Photochemistry After Fire: Application of Ultrahigh Resolution Mass Spectrometry and Two-dimensional NMR to the study of Pyrogenic Dissolved Organic Matter

Pyrogenic organic matter (pyOM), the solid-phase by-product of the combustion of biomass (e.g., wildfires, biochars), is now recognized as an important component of the global carbon cycle. Recent work has determined that large amounts of pyOM are exported by rivers to the open ocean as dissolved pyOM (pyDOM). However, the highly aromatic nature of many of the pyrogenic molecules renders them reactive to photo-degradation and transformation. Thus, pyDOM molecular composition likely changes during export. Understanding the molecular structures that are lost and/or produced during photo-irradiation is paramount to understanding the fate of pyDOM in the environment (e.g., export to the ocean, sequestration in soils, sediments, mineralization to inorganic carbon).

To simulate photo-alteration of pyOM, water-leachable pyDOM from a temperature series (250, 400, 525, 650 ^OC) of laboratory-produced oak biochars was photoirradiated. Molecular-level alteration of initial pyDOM was assessed using ultrahigh resolution mass spectrometry (FT-ICR-MS) and two-dimensional ¹H-¹H total correlation NMR spectroscopy (TOCSY). We find that pyDOM produced from oak biomass at higher pyrolysis temperatures (525 and 650 ^OC) is more labile to photo-transformation than lower thermal maturity (250 and 400 ^OC) pyDOM. Based on these data and previous studies, we propose a photo-degradation pathway which includes the oxygenation and ring opening reactions of condensed aromatic molecules. Further photo-oxidation yielded carboxyl groups, which are cleaved by decarboxylation, ultimately producing aliphatic residues and inorganic carbon. These aliphatic structures could be polymerized to larger molecules by reactive oxygen species if not consumed by bacteria and fungi. Results from these controlled experiments enhance our understanding of terrestrial-to-marine transfers of pyDOM, as well as its geochemical stability and fate in the environment.