地表臭氧和氮沉降增加对杨树林冠层/大气界面氨气交换通量的复合影响机制

Bi-directional exchange of ammonia between vegetation and the atmosphere is related to changes in climate, chemical composition and vegetation status. Currently, the increasing of atmospheric nitrogen (N) deposition and ozone (O3) concentration has been becoming significant in regional and global areas, and directly affects tree growth and nitrogen uptake. In this project, fast-growing poplar plantation is selected since the plants are widespread in northern China and sensitive to O3. Two O3 levels and two N addition levels will be applied under the field condition through the Free Air Ozone Concentration Enrichment system (O3-FACE), with aim to reveal the combined effect of enhanced N deposition and O3 concentration on NH3 exchange fluxes between the canopy and the atmosphere and its related eco-physiological mechanisms. The combined effect of enhanced O3 concentration and N deposition, as well as its key influencing factors on the canopy NH3 compensation point will be ascertained by observing physiological and biochemical indexes of poplar (Photosynthetic Characteristics, glutamine synthetase and Glutamate dehydrogenase activity), and calculating the stomatal and soil NH3 compensation points. Furthermore, we calculated the NH3 exchange fluxes between the canopy and the atmosphere using the two-layer NH3 exchange model, and then established response relationships between the estimated fluxes and the key physiological and biochemical indexes and gaseous NH3 concentrations over the canopy to reveal mechanism of combined effect of enhanced O3 concentration and N deposition on NH3 exchange fluxes, and its seasonal and annual variability as well. The outcomes of this project will provide the scientific data for evaluation of NH3 budget in N cycle of forest ecosystem in northern China under global environmental change and also for optimization of parameterizations of NH3 emission and deposition in atmospheric transport models.

植被与大气间NH3交换过程与气候、大气化学和植被生长状态有关。目前，区域或全球尺度的地表O3浓度和氮沉降不断上升，并直接影响林木生长和氮素利用。本项目选择我国北方分布广泛且对O3和氮沉降相对敏感的杨树林为对象，利用开放式O3熏蒸平台，研究O3和氮沉降增加对杨树林冠与大气间NH3交换通量的复合影响及相关生理生态机制。通过观测不同O3和氮添加处理下叶片生理生化指标（光和特性、呼吸速率、酶活性），并计算气孔和土壤的NH3补偿点，明确O3和氮沉降对冠层NH3补偿点的复合影响及其关键因子；在此基础上，利用NH3交换模型来推算植被/大气界面NH3通量，建立NH3通量与关键作用因子和林冠氨气浓度的响应模型，揭示O3和氮沉降增加对NH3通量的复合影响机制及其年季变化规律。研究成果为评估全球变化下我国北方森林生态系统氮循环中NH3收支提供数据支持，为优化大气传输模型中NH3排放和沉降的参数提供理论参考。

氨的气孔补偿点对地表和大气界面氨气交换起着重要作用，氨补偿点受到土壤氮素供应的影响，且在不同作物种类间波动较大，然而在当前大气氮沉降增加和臭氧浓度上升情况下，森林植物氨气孔补偿点如何响应仍不清楚，这导致大气化学模式中氨补偿点的参数化存在很大的不确定性。因此，本项目利用野外控制实验，通过氮添加和臭氧熏蒸实验模拟沉降增加和臭氧浓度上升对速生杨546叶片氨气孔补偿点的影响，同时探究植物光合参数与补偿点的关系。研究结果显示，臭氧浓度上升和氮沉降增加均显著减少氨气孔补偿点，降幅分别为41%和19%。臭氧和氮沉降之间存在明显的交互作用，氮添加可以减缓臭氧浓度上升对氨补偿点的负面影响。在光合指标响应方面，臭氧浓度上升可以显著减少光饱和光合速率和叶绿素含量，同时显著增加包间二氧化碳浓度，但其对气孔导度无显著影响。相比之下，氮沉降的增加对所有测定的光合参数均无显著影响。总的来说，氨气孔补偿点与光饱和光合速率、气孔导度和叶绿素含量呈显著正相关，而与包间二氧化碳浓度呈显著负相关。臭氧增加和氮添加对种植546杨树的土壤pH均无显著影响，不同处理pH均值在7.3-7.5之间；氮添加显著增加土壤铵态氮含量，但臭氧浓度上升与氮添加和臭氧浓度上升间土壤铵态氮含量无显著差异，说明臭氧熏蒸可能会影响546品种对土壤铵态氮的吸收，进而影响土壤气孔氨补偿点。本研究表明由臭氧引起的光饱和光合速率、包间二氧化碳浓度和叶绿素含量可能对氨气孔补偿点产生影响；土壤铵态氮含量的改变可能对土壤氨气补偿点产生影响。以上重要发现可以为大气化学模式中氨补偿点参数化的优化提供科学依据，更好的模拟在未来全球变化（例如氮沉降增加和臭氧浓度上升）在森林生态系统氨收支。