Chronic nitrogen deposition drives microbial community change and disrupts bacterial-fungal interactions along a subtropical urbanization gradient

Microbial responses to nitrogen (N) enrichment under chronic ambient N deposition conditions are understudied, especially in subtropical forests which are often not limited by N. We investigated variation in subtropical forest soil microbial biomass and composition, bacterial-fungal interactions (BFI), and their linkages to N cycling along a gradient of high-rate N deposition in Shanghai, China (estimated N deposition > 40-100 kg N ha(-1) yr(-1)). In contrast to global and temperate findings, arbuscular mycorrhizal (AM) fungal biomass and the ratio of AM to saprotrophic fungi increased with ammonium, indicating possibly increased P limitation following N enrichment in this subtropical ecosystem. Positive BFI (positive relationships between a particular fungal OTU and bacterial OTU) mostly decreased (by > 85%) with increasing N availability, as did negative BFI (by > 85%), suggesting that bacterial-fungal cooperation and competition both tended to weaken under nutrient-rich conditions. Ammonium and nitrate were significantly related to overall microbial community composition and microor-ganisms at different taxonomic levels. Increasing ammonium seemed to favor taxa from bacterial phyla Acid-obacteria, Actinobacteria, and Nitrospirae. Most pathogenic fungal OTUs increased with nitrate. Other microbial groups and taxa involved in litter decomposition and N cycling also had significant relationships with ammonium and/or nitrate. We found that N availability may drive significant microbial community change even when present in excess. Overall, we observed strong microbial linkages to N availability in subtropical forests under chronic high-rate N deposition, and specific relationships were often contrary to previous observations from temperate N addition experiments.

Automated summary

Microbial responses to nitrogen enrichment under chronic ambient N deposition conditions are not well studied in subtropical forests. This study found that nitrogen enrichment may lead to increased phosphorus limitation in subtropical ecosystems, and the cooperation and competition between bacteria and fungi tend to weaken under nutrient-rich conditions. Additionally, ammonium and nitrate were significantly related to overall microbial community composition and other microbial groups involved in litter decomposition and N cycling.