Structural characterization of carbon and nitrogen molecules in the Humeome of two different grassland soils

Background: The surface layers of two German grassland soils, a silt loam soil from Dortmund (Soil A) and a sandy loam soil from Hannover (Soil B), were subjected to the Humeomics fractionation to identify the molecular composition of the soil Humeome. Methods: The separated Humeomic fractions were analysed by high performance size exclusion chromatography hyphenated to a high-resolution electrospray-Orbitrap Mass Spectrometry (MS). Empirical formulae were obtained by mass data to describe the carbon and nitrogen molecular structures in the soil organic matter (SOM) of both soils. Results: Results showed 175 masses for Soil A and 139 for Soil B distributed among the Humeomic fractions. Masses were classified according to molecular weight, unsaturation degree, oxygenation, and presence of nitrogen in the formula. The molecular information obtained with MS was consistent with the physical-chemical properties and environmental condition of the two grassland soils: (i) nitrogenated compounds were generally more numerous and more relatively abundant in the Humeome of Soil B, which was more anoxic and lower in C/N ratio than Soil A, (ii) highly oxygenated compounds were more numerous and abundant in the more oxic Soil A, (iii) most unsaturated formulae were comparatively more abundant in Soil A than Soil B, in line with differences in environmental conditions. The empirical formulae were then matched with their molecular structures, based on the application of ChemSpider and PubChem databases, and were found to be distributed into 16 specific chemical groups. The Van Krevelen plots built on the resulting carbon and nitrogen molecular structures provided a visual comparison of the Humeome of the two soils. The organosoluble fractions in both soils were dominated by aliphatic amides and saccharide ethers, while the hydrosoluble fractions comprised mainly aromatic amides, heterocyclic nitrogen compounds, and saccharide ethers. Conclusions: Even though the two soils contained different compounds, 66 molecules were found to be common. Based only on differences in soil texture and oxic conditions, these findings indicate the existence of similar molecular dynamics in the stabilization of organic matter in these two grassland soils. The main common group in the Humeome of both soils was the saccharide ethers, which were additionally bound to aromatic compounds in the hydrosoluble fractions. Our detailed molecular study of the Humeome of these grassland soils confirms the potential of the Humeomic procedure to assess not only the carbon and nitrogen molecular composition in SOM, but also the mechanism of their long-term persistence.