Chemodiversity of soil dissolved organic matter complementally improved by stepwise elution and electrospray ionization modes

Abstract

The chemodiversity of dissolved organic matter (DOM) molecules can affect the biogeochemical cycling of carbon and nitrogen in terrestrial and aquatic environments. In this study, the negative and positive ion modes of electrospray ionization (ESI(−) and ESI(+), respectively) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were employed to reveal the chemical composition of seven soil DOM isolates by stepwise elution using different methanol/water solvents. The intensity percentage of CHON molecules in soil DOM detected in the ESI(+) mode was 4.6%–47.3% higher than that for the ESI(−) mode. The O/C ratio, H/C ratio, and the nominal oxidation states of the carbon value of soil DOM molecules linearly correlated (R² > 0.8 for 72% number of stepwise eluted DOM isolates) with the increasing volume of methanol in the SPE elution solvents in dual ion modes, demonstrating the selective fractionation of oxygen-rich and highly polar molecules by methanol-poor solvents. This polarity-dependent fractionation is expected to be universal across different soils. These results suggest that combining the ESI(−) and ESI(+) modes with stepwise elution enables the comprehensive characterization of soil DOM composition, favouring the in-depth understanding of the critical roles of DOM in the biogeochemical cycles of carbon and nitrogen.