Pyrolysis-field ionization mass spectrometry and nitrogen K-edge XANES spectroscopy applied to bulk soil leachates—a case study

Although dissolved organic matter (DOM) is an important component of C- and N-fluxes in the environment, its structural composition is still poorly understood due to methodological challenges. We explored the potential of combining pyrolysis-field ionization mass spectrometry (Py-FIMS) and N X-ray absorption near edge structure (N-XANES) spectroscopy to study the molecular–chemical composition of lyophilized bulk soil solution samples that were not subjected to pretreatment like dialysis. Soil leachates were collected at 90 cm and 220 cm depth from an arable and a fallow site. Py-FIMS spectra reflected differences in DOM composition related to land use and sampling depth. Land use effects were expressed in higher abundances of carbohydrates and peptides at the arable than at the fallow site. The relative proportions of carbohydrates decreased and the proportions of lignin-derived compounds increased with depth, indicating a relative enrichment to more stabilized DOM along the flow path. Nitrogen XANES spectra were dominated by the signal of NO3-salts but also indicated the presence of organic, non-amidic N as found in imidazoles, pyrazoles, purines and/or nitrile-N, whereas N-compounds like pyridines, pyrroles, quinoline and indole were detected by Py-FIMS. Thus, the combined application of Py-FIMS and N-XANES yielded complementary information regarding the molecular–chemical composition of DOM. Future applications of these techniques may benefit from selectively analyzing soil solution samples with lower nitrate concentrations collected in early spring.