Correlating bulk optical spectroscopy and ultrahigh-resolution mass spectrometry to determine the molecular composition of dissolved organic matter in Northern Peatlands

Northern peatlands store vast amounts of carbon. Although peatlands are at present net carbon sinks, they are also an important source for the greenhouse gases CO₂ and CH₄, and there is some concern that they may become carbon sources as global temperatures rise. In this work, we used Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS), PARAFAC modeling of excitation-emission fluorescence spectroscopy (EEMS), and specific UV absorbance (SUVA₂₅₄) to investigate the chemical quality and composition of DOM in porewaters collected from different bog and fen systems in northern Minnesota, USA. Four terrestrial humic-like components and one non-humic-like component were identified by EEM-PARAFAC, and their dynamics were evaluated based on their distribution with depth as well across different sites. We found an abundance of a microbial humic-like component in fen porewater DOM. This component was not as abundant in bog porewaters. SUVA, determined at 254 nm, was strongly correlated with lower aromaticity in deep fen samples relative to deep bog samples as determined by ¹H-NMR. Aromatic and carbohydrate components represented up to 70% of deep bog DOM but comprised a much smaller proportion of deep fen DOM, which was dominated by functionalized and non-functionalized aliphatics. Molecular formula data determined by FT-ICR mass spectrometry were consistent with results from optical and NMR spectroscopy experiments and showed that compounds with low O/C and high H/C were generated with depth in the fen. Such compounds were absent in both surface fen and in surface and deep bog samples, respectively, providing further evidence of qualitative and quantitative differences in the evolution of DOM in fens and bogs. These differences in reactivity suggest that bogs and fens will behave differently in response to global warming, with more CO₂ and CH₄ released from fens compared to bogs.