石生苔藓氮含量和氮同位素指示贵阳地区大气氮沉降的空间变化和来源

Nitrogen Concentration and Nitrogen Isotope in Epilithic Mosses for Indicating the Spatial Variation and Sources of Atmospheric Nitrogen Deposition at Guiyang Area

对贵阳市区到农村地区4个方向的石生苔藓氮含量和氮同位素组成进行了对比分析.苔藓氮含量变化范围为0.85%~2.97%,并从市区(2.24%±0.32%)往外明显降低(1.27%±0.13%),表明贵阳市区氮沉降最高、往外逐渐降低,但在较远的农村地区(>25 km)苔藓氮含量出现回升(平均1.33%~1.75%),反映了农村地区大气氮输入有所增加.苔藓氮同位素均为负值(-12.50‰^-1.39‰),并从市区到农村地区明显升高.市区苔藓较负的氮同位素比值(平均-8.87‰^-8.59‰)主要指示了城市排泄物和污水所释放的氨源贡献,而郊区和农村地区苔藓较高的氮同位素信号(平均-3.83‰^-2.48‰)主要反映了农业氨源的影响,苔藓氮含量回升可能与农业活动增强有关.此外,贵阳地区苔藓氮同位素的分布特征与受氧化态氮源控制的地区苔藓氮同位素变化相反,因而,贵阳地区大气氮沉降以铵(NHx)沉降为主,该结论有助于更加准确地认识城市地区大气氮沉降的来源和变化.

Tissue N concentration and δ15N signature in epilithic mosses were investigated along 4 directions from urban area to rural area at Guiyang city. Mosses N concentration ranged from 0.85 % to 2.97 % and showed significant decrease from urban area （2.24% ± 0.32 % ） to suburb （1.27%± 0.13% ）, reflecting the level of atmospheric N deposition decreased away from urban. While slightly higher tissue N reemerged at rural area beyond 25km （mean = 1.33%-1.75%）, suggesting increased N deposition occurred in the rural area. Mossesδ15N values varied from - 12.5‰ to - 1.39‰ characterized with getting less negative from urban to rural area. More negative δ15N signatures of urban mosses （mean = -8.8‰-8.59‰） mainly indicated the extensive NH3 sources released from excretory wastes and sewage, while mosses growing at farther suburb or rural area had less depleted δ15N values （mean = - 3.83‰- 2.48‰）, indicating the large sources of agricultural NH3 emission connected with fertilizer application. Moreover, the pattern of mosses δ15N variation in this study was opposite to areas where N deposition was dominated by oxidized form N （ NOx ） with generally positive mosses δ15 N. Accordingly, we concluded that reduced NH, was the dominant N form in atmospheric deposition at Guiyang area, which would be of great value for understanding the spatial variation and sources of atmospheric N deposition.