A Novel Method for Unraveling the Black Box of Dissolved Organic Matter in Soils by FT-ICR-MS Coupled with Induction-Based Nanospray Ionization

Dissolved organic matter (DOM) in soils plays important roles in carbon and nutrient cycling. However, our knowledge of the molecular composition of DOM is limited by selective loss during pretreatment [solid-phase extraction (SPE)] or small amounts of pore water. We proposed a novel method for characterizing DOM by coupling a Fourier transform ion cyclotron resonance mass spectrometer and a self-built induction-based nanospray ionizer. This approach reduces the ion suppression effect and avoids the need for SPE by substantially reducing the matrix effects. Moreover, highly diverse compound types were detected in Suwannee River natural organic matter. Cwt/Nwt was close to the actual C/N ratio. Only 0.5 μL of pore water (5 ng of C) was needed (vs ∼10 mL for the conventional method). This method was validated by analyzing forest and agricultural soil samples. Soil DOM was not dominated by lignin-like compounds, as determined using the conventional method. The novel method revealed more peptide/carbohydrate-like compounds, highly condensed aromatics, and more than double the number of CHNO compounds. A more complete understanding of DOM will allow us to elucidate the biogeochemical processes of nutrient and DOM–pollutant complexes in soils as well as to develop comprehensive policies for soil remediation.