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

Abstract

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 (gs). Nitrogen addition also significantly reduced the steady-state light-saturated gs 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 15.4ppmh and 17.1mmol O3 m-2 projected leaf area (PLA) for Accumulated O3 exposure Over an hourly Threshold of 40ppb (AOT40) and Phytotoxic Ozone Dose above a threshold 1nmol 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.