Isotope fractionation effect in denitrification process of typical grassland soil
Denitrification process is the main pathway for gaseous nitrogen loss in ecosystems,but it is difficult to quantify the rate of denitrification.The 15N natural abundance method is proved to be effective for quantifying deni-trification 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 deni-trification 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(△δ18O∶△δ15N)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 18ε ranged from 10.4‰ to 15.7‰(12.9‰±1.1‰).There was a certain difference in 15ε among 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 15 ε.Mean an-nual temperature and mean annual precipitation were important factors affecting 18ε value.The △δ18O∶△δ15N 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 in △δ18O∶△δ15N 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.
denitrificationstable isotope techniquegrasslandnitrogen and oxygen isotope fractionation effect△δ18O∶△δ15N
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