Relationship between soil bacterial communities and dissolved organic matter in a subtropical Pinus taiwanensis forest after short-term nitrogen addition

The interaction between soil bacterial communities and dissolved organic matter (DOM) is the central link of soil carbon and other nutrient cycle. However, how nitrogen (N) addition changes the relationship between forest soil bacterial community composition and DOM components (BCC-DOM) and which key bacterial taxa affect DOM components is not clear. After two years of N addition, we used parallel factor analysis and high-throughput sequencing to explore the changes of DOM fluorescent components, microbial community composition, and diversity at three N addition levels in a subtropical Pinus taiwanensis forest and presented the relationship between BCC-DOM through co-occurrence networks. The results show that low N addition, but not high N addition, transitions the surface bacterial community into a eutrophic state and reduces the Shannon index in the subsoil. Short-term N addition decreases the dissolved organic carbon (DOC) content but increases DOM humification degree and humus-like components (C1 and C2) in the topsoil. Network analysis revealed that the relationships between BCC-DOM are more complex after N addition and the negative associations were dominant. Random forest analysis showed that Chloroflexi (TK10, Ktedonobacteria, and JG37.AG.4) and Nitrospirae (Nitrospira) are key factors for predicting humus-like DOM. Protein-like DOM was mainly related to Firmicutes (Clostridia). Furthermore, structural equation models revealed that soil pH and DOC content do not determine the key bacterial taxa and DOM components, but total N content significantly affects these parameters. Short-term N addition reduces soluble substances in forest soil but improves the obstinacy of carbon substrates. Further, N addition helps to regulate forest soil bacterial communities and strengthen the interactions with DOM.