Objective This paper investigates the effects of long-term nitrogen (N) addition on resorption of plant leaf phosphorus (P) fractions, aiming to deepen our understanding of how N deposition regulates tree physiology and ecosystem nutrient cycling.

Method We conducted a 20-year continuous N addition experiment to simulate atmospheric N deposition, focusing on two key afforestation species of Larix gmelinii and Fraxinus mandshurica in northeastern China. Changes in leaf total P and concentrations of P fractions were measured to assess the impacts of long-term N addition on resorption characteristics of leaf total P and P fractions.

Result (1) Long-term N addition treatment significantly reduced the residual P concentration of L. gmelinii leaves by 23.66% and that of F. mandshurica by 24.47% (p < 0.05). The residual P and inorganic P concentrations of L. gmelinii were significantly higher than those of F. mandshurica under long-term N addition treatment (p < 0.05). (2) Long-term N addition significantly affected P resorption characteristics of the two tree species. At the total P level, the resorption efficiencies of L. gmelinii and F. mandshurica increased by 18.08% and 14.31%, respectively. At the fraction level, L. gmelinii showed significant resorption efficiency changes: lipid P resorption efficiency was significantly reduced by 27.62%, while metabolic P, nucleic acid P and inorganic P resorption efficiencies were increased by 6.82% to 11.68%, respectively (p < 0.05); in contrast, only metabolic P resorption efficiency of F. mandshurica was significantly reduced (p < 0.05). The comparison of P resorption proficiency showed that lipid P resorption in senescent leaves of L. gmelinii decreased significantly by 89.70%, while metabolic P and inorganic P resorption increased by 25.22% and 28.89%, respectively (p < 0.05); in contrast, in F. mandshurica, only the inorganic P resorption increased significantly by 28.89% (p < 0.05). (3) The two tree species exhibited distinct correlations between P fractions and resorption characteristics: L. gmelinii primarily demonstrated correlations between P fraction concentrations and resorption efficiency, whereas F. mandshurica showed correlations between P fraction concentrations and resorption proficiency. Under long-term N addition, leaf metabolic P, nucleic acid P, and inorganic P concentrations in L. gmelinii were significantly positively correlated with resorption efficiency (p < 0.05). In contrast, F. mandshurica maintained significant positive correlations between leaf total P, residual P, and lipid P concentrations with resorption proficiency. Additionally, long-term N addition resulted in significant positive correlations between leaf metabolic P, nucleic acid P and inorganic P concentrations of F. mandshurica and its resorption efficiency (p < 0.05).

Conclusion Long-term N addition significantly reduces the residual P concentration in the leaves of the two tree species. L. gmelinii shows a significant decrease in lipid P resorption efficiency and resorption proficiency and a significant increase in resorption efficiency and resorption proficiency of metabolic P and other fractions, while F. mandshurica shows a decrease in metabolic P resorption efficiency and an increase in inorganic P resorption proficiency. These differences reveal different P utilization strategies of the two tree species to long-term N addition.