High nitrogen addition decreases the ozone flux by reducing the maximum stomatal conductance in poplar saplings

Highlights: • O₃ flux of poplar was investigated under 5 O₃ levels and 4 N treatments. • High N reduced g_s and the O₃ flux. • E-O₃ inhibited the growth of poplar, but N addition stimulated it. • O₃ and N have no interactive effect on total biomass. • N addition did not affect O₃ flux-effect relationships for relative biomass. Ground-level ozone (O₃) and nitrogen (N) deposition are major environmental pollutants, often occurring concurrently. Ozone exposure- and flux-response relationships for tree biomass are used for regional O₃ risk assessment. In order to investigate whether soil N addition affects stomatal O₃ uptake of poplar, poplar saplings were exposed to treatment combinations of five O₃ levels and four N addition levels. High N addition treatment reduced the accumulated stomatal O₃ uptake in the leaf due to reduced maximum stomatal conductance (g_s). Nitrogen addition also significantly reduced the steady-state light-saturated g_s in August and September. Elevated O₃ significantly reduced and N addition increased total plant biomass; however, there were no significant O₃ × N interactions. The slopes of biomass-based O₃ exposure- and flux-response relationships did not differ significantly among N treatments. The critical levels for a 5% biomass reduction were estimated at 15.4 ppm h and 17.1 mmol O₃ m⁻² projected leaf area (PLA) for Accumulated O₃ exposure Over an hourly Threshold of 40 ppb (AOT40) and Phytotoxic Ozone Dose above a threshold 1 nmol O₃ m⁻² PLA s⁻¹ (POD₁). These results can facilitate the evaluations of O₃ effect on the carbon-sink capacity and productivity of forest.