Characteristics and influence factors of NO exchange flux between the atmosphere and P. nigra.

Nitrogen dioxide (NO) is an important substance in atmospheric photochemical processes and can also be absorbed by plants. NO fluxes between the atmosphere and P. nigra seedlings were investigated by a double dynamic chambers method in Beijing from June 15 to September 3, 2017. The range of NO exchange fluxes between P. nigra seedlings and the atmosphere was from -14.6 to 0.8 nmol/(m·sec) (the positive data represent NO emission from trees, while the negative values indicate absorption). Under ambient concentrations, the mean NO flux during the fast-growing stage (Jun. 15-Aug. 4) was -3.0 nmol/(m·sec), greater than the flux of -1.5 nmol/(m·sec) during the later growth stage (Aug. 8-Sept. 3). The daily exchange fluxes of NO obviously fluctuated. The fluxes were largest in the morning and decreased gradually over time. Additionally, the NO fluxes were larger under high light intensities than under low light intensities during the whole growth period. The effects of temperature on NO fluxes were different under two growth periods. The NO exchange fluxes were larger in a range of temperatures close to 44°C in the fast-growing stage, whereas there were no evident differences in NO exchange fluxes under widely differing temperatures in the later growth stage. Under polluted conditions, the uptake ability of NO was weakened. Additionally, the compensation point of NO was 5.6 ppb in the fast-growing stage, whereas it was 1.4 ppb in the later growth stage. The deposition velocities of NO were between 0.3 and 2.4 mm/sec.