Abstract: Aims Increased atmospheric nitrogen (N) deposition leads to changes in the proportion of different forms of N input in forest ecosystems, but the response mechanisms of plants to different N-form sources and their resource trade-off strategies have not been clarified. The effects of N deposition on plant growth and development are still unclear, restricting the improvement of forest productivity based on the regulation of N forms. This study aimed to reveal the response of leaf traits and growth of temperate tree species to different forms of N addition and elucidate the adaptive strategies of plants to the heterogeneity of ammonium, nitrate, and organic N. Methods We selected one-year-old seedlings of four common tree species in the temperate forests of Northeast China as the research objects, namely Betula platyphylla, Fraxinus mandshurica, Juglans mandshurica, and Quercus mongolica. We analyzed the effects of ammonium, nitrate, and organic N (glycine) additions and control (no N addition) on leaf traits of seedlings grown in pots over three growing seasons (2021-2023). At the end of the third growing season, changes in 11 indices, including leaf morphological traits, chemical traits, stoichiometric ratios, and biomass, were evaluated for their differences among N treatments and to investigate the relationship between leaf traits with sensitive plasticity in response to nitrogen treatments and growth. Important findings All three forms of N additions were found to reduce leaf P concentration and increase aboveground biomass, showing a “growth dilution effect” on P concentration due to rapid growth. Compared with control, the ammonium and nitrate additions did not have a significant effect on leaf N concentration, whereas glycine addition significantly increased leaf N concentration (+19%). Leaf N:P and C:P ratios showed sensitive plasticity to N addition (PPI≥0.3), and interspecific differences increased under ammonium and nitrate treatments. The species with high N:P and C:P achieved greater biomass, indicating that species capable of more effective resource utilization had greater growth competition potential. The addition of N affected growth strategies of temperate tree species by regulating leaf chemical traits and stoichiometric ratios. Organic N (glycine) addition boosted N concentration without exacerbating interspecific differences and facilitated seedling acclimatization to N- and P-limited environments. These results provide a theoretical basis for understanding the response of temperate forest species to N addition and guidance for the selection of N fertilizer forms in plantation forests under the background of N depletion. It is recommended to prioritize the application of organic N to balance productivity enhancement and maintenance of community stability.

Key words: Nitrogen addition, nitrogen form, phenotypic plasticity, leaf traits, aboveground biomass