北京地区典型落叶阔叶乔木叶片含氮量和δ^(15)N值对大气氮沉降的响应

Responses and Indications of Foliar Nitrogen Contents and δ^(15)N values to Atmospheric Nitrogen Deposition in Beijing,China

为探究植物对大气氮沉降的响应和对这部分氮素的来源指示作用,本研究通过对北京地区198个采样点,典型落叶阔叶乔木杨属(Populus)和柳属(Salix)植物叶片进行采样,测定其叶片样品含氮量和δ^(15)N值。结果表明:北京地区杨属植物叶片含氮量为16.5—38.6g/kg,平均(24.0±4.0)g/kg;柳属植物叶片含氮量为17.2—36.2g/kg,平均(25.9±4.1)g/kg。研究区域范围内杨属、柳属植物叶片的含氮量均呈现出西北低、东南高的对角线型分布,与该区域大气氮沉降的空间变异相吻合。由于研究区域范围内气候因子无明显的变异,植物叶片的含氮量变化反应了大气氮沉降对植物元素化学计量特征的影响和植物对大气氮沉降的响应。北京地区杨属植物叶片δ^(15)N值为-3.95‰—8.10‰,平均(1.15±2.48)‰;柳属植物叶片δ^(15)N值为-3.04‰—9.73‰,平均(2.31±2.60)‰。杨属和柳属植物叶片的δ^(15)N值均呈现出西北高、中部高、东南低的空间分布,与叶片含氮量空间分布趋势相反。中部城区较高的δ^(15)N值反应了交通污染对大气含氮化合物增加的影响;西北部较高的δ^(15)N值反应了该区域受人为活动排放源的影响较少,自然的氮循环是其较高δ^(15)N值的主要原因;东南部较低的δ^(15)N值则有可能是由农业活动和交通共同作用的结果。

With 60% of the total area located in the North China Plain, Beijing suffered with very high atmospheric nitrogen （N） deposition in the south part, which was as high as 100 kg hm^-2 a^-1 including both organic and inorganic nitrogenous species from both wet and dry deposition. To test the responses of foliar N contents of plants, a collection of deciduous broadleaf tree leaves was conducted across Beijing areas. Typical deciduous broadleaf trees, Populus and Salix were selected at 189 sites, which were firstly designed at 5-decimal minute in latitude and longitude grid resolution and then excluded sites without Populus or Salix species. Leaf sampling collection was carried out in the end of August corresponding to annual peak-standing biomass in late summer. Five trees of every species at each site were selected, and five mature leaves of every tree were collected. All the leaf samples were over-dried at 105℃ for 10 minutes and then 65℃ for 48 hours to constant mass. Samples of each species at each site were put together and grounded with a mill to pass 1-mm sieve for N contents analysis. Foliar N was determined following Kjeldehl digestion and δ^15N values were determined by continuous flow isotope-ratio mass spectrometer. The foliar N contents were （24.0±4.0） g/kg and （25.9±4.1） g/kg on average for Populus and Salix, respectively. Spatial variations of both the two genus showed similar trends, with higher foliar N contents in the southeast and lower foliar N contents in the northwest, consisting with the spatial variation of atmospheric N deposition in Beijing area. The foliar δ^15N values were （1.15±2.48）‰ and （2.31±2.60）‰ on average for Populus and Salix, respectively. However, the spatial variation trends of δ^15N values were contrary with the foliar N contents. Higher δ^15N values were found in the city center and northwest, while lower δ^15N values were found in the southeast. With the different δ^15N values of potential sources, the higher δ^15N values in the city center indicated the traffic emission sources, the higher δ^15N values in the northwest indicated the natural N cycling, while the lower δ^15N values indicated both the agricultural and traffic pollution.