Effects of different nitrogen levels on growth and rhizosphere soil microorganisms of Idesia polycarpa Maxim

Aims Idesia polycarpa Maxim is a valuable oil and timber species, yet scientific guidance for its fertilization management remains scarce, limiting its productivity. This study aims to investigate the effects of different nitrogen (N) fertilizer levels on the growth characteristics of I. polycarpa to provide a theoretical basis for its fertilization management. Methods Uniformly healthy one-year-old seedlings of I. polycarpa were treated with four nitrogen (N) application rates: 0 (control, CK), 1.2 (low nitrogen, LN), 2.4 (medium nitrogen, MN), and 3.6 (high nitrogen, HN) grams per plant. To assess the rhizosphere microbial community, high-throughput sequencing was performed targeting the bacterial 16S rRNA and fungal ITS gene regions. Results The results demonstrated that N fertilization significantly enhanced plant growth and soil physicochemical properties compared to the CK treatment. Specifically, the MN treatment significantly increased root length, root volume, and root surface area ( p < 0.05). The average root diameter was also higher in all N-fertilized groups than in CK. N application influenced soil properties: the HN treatment resulted in lower soil pH but higher alkali-hydrolyzable nitrogen (AN) and available potassium (AK) content, while the MN treatment exhibited higher soil organic matter (SOM) and available phosphorus (AP) content. The soil bacteria community was dominated by Proteobacteria, Chloroflexota, Acidobacteria, and Actinobacteria, While Ascomycota dominates the fungal community. Conclusion The study found that the primary metabolic pathway of bacteria in the rhizosphere soil of I. polycarpa was metabolism, while the main metabolic pathways of fungi were biosynthesis, precursor metabolism, and energy synthesis. Furthermore, an N application rate of 1.2–2.4 g per plant per month is recommended for optimal growth during the early rapid growth phase of I. polycarpa .