Fluxes of nitrogen oxides above a subtropical forest canopy in China.

Dry deposition of Nitrogen (N) in forests is commonly estimated from inferential method and/or throughfall measurements, with inevitable uncertainty. In this study, we applied an aerodynamic gradient method to directly measure the nitrogen oxides (NOx) flux above the canopy of a subtropical forest in southeastern China for two consecutive years. The flux and transfer velocity generally reached the maximum absolute values in the midday, with the largest diurnal maximum of absolute flux values observed in the winter of 2015 and that of transfer velocity in the autumn of 2015. The annual average transfer velocity was -0.79 and -0.38 cm s-1 in 2015 and 2016, respectively. Although the net downward NOx fluxes predominated for both years, upward flux (net emission) of NOx was observed during spring months, which reflected the possible bi-directional exchange balanced by soil-atmosphere and foliage-atmosphere exchanges. The NOx concentration seemed to be the most important factor controlling the NOx exchange above canopy, and could mainly explain the seasonal variation of N deposition. The linear regression between the NOx flux and concertation was explored, and it was observed that the deposition of NOx was offset by possible underlayer emission of NOx when the ambient NOx concentration below1.7 ppbv and 1.9 ppbv at night and in the day, respectively. The average dry deposition of NOx for the two years was 6.28 ± 0.06 kg N ha-1 a-1, >40% of which might be uptake by the canopy, estimated by comparing the wet/throughfall deposition measurement of nitrate with the observation of NOx flux. This indicated the importance of stomatal uptake of NOx in nitrogen budget in subtropical forests.