Effect Of Exogenous Nitrogen On The Background N₂O Emission In The Fluvo-Aquic Soil And Its Mechanism

Regulating soil N₂O emissions is essential to the sustainable development of agriculture in China.Recently,numerous studies showed that exogenous nitrogen（N） application can not only induce substantial direct N₂O emissions but also have a significant impact on the background N₂O emission.However,the mechanisms behind this are still unclear.In this paper,a cultivation study with ¹⁵N tracers was used to evaluate the influence of exogenous N on the source of N₂O emissions in fluvo-aquic soil and to assess the soil N dynamics under different background N levels.The N₂O emitted from different pathways and the abundances of functional genes related to N₂O emissions were qualified to evaluate the impacts of exogenous N on soil microorganism activates.The interactive effects of plant growth and exogenous N addition on background N₂O emissions were investigated via pot experiment.Finally,the impacts of exogenous N on background N₂O emission in fluvo-aquic soil and its mechanism are summarized from the view of 1)the change of N₂O emission substrate,2)the pathway of N₂O emission,and 3)plant regulation effect.The main results of this study are as follows:1.The cultivation experiment showed that exogenous N application could not only induce direct soil N₂O emissions but also significantly stimulated the background N₂O emissions.After N addition,the N₂O emissions from background with low（LN）and high（HN）background N levels were 1.41 and 5.65 times higher than that under control,respectively(increased by 8.68 and 39.61 ug N kg^-1 respectively).Under equivalent exogenous N addition,the background N₂O emission from HN was significantly higher than that from LN,indicating soils with higher background N levels had higher risks of background N₂O loss.The rate of gross N mineralization increased significantly after N application,and the amount of gross mineralization increased by 1.65-2.20 times(18.33-22.11 mg N kg^-1)in the first two days after N addition compared with non-N applied control.Correlation analysis showed that gross N mineralization had significant positive relationships with background N₂O emissions.The results indicate that the stimulation of gross N mineralization via exogenous N addition provides the substrates for background N₂O emissions.2.By exploring the effect of exogenous N on N₂O emission pathway and its functional microorganisms,it is found that 61.79%to 78.19%of the background N₂O emissions come from nitrification process.It is shown that the background N₂O emission is mainly produced by nitrification.Significant increase in soil AOB gene abundance after exogenous N application.Redundancy analysis showed that AOB gene abundance had a significant effect on the total N₂O emissions and the background N₂O emissions.It is shown that exogenous N application mainly promotes total N₂O emission and background N₂O emission by stimulating the growth of ammonia oxide bacteria.3.The pot experiment showed the increased background N₂O emissions after N fertilizer application account for about 13.69%to 18.14%of the total emissions.The interaction between N fertilizer application and crop growth has a significant effect on N₂O emissions and their sources.Compared with unplanted corn treatments,corn cultivation significantly reduced the total amount of soil N₂O emissions,but the proportion of background N₂O emissions increased significantly.This shows that the external N fertilizer and crops have a synergy effect on N turnover and background N₂O emissions.In summary,the application of N fertilizer causes the fluvo-aquic soil N turnover to accelerate,promotes the increase of soil N gross mineralization,provides the substrate for the background N₂O emission through the nitrification process mainly.Then under the dominance of ammonia oxide bacteria,the mineralized of native N is converted into N₂O emissions.Crop growth participation collaboration with exogenous N application could synergism promote the priming process of background N₂O emissions.Therefore,on intensively farmed fluvo-aquic soils,future N₂O mitigation strategies showed pay more attention to the emission from native soil.