Ultrahigh-resolution FT-ICR mass spectrometry for molecular characterisation of pressurised hot water-extractable organic matter in soils

Dissolved organic matter in soil is a highly reactive pool of organic matter and is of great importance for biogeochemical cycles in soil. A better understanding of its dynamics relies on its molecular characterisation. Here, the molecular composition of water-extracted organic matter at elevated pressure and temperature (PH-WEOM) obtained from 120 Burgundy soils was investigated using high-field Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR MS). Unsupervised multivariate statistical analysis (UMSA) was used to retrieve classes of samples with specific molecular characteristics. Accordingly, van Krevelen diagram, Kendrick mass defect (KMD), as well as aromaticity index (AI) and aromaticity equivalent (Xc), were applied to present a pool of ubiquitous molecular formulas and to evaluate the PH-WEOM molecular heterogeneity in the sample set. Based on UMSA, the PH-WEOM from forest soils revealed a clearly distinct molecular composition, with major contributions from lignin- and tannin-like compounds, and with its aromaticity related to soil characteristics, especially the soil pH. No clear evidence of land-cover influence on the PH-WEOM molecular composition was found for cropland and grassland soils, but the role of pH was also identified for these samples, and agrees with molecular patterns attributed to microbial activity, with the presence of compounds with high H/C ratio. A group of samples from cropland soils developed on residual formations is characterised by a very specific molecular composition, rich in aliphatic organosulfur-like compounds, highlighting the importance of specific soil processes in the molecular composition of PH-WEOM. This work demonstrates the potential of FT-ICR MS to resolve the high chemical complexity of PH-WEOM in soils and the intricate influences of both biotic and abiotic environmental factors on the molecular composition of PH-WEOM in soils.