不同区域旱地土壤氨挥发过程同位素δ^(15)N变化规律

Changes ofδ^(15)N Values during Ammonia Volatilization from Different Upland Soils in China

大气氨(NH_(3))是PM_(2.5)形成的重要前体物,明确和量化农田等排放源对大气NH_(3)的贡献,是大气污染治理的基础。农田NH3挥发是大气NH3的重要来源之一,氮同位素自然丰度(δ^(15)N)特征可以用来定量溯源大气NH3的来源,但目前对于农田土壤NH3挥发全过程δ^(15)N值动态变化规律的研究比较缺乏,且农田土壤NH_(3)挥发受土壤性质等不同因素的影响,会直接或间接影响NH3挥发的δ^(15)N值,进而影响溯源结果。旱地土壤NH3挥发在我国农田NH3挥发中占主导地位,选取我国4个不同区域旱地土壤(辽宁北票、河南新乡、河北唐山、西藏林芝),添加尿素后在受控条件下采用海绵吸收法开展为期15 d的室内培养试验,通过化学转化法测定不同区域土壤NH3挥发全过程δ^(15)N值并观察其变化规律。结果表明,辽宁北票、河南新乡、河北唐山、西藏林芝的土壤NH_(3)挥发过程δ^(15)N值变化范围分别为–26.14‰~–5.57‰、–31.92‰~–26.31‰、–24.41‰~–3.11‰、–29.17‰~–2.20‰,均值分别为–21.74‰±1.89‰、–29.31‰±1.72‰、–19.82‰±2.04‰、–23.25‰±2.16‰。不同区域旱地土壤NH3挥发过程δ^(15)N值的特征存在差异,新乡土壤的δ^(15)N-NH3值持续升高,而北票、唐山和林芝土壤的δ^(15)N-NH3值出现先降低后升高的趋势。综上所述,土壤性质、NH3挥发速率是影响NH3挥发δ^(15)N值的主要因素,其中土壤pH、NH3挥发速率和累积损失量与δ^(15)N-NH_(3)值显著负相关;此外,同位素分馏效应对δ^(15)N-NH_(3)值也有一定的影响。本研究结果可为大气NH3的定量溯源提供更好的支撑。

【Objective】Atmospheric ammonia(NH3)is an important precursor for the formation of PM2.5.Hence,identification and quantification of the sources of atmospheric NH3 are important for NH3 emission abatement and air pollution control.Farmland NH3 volatilization is one of the important sources of atmospheric NH3.The technology of natural abundance of nitrogen isotopes(δ^(15)N)has been used to trace NH3 sources in recent years.Despite these advances,studies on the dynamic change ofδ^(15)N values from the whole process of NH3 volatilization from farmland soils are lacking.Moreover,NH3 volatilization from farmland soils is affected by different factors such as soil properties,pH,which can directly or indirectly influence theδ^(15)N values of volatilized NH3 and may lead to uncertainties in sources traceability.Upland soil NH3 volatilization dominates total farmland NH3 volatilization in China.We selected four types of upland soil from different regions in China to study theδ^(15)N values of NH3 volatilization from the whole volatilization process to clarify the changes ofδ^(15)N-NH3 values and their impacting factors.【Method】Urea was applied to four types of soils from Liaoning,Hebei,Henan and Tibet,and NH3 volatilization was studied in a 15-day indoor culture experiment by the sponge absorption method under controllable conditions.Theδ^(15)N value during the whole process of NH3 volatilization was measured by the chemical transformation method.【Result】Results showed that the values ofδ^(15)N during NH3 volatilization from Beipiao soil from Liaoning Province ranged from–26.14‰to–5.57‰,with an average of–21.74‰±1.89‰.The variation range ofδ^(15)N values of Xinxiang soil(from Henan province)was from–31.92‰to–26.31‰,with an average value of–29.31‰±1.72‰while that of Tangshan soil(from Hebei province)and Linzhi soil(from Tibet)ranged from–24.41‰to–3.11‰with an average of–19.82‰±2.04‰,and from–29.17‰to–2.20‰with an average of–23.25‰±2.16‰,respectively.Overall,theδ^(15)N values of the NH3 volatilization process in upland soils from different regions are different.During the whole process of soil NH_(3) volatilization,theδ^(15)N-NH3 values of Xinxiang continued to increase,and theδ^(15)N-NH3 values of Beipiao,Tangshan and Linzhi first decreased and then increased.Soil properties and NH_(3) volatilization rate are the main factors affecting theδ^(15)N value.【Conclusion】Our results showed that soil pH,NH3 volatilization rate and cumulative NH_(3) loss were significantly negatively correlated with theδ^(15)N-NH_(3) values.In addition,isotope fractionation also impacts theδ^(15)N-NH3 values.The results of this study can provide better support for the quantitative traceability of atmospheric NH_(3).