Nitrogen addition and warming drive multilevel plant carbon allocation via storage-growth trade-offs: A meta-analysis

1. Atmospheric nitrogen (N) deposition and global warming are becoming increasingly severe, yet their combined effects on plant carbon allocation—a key factor influencing ecosystem carbon sequestration—remain poorly understood.2. Through a global meta-analysis (1,371 observations), we investigated how N addition and warming regulated carbon allocation strategies across multiple plant levels and functional types.3. Results revealed that N addition significantly enhanced above-ground nonstructural carbohydrate (NSC) storage, whereas both warming and N addition reduced below-ground NSC reserves. However, N addition and warming did not produce synergistic effects on whole-plant NSC storage. In contrast, the combined treatment of N addition and warming synergistically promoted structural growth at the above-, below-ground and whole-plant levels. Herbaceous plants exhibited NSC-driven plasticity under N addition, whereas woody plants demonstrated stronger biomass accumulation under warming, aligning with their distinct carbon economics. In addition, environmental factors (e.g., soil total nitrogen, soil organic carbon) also modulated these responses, showing that high soil nitrogen reduced below-ground NSC allocation.4. Synthesis. Our findings challenge the assumption of linear organ-level combined effects of N addition and warming on plant carbon allocation and highlight the need to incorporate plant functional types and environmental heterogeneity into carbon cycle models under multi-stressor global change.