Ozone (O₃) pollution and nitrogen (N) deposition/fertilization often simultaneously affect plant growth. However, research of their interactive effects on leaf N metabolism is still scarce. We investigated their interactive effects, aiming to better understand plant N metabolism processes and biogeochemical cycles under high O₃ pollution and N deposition/fertilization.

Poplar saplings were exposed to two O₃ levels (NF, non-filtered ambient air; NF60, NF + 60 ppb O₃) and four N treatments (N0, no N added; N50, N0 + 50 kg N ha−1 yr−1; N100, N0 + 100 kg N ha−1 yr−1; N200, N0 + 200 kg N ha−1 yr−1) in open-top chambers for 95 days. The indicators related to leaf N metabolism were analyzed, including the activities of N-metabolizing enzymes and the contents of total N, NO₃−-N, NH₄+-N, total amino acid (TAA) and total soluble protein (TSP) in the leaves.

NF60 stimulated the activities of nitrate reductase (NR) by 47.2% at August relative to NF, and stimulated glutamine synthetase (GS) by 57.3% when averaged across all N treatments and sampling times. In contrast, O₃ did not significantly affect TSP and even reduced TAA content in August. Relative to N0, N200 significantly increased light-saturated rate of CO₂ assimilation (A_sat) by 24%, and increased total N content by 70.3% and 43.3% in August and September, respectively, while it reduced photosynthetic N-use efficiency by 26.1% in August. These results suggest that the increase in A_sat and total N content are uncoupled, and that the surplus N is not used to optimize the capacity for carbon assimilation under high N treatment. Simultaneously, high N treatment significantly promoted leaf N metabolism by increasing NO₃−-N contents, NH₄+-N contents, TAA contents and the activities of NR and GS. There was no significant interaction between O₃ and N for all variables.