Contrasting mechanisms of poplar and rhizosphere soil influenced by urea and compound fertilizer

The intensive management of poplar plantation depended on nitrogen fertilizer to ensure rapid growth. It is a key prerequisite to optimize the application strategy of nitrogen fertilizer in plantation to clarify the biological mechanism difference of rhizosphere nutrient cycling driven by two nitrogen sources. Based on this, elemental analysis, high-throughput sequencing, transcriptome and metabolome were used to characterize the response of microbiome and metabolic function in poplar and rhizosphere soil to urea and compound fertilizers. The results showed that urea application significantly elevated ammonium (NH 4 + ) and soil organic carbon (SOC) content, enhancing urease and cellobiase activities to intensify carbon-nitrogen coupling. These conditions enriched Proteobacteria , likely through substrate-driven selection favoring taxa with nitrogen assimilation and cellulose degradation capabilities. In contrast, compound fertilizer prioritized nitrate (NO 3 − ) accumulation and stimulated acid phosphatase activity, promoting phosphorus mobilization and enriching Actinobacteria . Both fertilizers increased microbial α-diversity and induced β-diversity divergence. Functional profiling revealed that urea promoted cysteine metabolism and re-established galactose metabolism in roots. Compound fertilizer down-regulated the auxin catabolism gene and promoted the accumulation of auxin in roots. These findings demonstrated that urea and compound fertilizer created distinct microbial functional networks. Urea optimizes rapid nitrogen-carbon turnover, whereas compound fertilizer enhances phosphorus bioavailability. This providing a mechanistic basis for fertilizer-specific management in poplar plantations. • Fertilizers increased the alpha diversity of microorganisms in rhizosphere soil. • Soil N cycle was accelerated and C metabolism was rebuilt in fertilizer treatments. • Urea promoted cysteine metabolism and altered galactose metabolism in roots. • Compound fertilizer treatment resulted in the accumulation of auxin in roots.