Short-term nitrogen addition mediates nutrient allocation and resorption trade-offs in Populus koreana: insights for vegetation restoration on volcanic lava platform

Ecological stoichiometry serves as a foundational framework for understanding plant nutrient acquisition, allocation, and conservation strategies in extreme environments. The Wudalianchi volcanic lava platform represents a naturally oligotrophic habitat where Populus koreana functions as a keystone pioneer species during early ecological succession. However, the physiological and stoichiometric responses of P. koreana to nitrogen (N) enrichment remain poorly understood. This knowledge gap constrains both theoretical development of ecological stoichiometry in geologically extreme systems and the formulation of evidence-based ecological restoration strategies for volcanic substrates. To address this, we conducted a controlled short-term N addition experiment across 12 randomized plots (10 m × 10 m each), applying four N treatments: 0 (control), 4, 8, and 16 g N·m⁻²·yr⁻¹. N addition increased soil total organic carbon (TOC) (23.04–142.68%) and total N (TN) (2.94–95.59%), reduced total phosphorus (TP) (7.55–15.09%) and available N: P ratio. N and P concentrations in mature leaves increased significantly (by 28.76–49.87% and 11.13–24.75%, respectively), whereas P concentrations decreased in fine branches and fine roots. Senescent leaf N and P concentrations rose by 6.57–17.60% and 14.10–38.06%, respectively, with a greater increase in P leading to a lower senescent leaf N: P ratio. N resorption efficiency (NRE) increased significantly (by 23.34–33.26%), while P resorption efficiency (PRE) decreased (by 5.67–11.43%). Mature leaf N and P concentrations were positively correlated with soil TN and available P (AP), respectively. In contrast, P concentrations and N: P ratios in fine branches and fine roots correlated negatively with soil AP and the available N: AP ratio, but positively with soil TP and the TN: TP ratio. NRE correlated positively with leaf N concentration, while PRE correlated negatively with both mature leaves P concentration and soil AP. Short-term N addition reshapes soil nutrient availability, driving P. koreana to adopt a “photosynthetic organ-prioritized” nutrient allocation strategy and an N–P resorption trade-off. While N addition partially alleviates N limitation, it concurrently exacerbates P demand, thereby accelerating the transition from N limitation toward N–P co-limitation. These findings refine ecological stoichiometric theory in nutrient-impoverished volcanic ecosystems and offer mechanistically grounded, quantitative guidance for targeted vegetation restoration on lava platforms.