Dissolved organic matter molecular composition controls potential biodegradability

Abstract

Dissolved organic matter (DOM) is a heterogenous mixture of plant- and microbial-derived compounds at various stages of decomposition. However, there is a lack of detailed information about the biodegradability of DOM from different terrestrial sources that have varying molecular compositions. To circumvent this, incubation experiments were conducted for two months using DOM isolated from three different terrestrial sources: forest soil, peat and leonardite, all with distinct extents of diagenesis. The dissolved organic carbon (DOC), total dissolved nitrogen (TDN) contents, pH, and headspace CO₂ production were measured. DOM composition was monitored using solution-state ¹H nuclear magnetic resonance (NMR) and ultraviolet–visible (UV–VIS) analyses. To further understand microbial responses, microbial biomass and community composition was analyzed with phospholipid fatty acid (PLFA). DOM isolated from forest soil and peat demonstrated high biodegradability and decreased DOC concentrations, higher CO₂ production rates and higher microbial biomass over the course of incubation. Forest soil-derived DOM composition had decreased aliphatic, and carbohydrate and peptide components in the early and late stages, respectively. Peat-derived DOM exhibited lower carbohydrate and peptide concentrations and higher carboxyl-rich alicyclic molecules (CRAM). In contrast, CRAM and aromatic enriched leonardite-derived DOM had low CO₂ production and microbial biomass, and no major changes in DOM chemical composition, suggesting limited biodegradability. Further, CO₂ production, microbial biomass, and DOM molecular weight and degradation indices were positively correlated with carbohydrates and peptides, and inversely related to CRAM and aromatic components. Overall, these results corroborated distinct biogeochemical dynamics with varied DOM molecular composition, and highlight that DOM enriched in bio-recalcitrant components limited microbial processing, and exhibited high geochemical stability.