典型草地土壤反硝化过程同位素分馏效应

Isotope fractionation effect in denitrification process of typical grassland soil

反硝化过程是生态系统气态氮损失的主要途径,然而其速率一直难以量化。^(15)N同位素自然丰度法是量化陆地生态系统尺度反硝化速率的有效方法,但是该方法需要考虑反硝化过程中的氮氧同位素分馏效应。目前关于陆地土壤反硝化作用分馏系数的研究主要集中在森林和农田,对于草地土壤的研究十分有限。基于此,本研究以我国不同地区的4种典型草地土壤(大兴安岭、多伦、额尔古纳和刚察)为对象,通过室内厌氧培养实验测定土壤微生物群落反硝化作用的硝酸盐消耗速率,并借助瑞利分馏模型拟合出N和O同位素分馏效应(ε)及其比值(Δδ^(18)O∶Δδ^(15)N)。结果显示,草地土壤15ε为21.6‰~32.0‰(27.1‰±2.1‰),^(18)ε为10.4‰~15.7‰(12.9‰±1.1‰)。研究发现,不同环境之间15ε存在一定差异,可能归因于不同类型土壤理化性质和反硝化微生物群落组成的差异。此外,不同研究所用计算方法的差异、底物硝酸盐浓度、硝酸盐异化还原反应以及开放环境中可能发生的硝化作用等都会在不同程度上影响15ε的计算。对于^(18)ε来说,年均温和年均降水是影响^(18)ε的重要因素。草地土壤Δδ^(18)O∶Δδ^(15)N比值为0.38~0.49(0.46±0.03),低于以往陆地和水体生态系统以及微生物纯培养的研究。亚硝酸盐的再氧化和与水之间的氧同位素交换可能是造成Δδ^(18)O∶Δδ^(15)N差异的重要原因。综上所述,不同草地生态系统土壤反硝化分馏效应存在一定差异,在定量评估陆地生态系统反硝化作用速率时需综合考虑气候类型和生态系统等变量。

Denitrification process is the main pathway for gaseous nitrogen loss in ecosystems,but it is difficult to quantify the rate of denitrification.The^(15)N natural abundance method is proved to be effective for quantifying denitrification rates at the ecosystem scale.However,this method requires an understanding of the nitrogen and oxygen isotope fractionation factors during denitrification.Currently,studies on the isotope fractionation factors during denitrification mainly focused on forest and cropland soils,and only few studies have been conducted in grassland soils.Here,we determine microbial nitrate consumption rates of four typical grassland soils from different regions in China(Daxinganling,Duolun,Eerguna,and Gangcha)under anaerobic condition.The nitrogen and oxygen isotope effects(ε)and their ratios(Δδ^(18)O:Δδ^(15)N)were then fitted using the Rayleigh fractionation model.Our results showed that the 15εof grassland soils ranged from 21.6‰to 32.0‰(27.1‰±2.1‰),and the^(18)εranged from 10.4‰to 15.7‰(12.9‰±1.1‰).There was a certain difference in 15εamong different soils,which could be at⁃tributed to the differences in physical and chemical properties of different soil types and composition of denitrifying microbial communities.In addition,different calculation methods,initial nitrate concentration,nitrate dissimilatory reduction reaction,and possible nitrification in open environments may also affect the calculation of 15ε.Mean an⁃nual temperature and mean annual precipitation were important factors affecting^(18)εvalue.TheΔδ^(18)O:Δδ^(15)N ratios of grassland soils ranged from 0.38 to 0.49(0.46±0.03),which were lower than previous studies on terrestrial and aquatic ecosystems and microbial pure cultures.The potential reason for the difference inΔδ^(18)O:Δδ^(15)N might be the re⁃oxidation of nitrite and oxygen isotope exchange with water.In conclusion,there were certain differences in fractionation effects among different grassland ecosystems.Thus,variables such as climate and ecosystem type should be comprehensively considered in quantifying denitrification rates.