Insights into the composition of recalcitrant organic matter from estuarine sediments using NMR spectroscopy

peer-reviewedThe full text of this article will not be available on ULIR until the embargo expires on the 24/05/2018Sediment cores (ca. 6 m) from an estuarine environment gave insights into the composition and preservation of recalcitrant organic carbon (OC) in the environment. The coring locations provided organic matter (OM) of terrestrial origin and of marine origin. Our study specifically focused on the humin (HU), the OM fraction that is most difficult to isolate and to characterize. HU fractions were compared with the total OM recovered after removal of the associated mineral colloids. Solid state and multiphase (nuclear magnetic resonance) NMR experiments were carried out on dried and swollen samples to obtain comparative information about the whole samples and the fractionated samples. The total OM associated with the clay-size fraction provided a standard that allowed differences between the fractions to be observed. The NMR data provided new insights into the molecular structures that become part of the long term C sink in sediments. The recalcitrant OC in the sediments is composed mainly of aliphatic hydrocarbon material that may be protected from, or otherwise unavailable for, degradation. Microbial peptides and carbohydrates were also shown to be important contributors to the C sink and these biomolecules may be from living biomass or preserved necromass. Lignin residues formed only a small part of the OM in the surface sediments but made a greater contribution at depth. Highly ordered components in HU (that resists swelling by dimethylsulfoxide, DMSO) play a major role in C sequestration