Forest type and stand structure influence soil microbial network composition and stability in urban forests: Insights from Beijing, China

Urban forests are essential for enhancing urban livability, with their soil microbial communities playing a key role in ecosystem stability and function. However, how different urban forest types influence these microbial communities is not well understood. We investigated soil microbial communities in five distinct forests (dominated by Populus tomentosa, Robinia pseudoacacia, Salix matsudana, Eucommia ulmoides, and Ailanthus altissima) in Beijing's plain ecological forests. Using metagenomic sequencing, microbial co-occurrence network analysis, and structural equation modeling, we found that forest type significantly affected microbial diversity, functional gene abundance, and the structure of microbial networks. Salix matsudana soils exhibited the highest microbial diversity, and community structure differed significantly among forest types. Acidobacteria and Proteobacteria were the dominant bacterial phyla, indicative of their oligotrophic adaptation to carbon-rich forest soils and their specialized roles in nitrogen cycling, respectively. Populus tomentosa showed highest interconnectivity, while Robinia pseudoacacia had maximal topological integration. Network robustness was strongest in Salix matsudana, whereas Populus tomentosa and Ailanthus altissima were most vulnerable. A total of 430 key microbial functional genes (level 3 KEGG orthologues) were identified, primarily involved in carbon cycling and microbial metabolism. Structural equation modeling accounted for 76.4 % of the variance in network robustness (R² = 0.764) and 35.7 % of the variance in vulnerability (R² = 0.357). The analysis identified tree density and height as critical determinants of network stability, while bacterial diversity and ammonium nitrogen emerged as the primary factors influencing network vulnerability. These findings provide important insights into the complex interactions between forest type, environmental factors, and soil microbial communities, highlighting the critical role of microbial network stability in informing urban forest management strategies.