维管束植物桂花树叶片对大气氮沉降的吸收研究

本文通过测定维管束植物桂花树叶片的氮含量和氮同位素组成,分析其对大气氮沉降的截留吸收机制。为期一年(2009.03～2010.03)的监测数据显示,桂花树叶片氮含量为1.33%～3.09%,平均值为2.18%;叶片δ15 N为+0.54‰～+3.78‰,均值为+2.29‰。桂花树叶片N%、δ15 N的季节性变化趋势,总体呈现春、冬高,夏、秋低的规律,这与已有的贵阳市雨水监测数据一致。对比不同树冠厚度的叶片样品,发现叶片N%随树冠厚度增加而降低,顶部叶片N%表现为最高(2.39±0.4%),说明桂花树的树冠层对大气氮沉降有明显的截留吸收作用。桂花树顶部叶片δ15 N最偏负,而上覆树冠层最厚的下方叶片的δ15 N最偏正,反映了树冠层在吸收大气氮沉降过程中存在选择性吸收,引起同位素分馏,即树冠层越厚,穿冠水δ15 N越偏正,且分馏程度与树冠厚度成正比,导致下方叶片δ15 N最高。

Vascular Foliar N% and 15N Responses to Atmospheric Nitrogen Deposition in Guiyang

This article deals with the responses of ecological system to atmospheric nitrogen deposition.Leaves of Osmanthus Fragrans were collected since Mar.2009 till Mar.2010,in an urban area at Guiyang.We measured nitrogen content and isotopic composition of leaves with different canopy thickness,sampling positions were specifically identified.Results turned out that the temporal variation of foliar N% and δ15N showed higher levels in Spring/ Winter,lower levels in Summer/Autumn.That variation trend is consistent with the monitoring data of rainwater in Guiyang.In the case of upper and lower canopies,a decrease in N% of leaves was associated with an increase in canopy thickness,and the upper canopy has the most negative δ15N,while the lower one has more positive isotopic values.These results can be interpreted as that isotopic fractionation happened during uptake of atmospheric N by leaves.In canopy retention processes there is 15N discrimination,causing the isotope ratios to deviate from the atmospheric nitrogen deposition,and giving rise to the δ15N of lower canopy.This summary coupled with the results of other experiments supported the conclusion that vascular leaves can be used as a robust marker of atmospheric N sources,which can boost the prevention of eco-environmental deterioration.