Soil microbial community structure and assembly are more strongly associated with soil background nitrogen content than by short-term nitrogen addition in a salt marsh

Soil microbes drive nutrient cycling, and microbial community composition and assembly are often shaped by plant succession and environmental heterogeneity. Nitrogen (N) is a vital nutrient for microorganisms, yet it remains unclear whether microbial responses to elevated N inputs are consistent at different successional stages with variable soil nutrient conditions. We conducted a field experiment in the Yellow River Delta of China, applying N (NH 4 NO 3 ) at six levels (0, 2, 5, 10, 20, and 50 g N m⁻ 2 year⁻ 1 ) to three plant communities representing different successional stages. Following one year of N addition, herbaceous aboveground productivity increased in all three communities, but there was no significant effect on soil microbial community composition or assembly processes. However, soil bacterial and fungal diversity and community composition were correlated with background soil N content in all three stages. The assembly of these microbial communities was dominated by stochastic processes (drift and dispersal limitation), with a minor role for deterministic variable selection. Moreover, network analysis showed that there were strong positive bacteria-bacteria and fungi-fungi associations, indicating that cooperative relationships predominated in these communities. Additionally, microbial networks exhibited increased modularity and lower node numbers with advancing successional stage. Bacterial and fungal community composition were both shaped by plant characteristics and soil properties. These findings suggest that long-term shifts in soil nutrients during plant succession exert stronger influence on microbial communities than short-term fluctuations in N availability, likely due to the resilience of microbial communities.