Effects of Nitrogen Addition and Drought on Soil Microbial Diversity and Community Composition in a Young Tree Community

Soil microorganisms are well known to play a crucial role in carbon and nutrient cycling within terrestrial ecosystems. Numerous research efforts have demonstrated that nitrogen deposition can change forest soil microbial diversity and community composition; however, it is still unclear how nitrogen deposition will affect the soil microbial diversity and community composition in subtropical forests under the background of increasing drought. Consequently, over a period of 2.5 years, we carried out an experiment using two N addition regimes and three soil water treatment levels to reveal the effects of nitrogen, drought, and the influence of their interaction on the diversity and community composition of soil microorganisms. Overall, we found that both N addition and drought decreased the bacterial Shannon and Simpson indices yet had no significant effect on fungal diversity. In the well-watered treatments, nitrogen addition did not significantly reduce bacterial diversity, while in the moderate drought and severe drought treatments, N addition significantly decreased bacterial diversity, reducing the Shannon and Simpson indices by 27.05% and 0.13%, respectively, in the severe drought treatment. Drought significantly altered the community composition of bacteria regardless of N addition. N addition significantly changed the community composition of bacteria under moderate drought treatments, while both N addition and drought had less significant effects on the fungal community composition. The soil water content, fine root biomass, and soil pH were significantly correlated with bacterial community composition, which explained 53.3%, 11.1%, and 8.7% of the changes in soil bacterial community composition, respectively. These results suggest that drought may intensify the inhibitory effect of nitrogen on bacterial diversity and change the magnitude and direction of the impact of nitrogen on the composition of the bacterial community.