Canopy vs. soil nitrogen addition differentially modulates rhizosphere microbe-metabolite coupling in Populus euphratica under drought stress

Abstract

Nitrogen (N) deposition and water availability profoundly influence arid ecosystem functions; however, how different N input methods (canopy vs. soil) to shape plant–microbe–metabolite interactions remains poorly understood under drought. Integrating metagenomic and root metabolomic approaches, we conducted a pot experiment with Populus euphratica seedlings subjected to varying soil water conditions (well-watered vs. drought) and N addition methods. Our results identified soil water as a major factor influencing rhizosphere microbial assembly. Drought up-regulated the abundance of numerous carbon and N cycling genes, reflecting an adaptive shift in functional investment under stress (consistent with the significant increase in microbial acquisition strategies observed in this study), rather than accelerated biogeochemical process rates. Critically, the N input method significantly modulated these responses: canopy N addition buffered the adverse impacts of drought on microbial biomass and promoted a growth yield strategy. Conversely, soil N addition under drought conditions led to a decline in microbial biomass but fostered a highly complex and tightly coupled C-N functional gene network. Furthermore, fine-root endogenous metabolites—particularly carbohydrates, terpenes, and indoles—accounted for over 90% of the variation in microbial N-cycling genes, markedly outperforming traditional soil physicochemical indicators. Bipartite network analysis revealed that canopy N addition facilitated synergistic coupling between root metabolites and N-cycling genes, whereas soil N addition primarily triggered antagonistic decoupling. These findings suggest that N deposition methods qualitatively alter rhizosphere functional topology through root-metabolic mediation. Incorporating canopy processes into future N-deposition studies may improve our understanding of plant–microbe feedbacks in arid regions.