Insights into the complexity of dissolved organic matter in different soils using ultrahigh-resolution ESI-FT-ICR mass spectrometry

Abstract

Dissolved organic matter (DOM) significantly influences global carbon and nutrient cycles, drives microbial activity, and supports overall soil ecosystem functioning However, there is limited understanding of the molecular composition of soil DOM, particularly its distribution across different environments. In this study, we extracted and characterized DOM from paddy soils (DOMp), soils for vegetable planting (DOMv), and forest soils (DOMf) using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Structural differences were also observed using scanning electron microscopy (SEM). The mass spectra of all four DOM samples, including the standard Suwannee River Natural Organic Matter (SRNOM), ranged from 200 to 700 Da. CHO compounds were predominant, constituting over 41.86 % of the composition in the standard SRNOM. Notably, CHON compounds were most abundant in DOMv, accounting for 47.63 %, while CHOS compounds were higher in SRNOM and DOMp, reaching approximately 19.92 % in DOMp. The oxygen atom counts for CHO compounds varied from 2 to 16, with double bond equivalence (DBE) values between 2 and 9, suggesting low condensation levels. OxN1 chemicals were the most prevalent in CHON compounds, and OxS1 chemicals dominated CHOS compounds, reaching 96.11 % in DOMp. Furthermore, molecular agglomeration was observed, particularly in DOMv and DOMf. Despite the similar elemental composition of soil DOM, significant differences at the molecular level were evident, impacting soil properties.