气溶胶的辐射效应及冠层氮沉降效应对植物光合作用及生长的影响

气溶胶粒子能够散射太阳辐射，使更多散射光进入冠层内，缓解阴生叶光限制，从而促进冠层光合速率，提高冠层碳吸收；但高气溶胶浓度下，其对辐射的吸收作用导致总辐射下降，削弱散射光施肥效应，降低冠层碳吸收。以往的研究通常仅关注气溶胶的辐射效应，忽略了气溶胶沉降到生态系统后，其化学组分对碳循环的影响。我国多数区域气溶胶含氮量为20-30%，这些含氮颗粒物的沉降如何影响碳循环对气溶胶辐射效应的响应，相关研究还很缺乏。本研究拟利用城郊气溶胶浓度梯度，选取三个污染程度不同的站点，并设置15N标记的冠层氮沉降实验。通过对光合速率、叶片生理生化、个体生长等过程的监测，探讨气溶胶的辐射效应及氮沉降效应对植物光合及植株生长的影响。本研究的关键科学假设为冠层氮沉降上升将提高叶片含氮量，促进光合速率对散射辐射上升的响应强度。本研究将有助于分离气溶胶的光学特性及化学组分对碳循环影响的相对贡献，揭示相关过程的生理生态机制。

Anthropogenic activities have induced a dramatic increase in atmospheric aerosols. Although aerosol reduces total radiation, it increases the fraction of diffuse radiation. The more diffused sunlight penetrates into canopy more efficiently and canopy photosynthesis is then stimulated. But under very high aerosol loadings, the negative effect caused by total solar radiation reduction could offset the diffuse radiation fertilization effect, and therefore reduce canopy carbon uptake. Aerosol chemical composition depends on emission sources and varies in a wide range. Most previous aerosol studies are focused on the impacts of solar radiation changes, but ignore the effects caused by the chemical component deposition. Nitrogen compounds accounts for 20-30% of the aerosol mass in most regions in China. Deposition of these nitrogen compounds will profoundly alter the response of carbon cycle to radiation change induced by aerosols. However, this question has rarely been addressed. In this research, we will choose three sites along the urban-rural aerosol gradient in Beijing, and simulate canopy deposition using 15N labeled nitrogen. Leaf photosynthesis, leaf physiological and biochemical properties, plant growth and soil processes will be measured. The key hypothesis is that, canopy nitrogen deposition will increase leaf nitrogen concentration, and therefore enhance the response sensitivity of leaf photosynthesis and plant growth to increasing diffuse radiation induced by aerosols. The proposed study could partition the relative contributions of aerosol-induced radiation changes and aerosol-associated canopy nitrogen deposition on ecosystem carbon cycle, and improve the understanding of the underlying eco-physiological mechanisms.