Effects Of Biochar And Nitrification Inhibitors On Soil N₂O And NO Emissions In An Organically Managed Vegetable Soil

The vegetable field ecosystem is strongly disturbed by agricultural production activities and the long-term application of chemical fertilizers leads to soil degradation,reduced fertility,resulting in a high emission of active gaseous nitrogen,and a high risk of environmental pollution.China is rich in organic fertilizer resources,and the application of organic fertilizer to chemical fertilizer can help increase the soil carbon pool and improving soil fertility,while profoundly changing soil nitrogen conversion and active gaseous nitrogen emissions.Biochar is a carbon-rich substance produced by pyrolysis of biomass under oxygen-restricted conditions,and its farmland application can affect nitrogen conversion and microbial activity,thereby exerting multiple effects of carbon sequestration,emission reduction and yield increase,so it has attracted much attention.Nitrification inhibitors are an effective emission reduction measure by regulating the soil nitrogen conversion process to reduce the risk of nitrogen loss and reduce environmental pollution.In this study,long-term positioning test treatment was collected,and the influence mechanism of chemical fertilizer combined with organic fertilizer compared with chemical fertilizer alone on soil nitrification and denitrification process was compared.On the basis of organic fertilization of tropical vegetable plot soil,the soil nitrous oxide（N₂O）and nitric oxide（NO）emission characteristics of soil were studied by adding biochar and nitrification inhibitors,and combined with nitrogen conversion characteristics and molecular biology techniques,the soil active gaseous nitrogen emission and nitrogen conversion law of tropical vegetable field under organic fertilizer management and its response mechanism to biochar and nitrification inhibitors were systematically elucidated.The results of this study will elucidate the microbial mechanisms of reactive gaseous nitrogen emission from tropical vegetable soils,which will be of great theoretical value and practical significance for the construction of efficient and safe nitrogen management decisions and the prevention and control of ground source pollution in vegetable facilities.The main conclusions are as follows:（1）The different fertiliser treatments had a significant effect on soil physicochemical properties for six years.The application of chemical fertiliser alone and organic fertiliser with chemical fertiliser significantly increased the soil organic carbon（SOC）and Total Nitrogen（TN）content of the soil compared to the non-fertiliser treatment,with the organic fertiliser with chemical fertiliser treatment having a more significant increase.However,long-term application of chemical fertilizer alone led to soil acidification,soil p H decreased by 1.27 units,and the application of chemical fertilizer with organic fertilizer treatment could significantly increase soil p H and effectively alleviate soil acidification.（2）Compared with chemical fertilizer treatment,the application of organic fertilizer significantly increased the soil nitrification potential and denitrification potential,which increased by 25 times and 40 times,respectively.Compared with chemical fertilizer treatment,the abundance of nitrification genes and denitrification genes was significantly increased by the combined application of organic fertilizer:the abundance of nitrification microorganisms ammonia-oxidizing archaea（AOA）and ammonia-oxidizing bacteria（AOB）genes increased by 17.9%and 36.2%.The denitrification genes nir K,nir S,nar G and nos ZI.gene abundances increased by 7.0%,6.2%,6.7%and 9.2%,respectively.This indicates that the combination of chemical fertilizer with organic fertilizer promotes the nitrogen conversion process to a certain extent,which in turn increases the potential risk of soil N₂O and NO emissions.（3）The addition of biochar and nitrification inhibition significantly reduces N₂O and NO emissions under organic management models.After adding biochar,soil N₂O and NO emissions were reduced by 38%and 35%,respectively.After adding nitrification inhibitors,soil N₂O and NO emissions were reduced by 82%and 67%,respectively.The combined application of the two reduced soil N₂O and NO emissions by 85%and 76%.（4）Emissions of N₂O and NO are strongly influenced by soil environmental factors.Correlation analysis showed that N₂O and NO were significantly negatively correlated with p H,electric conductivvity（EC）,dissolved organic carbon（DOC）,NH₄⁺-N intensity,AOB,nos ZI.（P<0.05）,and NO₃^--N intensity,Potential nitrification rate（PNR）and complete ammonia oxidizers（Comammox）（P<0.05）.The main emission factors of N₂O and NO were predicted by random forest model,and the results showed that N₂O was mainly regulated by PNR,NO₃^--N intensity,NH₄⁺-N intensity and EC,and the explanatory degrees were 8.9%,8.9%,8.7%and 8.6%,respectively.NO was mainly affected by DOC,NO₃^--N intensity,EC and PNR,and the interpretive degrees were 9.2%,8.9%,8.3%and 8.1%,respectively.（5）High-throughput sequencing detected a total of 11 phylums and 7 genera with abundances greater than 1%.Firmicutes,Proteobacteria,Acidobacteria,Actinobacteria and Phylum Chlorofelexi were the dominant flora of tropical vegetable floor tiles in red soil,accounting for 73.5%～90.7%of the total.Bacillus,Candidatus-Nitrososphaera,Katistobacter,and Nitrospira were the dominant genus,accounting for 13.3%～24.3%of the total.Compared with the control,the addition of biochar or nitrification inhibitors and the combined administration of the two significantly increased the abundance of Firmicutes by 17.1%～22.7%,and the abundance of Actinobacteria and Chlorofelexi decreased by 72.6%～77.9%and 18.2%～28.7%.Compared with the control,the addition of biochar and nitrification inhibitors and the combination of the two increased the relative abundance of Bacillus,Candidatus-Nitrososphaera,Katistobacter and Nitrospira,which increased by 45.4%～52.0%,25.0%～41.7%,4.2%～16.6%,25.0%～30.3%,respectively.（6）The addition of biochar or nitrification inhibitors and the combined administratio n of both reduced the richness and diversity of bacterial communities.The correlation ana lysis showed that the diversity index of bacterial community was closely related to DOC a nd p H,indicating that bacterial community diversity was directly or indirectly affected by soil properties.Redundancy analysis（RDA）showed that DOC and SOC were the main factors affecting the level of soil bacterial community phylum,which explained a total of24.9%and 21.0%of the bacterial community differences,respectively.PNR and potential denitrification rate（PDR）were the main factors affecting the level of soil bacterial comm unity,which explained 55.2%and 12.1%of the differences in bacterial community level,respectively.