Concentration-Dependent N-P Interactions Cause Organ-Specific Responses and Nutrient Allocation in Poplar Seedlings

This study explores the complex regulatory mechanisms of nitrogen (N) and phosphorus (P) supply interactions on the growth, root architecture, and nutrient uptake of Populus × euramericana ‘Neva’ seedlings. It shows that these responses depend on nutrient concentrations and exhibit organ-specific patterns. Low P (0 mM) and sufficient N (15–30 mM) enhances plant height and aboveground biomass by promoting P acquisition processes. At moderate N levels (5–15 mM), P supply is sufficient (0.5–1.5 mM) for root and stem growth. Nitrogen application prioritizes aboveground biomass, reducing the root-to-shoot ratio. Root architecture also responds organ-specifically: sufficient N under low P promotes fine root growth to increase P absorption; under moderate P (0.5 mM), balanced N optimizes branching; and under sufficient P (1.5 mM), N increases root thickness while reducing fine root investment. In terms of P metabolism, moderate N under low P increases P concentrations by upregulating phosphate transporter genes, while sufficient N maintains P use efficiency (PUE). For N metabolism, added P under low N (0 mM) maintains N use efficiency (NUE), while higher N levels (15–30 mM) reduce NUE due to interference in nitrogen transport and enzyme activity. This study highlights the importance of organ-specific resource allocation in adapting to N–P interactions and suggests optimizing fertilization strategies based on soil nutrient status to avoid physiological imbalance.