractions.