Effects Of Biochar On N₂O Emission From Calcareous Arable Soil Under The Condition Of Facilitating Denitrification

Biochar is the solid product of biomass pyrolyzed at high temperature under no oxygen condition.Biochar added to soil can reduce nitrous oxide(N₂O)emission.Therefore,biochar return to the field has become an important way to deal with global climate change.Nitrification and denitrification are the two main processes of N₂O emission from soil.Arable soil in the semi-arid region of Loess Plateau is in a relatively arid state in most of the time.However,the moisture content of the soil would be at relatively high level under the conditions of rainfall,irrigation,or soil freezing-thawing cycles in spring.At these moments,denitrification may be the main pathway of soil N₂O emission.However,the effect of biochar on soil N₂O emission from calcareous arable soil of Loess Plateau is still unclear under the condition of facilitating denitrification.In addition,the role of dissolved organic carbon(DOC)in denitrification is also needed to be explored.In order to explore the effect of biochar on N₂O emission through denitrification in calcareous arable soil,maize straw-derived biochars(BC400,BC600 and BC800)prepared at400,600 and 800?respectively were selected for the experiment.The indoor static soil incubation experiment was carried out for 15 days after evenly mixing biochar with the soil sample at the mass ratio of 0%(control),2%and 5%,respectively to investigate the diurnal variation of N₂O emission rate and the cumulative N₂O emission of the soil.Meanwhile,the pH,inorganic nitrogen,and DOC contents of the soils were determined at the beginning and end of incubation.The results indicated that the three kinds of biochar significantly promoted N₂O emission during the process of denitrification in calcareous arable soil,especially for the BC400-5%treatment.Compared with the control,the cumulative N₂O emission of BC400-2%,BC600-2%,and BC800-2%treatments increased by 123.08%,64.22%,and 24.28%,respectively;the cumulative N₂O emission of BC400-5%,BC600-5%,and BC800-5%treatments increased by 566.98%,197.34%,and 161.51%,respectively.The changes of pH and NO₃^--N content of the soil before and after incubation showed that maize straw-derived biochar promoted soil denitrification process,and the promotion effect enhanced as the biochar addition rate increase.Compared with BC800,BC400 showed stronger promotion effect on soil N₂O emission,which could be due to the fact that the biochar prepared at relatively low temperature contained much more DOC and was rich in phenolic C-OH.Based on the above studies,the effect of different exogenous carbon-containing substances on arable soil N₂O emission through denitrification pathway was further studied by adding BC400 and its carbon skeleton with large difference in DOC content to soil.Meanwhile,glucose addition treatment was set as the control to identify the effect of biochar DOC structure on N₂O emission.The addition rate of BC400 and its carbon skeleton was set as 2%and 5%(mass ratio),respectively.Moreover,two levels of glucose addition rate(high and low)were set to reach the same DOC content of BC400-2%and BC400-5%possessed,respectively.A15-day indoor static soil incubation experiment was carried out to measure the dynamic change of N₂O emission rate during the incubation period and calculate the cumulative N₂O emission.At the same time,pH,inorganic nitrogen,and DOC contents of the soil samples collected at the beginning and end of incubation were also determined.The results showed that both carbon skeleton and glucose promoted N₂O emission in the calcareous arable soil,and the promotion effect enhanced with the increase of exogenous material addition rate.Compared with BC400,carbon skeleton showed relatively weaker promotion effect on soil N₂O emission,mainly because a large amount of DOC was lost during the process of carbon skeleton preparation.Compared with glucose,BC400 promoted soil N₂O emission more effectively,mainly because the DOC of biochar possessed more reductive phenolic C-OH.In order to explore the role of DOC of biochar in N₂O emission from arable soil.Rice husk-derived biochars BC1 and BC2,which DOC contents were significantly lower than the maize straw-derived biochars,were selected for the experiment.The indoor static soil incubation experiment was carried out for 15 days after evenly mixing biochar with the soil sample at the mass ratio of 0%(control),2%and 5%,respectively to investigate the diurnal variation of N₂O emission rate and the cumulative N₂O emission of the soil.At the same time,pH,inorganic nitrogen,and DOC contents of the soil samples collected at the beginning and end of incubation were determined.The results indicated that the rice husk-derived biochar(BC1 and BC2)significantly inhibited arable soil N₂O emission through denitrification pathway,and the main reason was that the rice husk-derived biochar reduced the effectiveness of DOC in the incubation system.Compared with BC1,BC2 showed stronger inhibition effect on N₂O emission,mainly due to the lower content of phenolic C-OH on its surface.Based on the study,it is expected to explore the potential mechanisms of the effect of biochar on N₂O emission through denitrification pathway in calcareous arable soil of Loess Plateau,and provide a theoretical reference for the agricultural application of biochar in arable soils of this region.