October 8, 2024
Journal Article
Improved characterization of soil organic matter by integrating FT-ICR MS, liquid chromatography tandem mass spectrometry and molecular networking: a case study of root litter decay under drought conditions
Abstract
Understanding of how soil organic matter (SOM) chemistry is altered in a changing climate has advanced considerably, however most SOM components remain unidentified, impeding the ability to characterize a major fraction of organic matter and predict what type of molecules, and from which sources, will persist in soil. We present a novel approach to better characterize SOM extracts, by integrating information from three types of analyses, and we deploy this method to characterize decaying root-detritus soil microcosms subjected to either drought or normal conditions. To observe broad differences in composition we employed direct infusion Fourier transform ion cyclotron resonance mass spectrometry (DI-FT-ICR 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 integrated approach allowed us to corroborate DI-FT-ICR MS molecular formulas using library matches, which included fungal metabolites and related polyphenolic compounds. We also inferred structures of unknowns from molecular networks, and improved LC-MS/MS annotation rates from ~5 to 35% by considering DI-FT-ICR MS molecular formula assignments. Under drought conditions, we found greater relative amounts of lignin-like vs condensed aromatic polyphenol formulas, and lower average nominal oxidation state of carbon, suggesting reduced decomposition of SOM and/or microbes under stress. Our integrated approach provides a framework for enhanced annotation of SOM components that is more comprehensive than performing individual data analyses in parallel.Published: October 8, 2024