Effect Of Organic Acids And Nitrogen Fertilizers On N₂O Emission From Soil

N₂O,as an important greenhouse gas,mainly comes from soil.There are many factors that could affect the release of N₂O in soil,and the mechanism of some factors is still unclear.In order to know the mechanism of the influence of organic acid and nitrogen fertilizer types on the emission of N₂O from soil,it was planned to add organic acid and nitrogen fertilizer to the soil for 83 days to carry out a culture study.The N₂O emission rate during the culture period and the physical and chemical properties of the soil after the culture were measured by gas chromatography and conventional methods,respectively.The experimental results were as follows:(1)Organic acids had significant effects on the emission of N₂O,and the effects were different due to different types of organic acids.Under the condition that no nitrogen fertilizer was added to the soil,oxalic acid inhibited N₂O emission;malic acid had no significant effect on its emission;while citric acid and salicylic acid could significantly promote the emission of N₂O,and the total emissions were 5.5 and 13.7 times of that of the control group respectively.Under the condition of adding the same fertilizer,different types of organic acids all promoted the emission of N₂O from the soil.However,the total emission of N₂O from salicylic acid treatment was the highest under different nitrogen fertilizer treatments.When carbamide,ammonium sulfate and potassium nitrate fertilizers were applied to the soil,oxalic acid treatment had the lowest emission of N₂O,and citric acid treatment also had the lowest emission under the condition of adding ammonium nitrate.(2)Nitrogen fertilizer application in soil had significant effect on N₂O emission,and the total amount of N₂O emission varied with different nitrogen fertilizer types.Under the condition of no organic acid,the addition of ammonium sulfate,carbamide and ammonium nitrate fertilizers significantly increased the emissions of N₂O,and the total emissions were 21.3,20.8and 7.2 times of the control group respectively;while the addition of potassium nitrate fertilizer reduced the total N₂O emission by 67.8%in soil compared with the control.When different types of organic acids were added to the soil,the total amounts of N₂O emission in different nitrogen fertilizers were also different,of which potassium nitrate had the weakest promotion effect on the emission of N₂O from soil,while ammonium sulfate and carbamide had the most obvious promotion effect.(3)The emission rates of N₂O were negatively correlated with time under different treatment conditions during the culture period,but the types of maximum correlation coefficients were different,indicating that organic acid addition could affect the N₂O emission mode of nitrogen fertilizer in soil.The emission rates of N₂O decreased rapidly with the extension of culture time under different treatment conditions in the early stage of culture,after about 30 days,the emission rate remained at a low level with time,indicating that organic acids and nitrogen fertilizer had phased effects on the emission of N₂O.(4)The addition of organic acid reduced the nitrate nitrogen content from soil in the groups without fertilizer,adding potassium nitrate and ammonium nitrate fertilizer,while the addition of salicylic acid reduced the nitrate nitrogen content in all control groups and experimental groups.The addition of organic acid and ammonium sulfate fertilizer increased the ammonium nitrogen content in the soil,while adding salicylic acid could promote the ammonium nitrogen content in all non-fertilizer and fertilized soils.These results showed that the addition of organic acid would affect the transformation of inorganic nitrogen,and adding salicylic acid significantly reduced the content of nitrate nitrogen and increased the content of ammonium nitrogen in soil.(5)The addition of nitrogen fertilizer reduced the pH value of soil,of which potassium nitrate was more obvious.When the concentration of organic acids was 20 mmol L^-1,salicylic acid increased the pH value of soil,while oxalic acid has the opposite effect.(6)The addition of oxalic acid significantly increased the content of available phosphorus,while salicylic acid reduced its content in soil.(7)The addition of citric acid,malic acid and salicylic acid all improved the activity of urease in soil,but oxalic acid had no significant effect.Adding potassium nitrate and ammonium nitrate fertilizers also improved urease activity,of which potassium nitrate was more effective.(8)The addition of organic acid inhibited the activity of nitrate reductase in the treatment groups of no fertilizer,carbamide,ammonium sulfate and ammonium nitrate,but promoted the activity of nitrate reductase in soil added with potassium nitrate fertilizer.Adding oxalic acid improved the activity of nitrite reductase,while the addition of salicylic acid had inhibitory effect.In addition,adding different types of nitrogen fertilizers promoted the activity of nitrite reductase.Overall,organic acid mainly affected the emission of N₂O by changing nitrification and denitrification reaction through the influence of inorganic nitrogen transformation,pH and urease,nitrate reductase and nitrite reductase activities in the soil applied nitrogen fertilize.In this study,N₂O emission in soil mainly came from inorganic nitrogen produced in nitrification reaction,so the application of nitrate nitrogen fertilizer could reduce its emission.However,the addition of organic acid increased N₂O emission from nitrogen-applied soil,in order to reduce N₂O emission during production time,it was better not to apply inorganic nitrogen and organic fertilizer at the same time.