Abiotic Transformation of Terrestrial Natural Organic Matter Probed by Multimodal Spectroscopy

Abstract

Natural organic matter (NOM) is a major component of soil and aquatic environments, impacting the global carbon cycle. Its formation and breakdown are modulated by abiotic transformations of plant and microbial materials, yielding thousands of poorly characterized organic molecules. The complexity of NOM poses substantial analytical challenges, impeding the molecular-level examination of its abiotic chemistries. To address these uncertainties, we employed complementary spectroscopic and spectrometric methods to explore the alterations of terrestrial NOM in key abiotic reactions, including mineral–organic matter interactions, and chemical and photochemical oxidations. Metal complexation and mineral sorption of NOM are impacted by carboxyls and their structures. The structural environments of carboxyls were resolved using vibrational spectroscopy at low pH, confirming that carboxyls in NOM from various environments were structurally similar and mainly composed of α-substituted aliphatic carboxylic acids. Additionally, mineral–organic matter interactions were investigated between dissolved NOM (from the New Jersey Pine Barrens) and soil minerals (Fe and Al oxides, 1:1 and 2:1 clay minerals, quartz, and calcite). The extent of molecular fractionation followed the magnitude of mineral sorption: metal oxides > clay minerals > quartz ≈ calcite. This manifested as different degrees of the sorption of highly unsaturated and aromatic compounds associated with redshifted chromophores/fluorophores possibly containing ketones and carboxyls. Importantly, long-term mineral sorption persisted for up to six months, particularly for the Fe oxide and clay minerals. Besides adsorption, mineral surfaces catalyzed the breakdown of organic molecules, primarily through ring-opening reactions and decarboxylations. These transformations mirrored abiotic oxidations of terrestrial NOM, like through Fenton chemistry and photochemistry. Using a model β–O–4 lignin polymer and Pine Barrens NOM (rich in lignin and lignin degradation products), congruent oxidative modifications were revealed at the bulk (mineralization coupled with production/destruction of redshifted chromophores and fluorophores), structural (increased abundances of ketones and carboxyls), and molecular levels (formation of O-rich aromatic and aliphatic compounds, and condensed formulas). Chemical and photochemical degradation of lignin-like molecules emerged as a controlling factor in the flux and quality of NOM in soils and inland waters, thereby regulating the transport of carbon from land to the ocean.