Decreasing molecular diversity of soil dissolved organic matter related to microbial community along an alpine elevation gradient

Characterization of soil dissolved organic matter (DOM) and understanding of the interactions between soil microbial communities and DOM molecules along elevation gradients in alpine ecosystems are still limited. To unravel these interactions and how they change along alpine elevation gradients, we sampled topsoil in the Sygera Mountains (Tibet, China) at elevations between 3800 and 4600 m. The molecular characteristics of soil DOM were determined using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and soil microbial composition was identified by high-throughput sequencing. Among the seven components of DOM, the lignins/CRAM (carboxyl-rich alicyclic molecules)-like structure dominated at all elevations, followed by tannins, while the relative abundance of unstable substances, including lipids, aliphatic/protein, and carbohydrates, was lower. As elevation increased, the molecular diversity, degree of oxidation, aromaticity, and unsaturation of soil DOM decreased. The abundance and diversity of soil bacteria and fungi also generally decreased with elevation. Both bacteria and fungi play an important role in the degradation of DOM molecules, but bacteria appear to have greater degradation ability. Among them, Proteobacteria and Bacteroidetes mainly promote the degradation of lignins/CRAM-like structure molecules, while Basidiomycota mainly degrade more unstable substrates. Co-occurrence network analysis revealed complex correlations between specific microbial groups and DOM molecules. Our results suggest that more active cycling of soil DOM could occur in alpine ecosystems due to climate warming, as the result of increased vegetation productivity and litter input in response to rising temperature promoting the relative abundance of microbial groups capable of degrading lignins/CRAM-like structures in soil DOM.