Performances of a system for free-air ozone concentration elevation with poplar plantation under increased nitrogen...

The increasing emission of nitrogen oxides exerts large impacts on vegetation by raising surface ozone (O₃) concentrations and enhancing atmospheric nitrogen (N) deposition. We established a free-air O₃ concentration elevation and enhanced N deposition system (O₃-N-FACE) in Beijing, China, to investigate long-term effects of elevated O₃ and N deposition on poplar plantation. Eight square plots with a side length of 16 m were randomly allocated to elevated O₃ (E-O₃) and ambient air (AA) treatments. Ozone generated by electric discharge in pure oxygen is mixed with clean and dry air, and released from small holes on the tubes installed above the plant canopy at a rate controlled to keep O₃ concentration in E-O₃ plots by 50% higher than that in AA plots. Each O₃ treatment plot consisted of four subplots with a factorial combination of 2 lines of poplar clones and 2 levels of N deposition rate. In enhanced N deposition subplots, we sprayed urea solution on the plantation floor at a rate of 60 kg ha^-1 year^-1. We hereby present the system performances during the growing seasons of 2018 and 2019: the first 2 years of experiment. The mean daytime O₃ concentrations of E-O₃ plots were 38% and 31% higher than AA plots in 2018 and 2019, respectively. And, in 2019, the accumulated O₃ exposure over 40 ppb (AOT40) in E-O₃ plots was 70% higher than that in AA plots. The hourly mean O₃ concentrations in E-O₃ plots were within 20% of the target for 83% of time on average across the four E-O₃ plots. Within the E-O₃ plots, spatial distribution of the hourly O₃ concentration exhibited the maximum deviation at 24% in 2019. We concluded that performance of this system is better than other similar facilities for trees and suitable for a long-term experiment of enhanced O₃ and N.