Effects Of Modified Rice Husk-derived Biochar On Denitrification And N₂O Emission And The Potential Mechanisms

In recent years,the global greenhouse effect has become more and more serious.As the third potent greenhouse gas next to CO₂ and CH₄,N₂O has an extremely important impact on the global warming.As a large agricultural country,China produces a large amount of agricultural waste annually.In response to the ecological agriculture concept proposed by the national sustainable development strategy,it is an effective way to transform the agricultural waste into biochar and then return it to the field.Biochar possesses the potential of inhibiting N₂O emission.However,the properties of biochar vary with the different feedstocks,pyrolyzing temperatures and preparation methods.The effects of biochar on denitrification and N₂O emission and the potential mechanisms remain unclear.The object of the study was to explore the effect of original（BC）and Fe Cl₃-modified rice husk-derived biochar（BC-Fe）and its application rate on denitrification and N₂O emission in calcareous arable soil and the potential mechanisms.After BC-Fe preparation and the physicochemical property determination of BC and BC-Fe,the biochars were thoroughly mixed with the soil at the mass ratio of 2%and 5%respectively,and a control without any biochar addition was also arranged.After 7-day preincubation,an 8-day microcosm incubation experiment was carried out consecutively under the condition of facilitating denitrification.N₂O and N₂O+N₂ emission rates were determined,and the distribution pattern of N₂O and N₂,as the end products of denitrification,was explored by the acetylene inhibition method.The dynamic changes of soil physicochemical properties and the activities of nitrate reductase and nitrite reductase during incubation were explored.Results showed that both JBC and JBC-Fe promoted soil cumulative N₂O and N₂O+N₂ emission during the 8-day incubation.Compared with BC-2%,the cumulative emissions of N₂O and N₂O+N₂ in BC-Fe-2%significantly increased by 465.98%and 9.83%,respectively.Compared with BC-5%,the cumulative emissions of N₂O and N₂O+N₂ in BC-Fe-5%significantly increased by 147.68%and 41.21%,respectively.Compared with BC,BC-Fe promoted soil NO₃^--N reduction and inhibited N₂O reduction.Compared with 2%biochar addition,5%biochar addition increased soil C/N,decreased the bioavailability of nitrogen and inhibited nitrate reductase activity,and subsequently decreased the cumulative N₂O+N₂ emission.Moreover,5%biochar addition also inhibited soil N₂O reductase activity,resulting in the increased cumulative N₂O emission.After biochar application into soil,its soluble components will dissolve out due to the precipitation and other reasons,and the oxygen-containing functional groups on the surface of the remaining insoluble components will also change.In addition,because of the high heterogeneity and microbial diversity of soil,it was difficult to distinguish the key factors of biochar influencing microbial denitrification.Therefore,based on the preparation of biochar skeleton（WBC）and the extract of biochar（BCE）by water-washing method and the enrichment and screening of the anaerobic denitrifiers（DB）,the microcosm incubation experiment was carried out to explore the removal efficiency of nitrate with low concentration(about 10 mg·L^-1)in simulated wastewater by DB.The N₂O and N₂O+N₂ emission rates were determined,and the dynamic changes of the physicochemical properties and the nitrate reductase and nitrite reductase activities of the simulated wastewater during incubation were investigated.Results showed that the N₂O emission from the simulated wastewater was entirely caused by microbial pathway.The peak denitrification rate（N₂O+N₂ emission rate）of DB appeared at 48 h,while that of DB+BC appeared at 36 h,and that of DB+WBC even appeared at 24 h.The peak denitrification rates of DB+BC and DB+WBC(44087.79 and 46826.27 ng N·h^-1,respectively)were significantly higher than that of DB(38097.45 ng N·h^-1).Compared with DB,the cumulative N₂O+N₂ emissions of DB+BC and DB+WBC increased by 30.17%and 2.86%respectively,while the cumulative N₂O emissions decreased by 83.04%and 74.52%respectively,indicating that both BC and WBC increased the denitrification rate and promoted N₂O reduction.Compared with DB+BC,the cumulative N₂O+N₂ emission of DB+WBC decreased by 20.98%,which indicated that the capacity of denitrification promotion of the biochar skeleton was not as good as the biochar,but it still played a major role.Moreover,the effect of biochar on N₂O emission in denitrification process is also related to the redox active functional groups on biochar surface.Therefore,based on the above-mentioned studies,the influence of rice husk-derived biochars modified by H₂O₂ and Na BH₄（BC-H₂O₂ and BC-Na BH₄）on the denitrification process and N₂O emission and the potential mechanisms were further explored.Firstly,BC-H₂O₂ and BC-Na BH₄ were prepared,and their physicochemical properties and the contents of surface redox active functional groups were determined.Then after BC,BC-H₂O₂ and BC-Na BH₄ were added into the incubation system at the ratio of 1%（w/v）respectively,the microcosm incubation experiment was carried out to explore the removal efficiency of nitrate with low concentration(about 10 mg·L^-1)in simulated wastewater by DB.The N₂O and N₂O+N₂ emission rates were determined,and the dynamic changes of the physicochemical properties and the nitrate reductase and nitrite reductase activities of the simulated wastewater during incubation were investigated.Results showed that H₂O₂ modification increased the carboxyl group content on the surface of biochar,while Na BH₄modification increased the contents of lactone group and phenolic hydroxyl group on the surface of biochar（P<0.05）.In addition,Fourier transform infrared spectra analysis showed that the C=O content of BC-H₂O₂ significantly increased compared with BC.The denitrification rate peaks of DB+BC-H₂O₂ and DB+BC-Na BH₄ appeared 12 h earlier and the values were17.50%and 6.32%higher than DB+BC,respectively.Compared with DB+BC,the cumulative N₂O emission of DB+BC-H₂O₂ significantly increased by 165.54%and N₂O/（N₂O+N₂）significantly increased by 170.00%,but there was no significant difference in the cumulative N₂O+N₂ emission（P<0.05）,which indicated that BC-H₂O₂ inhibited the reduction of N₂O to N₂,leading to the increase of N₂O emission.Furtherly,there was a significant positive correlation between N₂O emission rate and NO₂^--N removal rate（R²=0.48,P<0.05）.This study is conducive to understanding the effects of rice husk-derived biochar on denitrification and N₂O emission and the related potential mechanisms,and will provide a theoretical basis for the preparation and application of the functional biochar and a theoretical reference for alleviating the global greenhouse effect.