Spatial And Temporal Variation Characteristics Of Greenhouse Gas Fluxes In Soils Profiles And Influencing Factors Under Biochar And N Application In Dry Farmland

Agroecosystems are one of the major sources of greenhouse gases（GHGs）emissions.To increase crop yield,nitrogen（N）fertilizer is widely used,but excessive N fertilizer application usually causes a series of environmental problems such as N leaching,water eutrophication and GHGs emission.Biochar application is considered to be an effective management for achieving soil carbon（C）sequestration and reducing GHGs emissions,the combination of N fertilizer and biochar application may be an effective management to improve soil fertility and mitigate GHGs emissions.To explore the impacts of the biochar and N application on the soil GHGs fluxes at the atmosphere-soil interface and soil profile,we conducted an experiment in wheat farmland.We evaluated the effects of biochar and N application on global warming potential（GWP）and GHGs emission intensity（GHGI）,analyzed the impact of biochar and N application on soil C and N stocks and enzyme activities,clarified the vertical distribution pattern of soil C and N fractions in the soil profile and further revealed the responses of soil GHGs fluxes on biochar and N application and its driving mechanism by combining in situ field monitoring and global meta-analysis.Our results have important theoretical implications for the application of biochar in agricultural fields to achieve soil C sequestration and reduce GHGs emissions under the background of climate change.The main research results obtained are as follows:（1）The combination of biochar and N application improved soil C and N fractions and enzyme activities（0-15 cm）in dry farmlandBiochar application for 3 consecutive years significantly reduced soil easily oxidizable organic carbon（EOC）and increased dissolved organic carbon（DOC）,particulate organic carbon（POC）,mineral-associated organic carbon（MAOC）,and microbial biomass carbon（MBC）,and was positively correlated with biochar application rate;Biochar reduced soil inorganic nitrogen（SIN）but increased dissolved organic nitrogen（DON）,particulate organic nitrogen（PON）,mineral-associated organic nitrogen（MAON）and microbial biomass nitrogen（MBN）content,and N application increased SIN and DON content;biochar application alone reduced C:N acquisition enzyme ratio,and the combination of N(N fertilizer,360 kg hm^-2)and B2(biochar,40 t hm^-2)application facilitated soil C sequestration,while N application alone or the combination of N1(180 kg hm^-2)and B1(20 t hm^-2)application was more beneficial to soil quality improvement.（2）The combination of biochar and N application increased C and N stocks in 0-60 cm soil layer in dry farmlandBiochar and N application increased soil organic carbon（SOC）and MBC content in 0-20 cm soil layer and decreased DOC content in 10-20 cm soil layer.SOC in the surface layer（0-10 cm）and DOC in the deep layer（40-60 cm）were more sensitive to the combination of biochar and N fertilizer;SIN was positively correlated with N application rate,and NH₄⁺-N was more sensitive to the combination of biochar and N fertilizer than NO₃^--N.Overall,biochar reduced NO₃^--N content in surface soils and had no significant effect on deeper soils.Under B1(20 t hm^-2)treatment,DON and MBN leaching occurred at the 20-40 cm soil layer;biochar and N application increased C and N stocks in the 0-60 cm soil layer,with the highest increment at the N1B2 level,and belowground biomass and SOC were the key influencing factors.The combination of biochar(42.35 t hm^-2)and N(277.78 kg hm^-2)application was most beneficial for soil C sequestration from 0-60 cm.（3）The combination of biochar and N application mitigated the net GWP and enhanced crop yield in dry farmlandThe effect of biochar on CO₂ emission shifted from promoting to inhibiting with increasing duration of the experiment,and N fertilizer consistently increased soil CO₂emission,with soil N fraction and N cycle-related enzymes being the key influencing factors;biochar reduced the rate of CH₄uptake.B1 treatment reduced more CH₄ uptake compared to B2treatment.N fertilizer promoted CH₄uptake in the later stages of the experiment,and soil NH₄⁺-N was the key factor affecting CH₄ uptake;N application promoted N₂O emission and biochar inhibited N₂O emission,and DON was the key factor affecting N₂O emission（64.4%）during 2 consecutive growing seasons;the reduced NGWP and NGHGI were mainly affected by biochar application,and grain yield was regulated by both biochar and N fertilizer.Our results showed that N1B2 treatment was the most beneficial fertilization measure for soil C sequestration and yield enhancement.It cannot be ignored that the average fluxes of CO₂,CH₄and N₂O during the fallow period contributed more than 42%to the total annual fluxes.（4）The co-application of biochar and N with high addition rate reduced the total CO₂,CH₄ and N₂O fluxes from 0-60 cm in dry farmlandThe CO₂,CH₄and N₂O diffusion fluxes in the soil profile showed a decreasing trend with the deepening of the soil layer,and soil temperature was the most critical influencing factor.The inhibitory effect of biochar on total CO₂ flux was mainly derived from the contribution of0-10 cm and 40-60 cm soil layers,with SOC and aboveground biomass as key influencing factors;similarly,biochar-induced reduction in the total CH₄ flux in 0-60 cm was mainly related to the changes in the 0-10 cm and 40-60 cm soil layers,where SOC and TN were negatively correlated with CH₄ flux in 0-10 cm soil layer.The CH₄ flux in the 20-60 cm soil layer occupied 36.4%of the total CH₄ flux in 0-60 cm;the promoting effect of N application on total N₂O flux from 0-60 cm was mainly related to SOC and NO₃^--N.The inhibitory effect of the combination of biochar and N application on total N₂O flux was mainly associated with soil NO₃^--N in the 0-20 cm soil layer.（5）Antagonistic and synergistic effects of biochar and N interaction on GHGs fluxes and crop yieldThe meta-analysis showed that biochar,N and their combined application increased CO₂emissions（8.5%-29.6%）,SOC（1.8%-50.4%）,DOC（2.7%-30.0%）,TN（6.8%-15.6%）and crop yield（4.2%-58.2%）,but had no significant effect on CH₄ flux.N and their combined application stimulated N₂O emission and GWP,whereas biochar application alone inhibited them.The interaction between biochar and N showed mainly antagonistic effects on soil GHGs and synergistic effects on yield.However,soil GHGs responses were also influenced by geographic climate,soil properties,biochar and N fertilizer parameters,and experimental conditions.Co-application of herbaceous biochar and nitrogen fertilizers to fine-textured soils was most beneficial for mitigating GHGs emissions and increasing yields.In conclusion,the combination of biochar and N application can effectively mitigate GHGs and improve crop yields.In addition,the long-term impact of biochar and N application on climate change mitigation remains to be verified.