Pilot Study of Grassland Soil Soluble Organic Matter with High-Resolution Mass Spectrometry ^†

Soil is the largest terrestrial carbon pool and regulates carbon, water, and nutrient cycles [1,2]. The diversity of the soil environment prevents it from having one certain parameter to assess the condition of the soil. Instead, different physical, chemical, and biological parameters are used to assess soil health in general [1,3]. A non-targeted method was proposed for fingerprinting soil soluble organic matter (SOM) with high-resolution mass spectrometry (HRMS) and making assessments of soil health from collected data. Soil samples were collected from a permanent grassland in Tõravere over the course of April 2022 to June 2022. The samples were dried with a vacuum and extracted with organic solvents. Three solvents were compared for extraction—acetonitrile, methanol, and toluene. The extracts were analyzed with HRMS by using a Fourier-transform ion cyclotron resonance mass spectrometer (FT-ICR MS). Flow infusion together with two ionization sources were compared—nano-electrospray ionization (nESI) and atmospheric pressure chemical ionization (APCI) [4]. The results demonstrated that the mass spectra of the methanol extracts were most abundant in peaks. Further data analysis of the elucidated molecular formulas revealed that the identified compounds in each extract consisted mostly of various lipids, although some peptides were identified in extracts of all three solvents and some carbohydrates were identified in the methanol and toluene extracts. The results also suggest nESI to be more suitable for HRMS analysis since it used up smaller amounts of extract but gave more peaks on the mass spectrum [4]. Further research on more diverse sample types and variations of additional parameters (time-series for observation of the effects of the plant development cycle and soil microorganisms’ population dynamics, changing weather conditions, etc.) are needed for looking at relationships between SOM chemical composition and soil health.