Composition and cycling of natural organic matter: Insights from NMR spectroscopy

Abstract

Different aspects of natural organic matter composition and cycling have been studied using solid-state 13C and 31P Nuclear Magnetic Resonance (NMR) spectroscopy. Depending on the specific study, complementary analytical techniques such as elemental, isotopic and molecular analyses have also been applied. Samples from a variety of environments were examined including ocean waters, marine sediments and atmospheric aerosols. Studies from all these environments illustrate differences in natural organic matter composition resulting from various factors such as sources, cycling mechanisms and redox conditions. In the marine water column, organic matter of two different size fractions (dissolved and particulate) is found to have distinctly different bulk chemical and isotopic compositions. Overall, this indicates that particulate organic matter does not form from the simple physical aggregation of dissolved organic matter, and dissolved organic matter is not the primary source for particulate organic matter. Comparison of carbon and phosphorus compositional changes with depth in the ocean within the dissolved and particulate fractions reveals differences in cycling mechanisms. In the marine water column, selective mineralization of specific carbon compounds such as carbohydrates and amino acids occurs relative other species such as lipids. Whereas for phosphorus, the relative proportion of the different functional groups are unvarying with depth. In marine sediments, NMR spectroscopy reveals P cycling for specific phases such as polyphosphates is a function of sediment redox conditions. In atmospheric aerosols 13C NMR spectroscopy shows differences in water-soluble organic carbon composition from urban versus biomass burning sources. Urban aerosols have higher aliphatic and lower aromatic compound contents relative to samples derived from biomass burning. The results of these studies provide new insights into carbon and phosphorus cycling in the environment and demonstrate the capabilities of solid-state NMR as a tool for investigating natural organic matter composition.