Nitrogen-driven shifts in molecular composition of soil dissolved organic matter linked to rare bacterial sub-communities

The interaction between soil dissolved organic matter (DOM) and bacterial communities is critical for understanding key processes in the global carbon cycle. However, the molecular-level associations between these components remain poorly understood. To address this gap, high-resolution Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was combined with high-throughput sequencing to examine how DOM composition and bacterial sub-community diversity respond to different levels of nitrogen (N) addition (0, 40, and 80 kg N ha^-1 yr^-1) and to explore the relationships between them. The results demonstrate a significant decline in carbohydrate molecules within DOM under low N conditions. Conversely, the β-diversity and double bond equivalents of DOM molecules increased, indicating greater heterogeneity and stability in DOM composition under this treatment. Rare bacterial sub-communities, as opposed to abundant taxa, were more sensitive to N addition, exhibiting narrower ecological niches and weaker phylogenetic signals. β-Diversity decomposition analysis indicated that compositional differences in abundant taxa were primarily driven by richness differences, whereas those in rare taxa were predominantly influenced by species replacement. Co-occurrence network analysis revealed that DOM molecules were more frequently linked with rare taxa than with abundant taxa. Furthermore, a strong positive relationship was observed between the β-diversity of rare taxa and that of DOM molecules along the N gradient. These findings underscore that rare bacterial sub-communities are the primary drivers of changes in DOM molecular composition under N enrichment, emphasizing their potential role in shaping chemical diversity.