Long-term Effects Of Biochar Application On Soil N₂O Emission In A Poplar Plantation System

China’s plantation area ranks first in the world,with Populus accounting for more than 90%of the world’s poplar plantation area and 18.1%of the total national plantation area.Poplar plantations have great potential to increase forest carbon sequestration and reduce greenhouse gas emissions.However,soil fertility decline often occurs in fast-growing and high-yield forests represented by poplars.Fertilization is a conventional management measure for poplar plantations to increase soil fertility and promote plant growth,which may lead to an increase in soil nitrous oxide（N₂O）emission and offset the ecological benefits of poplar plantations in mitigating climate change.Biochar,as a soil conditioner,can improve soil fertility by improving soil physical and chemical properties,and mitigate climate change by increasing soil carbon sequestration and reducing greenhouse gas emissions.However,the long-term effects of biochar application on soil N₂O emission in plantations and its mechanism remain unclear.In this study,relying on a long-term field experiment of biogas slurry and biochar combined application in Dongtai Forest Farm,Yancheng City,Jiangsu Province,we investigated the combining effects of different rates of biogas slurry(0,125,250,375m³hm^-2)and biochar application(0,80,120t hm^-2)on soil N₂O emission,soil physical and chemical properties,soil potential nitrification rates,denitrification enzyme activity,soil nitrifying and denitrifying microbial functional genes through in-situ field observation and incubation experiments.We explored the relationship between soil N₂O emission and those factors and revealed the long-term effects of biochar application on soil N₂O emission in the poplar plantation system.The main results are as follows:（1）Biogas slurry promoted soil N₂O emission,while biochar had no significant effect on soil N₂O emission,soil potential nitrification rates,denitrification enzyme activity and N₂O reduction rates after 7 years of a single application.Biochar,biogas slurry and their interactions significantly affected soil nitrifying and denitrifying microbial functional genes.Specifically,the nir S and nir K gene abundance and（nir S+nir K）/nos Z ratio were significantly increased by high concentration biochar application,and the（nir S+nir K）/nos Z ratio in the treatments of biochar combining with biogas slurry was higher than that of biogas slurry application alone.Soil N₂O flux was significantly positively correlated with NO₃^--N,MBC content,PNR,DEA.（2）When fresh biochar was applied alone,low concentration biochar treatment reduced soil N₂O emission,but high concentration biochar treatment increased soil N₂O emission.The combined application of biochar with biogas slurry significantly reduced soil N₂O emission compared with biogas slurry alone during the 42 days incubation.Biochar significantly reduced soil potential nitrification rates and denitrification enzyme activity,and increased N₂O reduction rates.Biochar inhibited the ammonia oxidation process by reducing the abundance of amo A gene in nitrifying bacteria and archaea.Meanwhile,the abundance of nos Z gene was significantly increased and the（nir K+nir S）/nos Z ratio was decreased,which promoted the complete denitrification process and reduced N₂O emission.Soil N₂O flux was significantly correlated with TOC,MBC,NO₃^--N contents,PNR and DEA.（3）Seven-year field application of biochar,biogas slurry and their interaction significantly affected soil N₂O emission.Biogas slurry significantly promoted soil N₂O emission in the first 8hours after application,and there was a significant interaction between biogas slurry and biochar.The combined application of 7-year field application of biochar and biogas slurry significantly reduced soil N₂O emission during the one week incubation.The effect of biochar on soil potential nitrification rates and denitrification enzyme activity depended on biogas slurry application,with which biochar significantly inhibited soil potential nitrification rates and denitrification enzyme activity.Soil N₂O flux was positively correlated with NH₄⁺-N,TOC content and DEA.In conclusion,fresh biochar and 7-year field application of biochar can significantly reduce soil N₂O emission stimulated by biogas slurry during incubation period.After 7 years of a single application,biochar can no longer inhibit soil N₂O emission stimulated by biogas slurry,which is inconsistent with incubation results,possibly due to improper field sampling frequency or field vegetation interference.Therefore,in order to accurately evaluate the impact of long-term application of biochar on soil N₂O emission stimulated by nitrogen fertilizer（biogas slurry）,it is suggested to increase longer-term experimental research and more frequent gas sampling.