State Key Laboratory of Environmental Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China.
CAS Key Laboratory of Forest Ecology and Management, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China.
Sci Total Environ. 2020 May 1;715:136993. doi: 10.1016/j.scitotenv.2020.136993. Epub 2020 Jan 28.
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 (NO) 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 in 2015 and 2016, respectively. Although the net downward NO fluxes predominated for both years, upward flux (net emission) of NO was observed during spring months, which reflected the possible bi-directional exchange balanced by soil-atmosphere and foliage-atmosphere exchanges. The NO concentration seemed to be the most important factor controlling the NO exchange above canopy, and could mainly explain the seasonal variation of N deposition. The linear regression between the NO flux and concertation was explored, and it was observed that the deposition of NO was offset by possible underlayer emission of NO when the ambient NO concentration below1.7 ppbv and 1.9 ppbv at night and in the day, respectively. The average dry deposition of NO for the two years was 6.28 ± 0.06 kg N ha a, >40% of which might be uptake by the canopy, estimated by comparing the wet/throughfall deposition measurement of nitrate with the observation of NO flux. This indicated the importance of stomatal uptake of NO in nitrogen budget in subtropical forests.
森林氮的干沉降通常通过推断法和/或穿透雨测量来估算，存在不可避免的不确定性。在本研究中，我们应用气动梯度法直接测量了中国东南部亚热带森林冠层上方的氮氧化物 (NO) 通量，连续两年进行测量。通量和传递速度通常在中午达到最大绝对值，2015 年冬季观察到最大的绝对通量值的日变化，2015 年秋季观察到最大的传递速度的日变化。2015 年和 2016 年的年平均传递速度分别为-0.79 和-0.38 cm s。尽管两年中净向下的 NO 通量均占主导地位，但在春季仍观察到向上的 NO 通量（净排放），这反映了可能由土壤-大气和叶-气交换平衡的双向交换。NO 浓度似乎是控制冠层上方 NO 交换的最重要因素，主要可以解释氮沉降的季节性变化。还探索了 NO 通量与浓度之间的线性回归，观察到当环境中的 NO 浓度在夜间分别低于 1.7 ppbv 和白天低于 1.9 ppbv 时，NO 的沉积可能会被下层可能的 NO 排放所抵消。两年的平均 NO 干沉降为 6.28 ± 0.06 kg N ha a，其中 >40%可能被冠层吸收，这是通过将硝酸盐的湿沉降/穿透雨测量与 NO 通量的观测进行比较来估计的。这表明在亚热带森林的氮收支中，NO 通过气孔吸收的重要性。
