Improved characterization of soil organic matter by integrating FTICR-MS, liquid chromatography tandem mass spectrometry and molecular networking: a case study of root litter decay under drought conditions

Abstract Knowledge of the type of carbon contained in soils is important for predicting carbon fluxes in a warming climate, yet most soil organic matter (SOM) components are unknown. We used an integrated three-part approach to characterize SOM from decaying root-detritus microcosms subject to either drought or normal conditions. To observe broad differences in SOM compositions we employed direct infusion Fourier transform ion cyclotron resonance mass spectrometry (DI-FTICR-MS). We complemented this with liquid chromatography tandem mass spectrometry (LC-MS/MS) to identify components by library matching. Since libraries contain only a small fraction of SOM components, we also used fragment spectra cosine similarity scores to relate unknowns and library matches through molecular networks. This approach allowed us to corroborate DI-FTICR-MS molecular formulas using library matches and infer structures of unknowns from molecular networks to improve SOM annotation. We found matches to fungal metabolites, and under drought conditions, greater relative amounts of lignin-like vs condensed aromatic polyphenol formulas, and lower average nominal oxidation state of SOM carbon, suggesting reduced decomposition of carbon and/or microbes under stress. We propose this integrated approach as more comprehensive than individual analyses in parallel, with the potential to improve knowledge of the chemical composition and persistence of SOM. Synopsis Structural characterization and identifications are lacking for soil organic matter components. This study integrates molecular formula assignments and structural information from fragment ion spectra into molecular networks to better characterize unknown soil organic matter components. For Table of Contents Only