Effects Of Biochar On N₂O Emission And Nitrification And Denitrification Process In A Farmland Soil

Nitrous oxide（N₂O）is one of the main greenhouse gases.Farmland soil is the most important source of emissions and one of the important ways to lose soil nitrogen.The content of mineral nitrogen,soil p H and the number and diversity of nitrifying and denitrifying microorganisms in soil are important factors affecting soil N₂O emissions.Biochar is produced by pyrolysis of biomass materials such as agricultural waste under anoxic conditions,and has great application prospects in improving soil properties and mitigating climate change.Studies have shown that the application of biochar can improve soil quality,increase nitrogen use efficiency,change soil nitrogen cycle,and thus affect soil nitrification and denitrification process,and indirectly affect soil N₂O emissions.In this study,the soil of winter wheat-summer maize rotation area in North China Plain was selected as the research object.The effects of biochar on soil nitrification and denitrification were studied by isotope labeling combined with acetylene inhibition technology.Effects of N₂O emission rate,soil p H and soil mineral nitrogen content on soil nitrification and denitrification,and five functional genes of soil AOA-amo A,AOB-amo A,nir K,nir S and nos Z were performed by q PCR and Mi Seq high-throughput sequencing.Absolute quantification and microbial community structure diversity analysis,clarifying the microbial mechanism of biochar on N₂O emission from farmland soils in North China,and providing a theoretical basis for the application of biochar in farmland in North China.The main conclusions are as follows:1.During the indoor culture period,the dynamic change trend of ¹⁵N₂O emission rate is basically the same in different treatments.The ¹⁵N₂O emission of nitrification treatment is mainly concentrated in the first 24 h,and the emission reduction effect of 2 % biochar treatment is also in this period;denitrification The ¹⁵N₂O emissions from each treatment were concentrated in the first 36 h and peaked at 24 h,during which 2 % biochar treatment was significantly lower than the control treatment,while 5% biochar treatment was significantly lower than the control at 36 h;heterotrophic nitrification The ¹⁵N₂O emission rate in the treatment was at a low level,and the two proportions of biochar significantly reduced the ¹⁵N₂O emission rate in the first 24 hours.2.From the perspective of cumulative emissions,2 % biochar in autotrophic nitrification reduced 20.6 % of ¹⁵N₂O emissions compared with the control,while 5 % biochar did not differ from the control;2% biochar in heterotrophic nitrification Compared with the control,5% biochar significantly reduced ¹⁵.7 % and 13.2 % of ¹⁵N₂O emissions,respectively;in denitrification,2 % biochar and 5 % biochar decreased significantly by 40.9 % and 11.7 %,respectively.¹⁵N₂O emissions.Overall,2 % biochar has a better emission reduction effect on nitrification and denitrification than 5 % biochar,especially for ¹⁵N₂O produced by denitrification.3.Under the soil and environmental conditions,N₂O from autotrophic nitrification accounts for 77.4 %⁸1.9 %,heterotrophic nitrification accounts for 0.68 %⁰.85 %,and denitrification accounts for 17.1 %²5.7 %.The effect of adding biochar on the relative contribution of N₂O produced by autotrophic nitrification,heterotrophic nitrification and denitrification was not significant.4.The application of biochar has a certain potential effect on the adjustment of soil p H.The soil p H of each treatment only significantly different at several time points,but the overall range of soil p H of each treatment is only 0.1⁰.2 units.The difference is not significant in the hole incubation time,indicating that biochar has no effect on soil p H in a short time.5.The change trend of soil ¹⁵NH₄⁺-N content between biochar and control was basically the same.The addition of 5% biochar significantly reduced the content of ¹⁵NH₄⁺-N in the soil during nitrification and denitrification treatment.The addition of 2% biochar also showed the content of ¹⁵NH₄⁺-N decreased in the soil,indicating that biochar had certain soil ¹⁵NH₄⁺-N.Holding ability.6.The change trend of ¹⁵NO₃^--N content in soil between treated biochar and control was different,but adding 2% biochar and 5% biochar significantly reduced the content of ¹⁵NO₃^--N in soil during nitrification and denitrification treatment.It indicates that biochar has a certain ability to retain ¹⁵NO₃^--N in the soil,which may be the reason why biochar reduces denitrification to reduce ¹⁵N₂O emissions.7.The application of biochar significantly increased the copy number of AOA-amo A,and showed a tendency to increase with the increase of carbon application,while the effect of AOB-amo A was significantly reduced by biochar,and the increase of AOA-amo A did not appear as N₂O.The increase in emissions indicates that AOB-amo A plays a major role in ammonia oxidation under this condition.The application of biochar significantly increased the copy number of nir K gene,and the increase of 5 % biochar was larger,while the copy number of nir S gene was slightly decreased.The regulation of biochar on nir S gene and nir K gene may be biochar reduction.The cause of denitrification of N₂O emissions.The nos Z gene copy number is less affected by biochar and may not be the cause of biochar reduction in short-term experiments.8.The community structure of nitrifying and denitrifying microorganisms in different treatments has little change.For AOA,its diversity is relatively simple,and its dominant gates,classes and genera are Thaumarchaeota,Nitrososphaeria and Nitrososphaera.The proportion is about 94.5 %,and biochar has no effect on its community structure.For AOB,the dominant gates,classes,and genera are Proteobacteria,Betaproteobacteria,Nitrosospira,and Nitrosovibrio,among which biochar improves Nitrosospira and reduces Nitrosovibrio.For nir K and nir S denitrifying bacteria,nir K is sensitive to biochar at the genus level,among which Bradyrhizobium,Rhodopseudomonas,Rhodococcus and Mesorhizobium are the main dominant genus,and Rhodococcus genus is significantly improved in 2 % biochar.The relative abundance of the genus of the other class increases with the amount of biochar applied.Biochar did not change the community structure of nir S-type denitrifying microorganisms.Azospirillum,Rubrivivax,Herbaspirillum,Cupriavidus and Sulfuritalea were the main dominant genus of nir S-type denitrifying microorganisms.The main advantages of nos Z type denitrifying bacteria are Pseudogulbenkiania,Cupriavidus and Burkholderia,in which biochar has a significant effect on the relative abundance of the genus Cupriavidus and Burkholderia.Unidentified genus and relative abundance of nos Z type denitrifying bacteria are less than 1 %.The genus accounts for a large part,and its specific role in N₂O reduction remains to be studied.9.¹⁵N₂O produced by nitrification was significantly positively correlated with soil ¹⁵NH₄⁺-N,and negatively correlated with soil ¹⁵NO₃^--N,and had no significant correlation with soil p H.The ¹⁵N₂O produced by denitrification was significantly positively correlated with soil ¹⁵NO₃^--N,and had no significant correlation with soil ¹⁵NH₄⁺-N and p H.The ¹⁵N₂O produced by heterotrophic nitrification was significantly positively correlated with soil ¹⁵NH₄⁺-N,and negatively correlated with soil ¹⁵NO₃^--N,and had no significant correlation with soil pH.