贵阳地区主要大气氮源的沉降机制与分布：基于石生苔藓氮含量和氮同位素的证据

Deposition mechanism and distribution of the dominating atmospheric N sources in Guiyang area:Evidences from tissue N and δ ^(15)N in epilithic mosses

对贵阳市区到农村地区4个方向的175个石生苔藓样品氮含量和氮同位素(δ15N)进行了分析。苔藓δ15N表明贵阳地区大气氮沉降以铵沉降为主,主要来源为城市废水氨释放以及农村地区的农业氨挥发。苔藓氮含量从市区往外随距离呈指数降低,而δ15N随距离对数升高,揭示了城市来源的铵沉降随距离指数降低,该变化模式与其他点源氨的沉降分布以及点源氨附近苔藓氮含量的变化规律相同。因此,贵阳市区可以看成整个研究区的一个点状氨源。不同方向苔藓氮含量和δ15N随距离的变化梯度存在差异,表明城市铵向周边不同方向的扩散迁移具有非均一性,其原因可能与地形条件、植被状况、风向、城市化程度等有关。苔藓δ15N平均值在15km以外没有明显差异,反映了农业面源氨的影响。此外,运用极限法计算,苔藓氮含量随距离的变化关系(y=1.5e-0.13x+1.26)表明城市铵的影响范围总体小于41km,而苔藓δ15N的变化(y=2.54lnx-12.23)反映了在17.2km以内的地区,大气氮沉降以城市铵为主,17.2km以外的地区主要受农业铵沉降的控制。本研究为城市氮沉降的变化提供了新的证据,促进了苔藓氮含量和δ15N方法研究大气氮沉降从定性向定量的发展。

The study investigated the N concentrations and δ^15N values in 175 epilithic mosses collected along four directions from urban sites to rural sites in Guiyang. Mosses δ^15N showed that atmospheric N deposition was dominated by ammonium （NH,）, which was mainly derived from city sewage NH3 release and agricultural NH3 in rural area. Away from city center, mosses N concentrations decreased exponentially with distance, while δ^15N values showed a significant logarithmic increase with distance. These evidences revealed that the deposition of city NH, declined exponentially with distance, which was consistent with the pattern of NH, deposition and the variation of mosses N concentrations around point NH3 sources, thus the urban area could be seen as a point NH3 source within the whole studying area. Different varying gradients of mosses N concentrations and δ^15N were observed for different directions, indicating the diffusion and transport of city NH, was not equipotent around the urban area, which was mainly related to local factors such as geographic conditions, vegetation coverage, wind direction and urbanization etc. However, no significant difference was seen for mosses δ^15N at sites beyond 15 km from city center, showing the predominance of non-point agricultural NH3 sources. Moreover, according to limiting calculation, the equation between mosses N concentrationa and distance （ y = 1.5^e-0.13 x ＋ 1.26） indicated that the influence of city NH 2 was generally less than 41 km, and the equation between δ^15N and distance （ y = 2. 541nx - 12.23） showed atmospheric N deposition was dominated by city NH, within 17.2 kln, but was controlled by agricultural NH2 beyond 17.2 km. This study has provided new evidences for city N deposition and was helpful for the development of mosses N concentration and δ^15N from a qualitative technique to a quantitative one in N deposition study.