Simulation Of Greenhouse Gas Emissions In Rainfed Farmland Under Extreme Precipitation

Precipitation is an important driving factor of greenhouse gas emission from farmland soil in arid and semi-arid regions.Although the duration is short and the occurrence is sudden,with the occurrence of precipitation,the release of soil CO₂ and N₂O accounts for more than50%in the whole growth period.Surface CO₂ and N₂O emissions are the net result of CO₂and N₂O production,transport and consumption in soil.Soil O₂ concentration,carbon and nitrogen substrates and other factors affect the production of soil CO₂ and N₂O.The frequency of extreme precipitation events is increasing under global warming.However,the mechanism of soil CO₂ and N₂O emissions is not very clear under extreme precipitation.Based on the simulated extreme precipitation experiment,this study set two single precipitation conventional precipitation(10 mm-P₁₀)and extreme precipitation(75 mm-P₇₅)on the basis of controlling the same total precipitation（300 mm）.Surface CO₂ and N₂O fluxes were monitored by four management measures,namely control（CK）,nitrogen application（N）,nitrogen application plus a small amount of straw（N+LM）,and nitrogen application plus a large amount of straw（N+HM）.Dynamic changes of soil CO₂,N₂O,CH₄ and O₂concentrations,soil temperature and moisture,soil DOC,NH₄⁺-N and NO₃^--N contents;The following three aspects are studied:1)The impact of extreme precipitation on greenhouse gas emissions;2)Effects of O₂ on soil greenhouse gas concentration under extreme precipitation;3)Effects of substrate on soil greenhouse gas concentrations under extreme precipitation.The main results are as follows:（1）Extreme precipitation and management measures significantly affect the dynamic changes of surface CO₂ and N₂O fluxes.Surface CO₂ and N₂O fluxes showed a dynamic change trend of first increasing and then decreasing after precipitation,and the change was more drastic（reaching the peak within 2-5 days）under extreme precipitation.With the increase of the number of precipitation,surface CO₂ and N₂O fluxes gradually decreased.The increasing range of surface CO₂ and N₂O fluxes in the four management measures is N+HM>N+LM>N>CK.Extreme precipitation can reduce the warming potential of CO₂ and N₂O.Surface CO₂ and N₂O cumulative emissions were significantly different in N+LM（except N₂O cumulative emissions）and N+HM treatments under extreme precipitation（P<0.05）.Compared with P₁₀,CO₂ cumulative emissions treatment decreased by 32%and 40%respectively in N+LM and N+HM,while N₂O cumulative emissions increased by 89%in N+HM under P₇₅.（2）Soil O₂ consumptions,CO₂ and N₂O accumulations were promoted in four management measures under extreme precipitation,but the effect on CH₄ concentration is small.Soil O₂ concentrations first decreased and then increased after precipitation,while soil CO₂ and N₂O concentrations showed an opposite trend to O₂ concentration under extreme precipitation.The dynamic changes of the three gases are more drastic,and the peak time（1-5 days）is earlier under extreme precipitation.In the four management measures,the variation range of the three gas concentrations is N+HM>N+LM>N>CK under extreme precipitation.There were significant negative correlations between soil CO₂,N₂O and O₂ concentrations（P<0.05）.However,with the decrease of soil O₂ concentration,soil CO₂ concentration increased in a growth curve pattern,and soil N₂O concentration increased in an exponential pattern.Surface CO₂ and N₂O fluxes are related to soil CO₂ and N₂O concentrations.The dynamic trend of surface CO₂ and N₂O fluxes is basically the same as that of soil concentrations,but the peak time is a little later（1-3 days）under extreme precipitation.The surface CO₂ and N₂O fluxes were significantly positively correlated with soil concentrations（P<0.05）.（3）Extreme precipitation had no significant effect on soil DOC content,but reduced soil NH₄⁺-N and NO₃^--N contents.Under P₁₀ and P₇₅,soil DOC content decreased within 3 days after the first precipitation.Compared with P₁₀,the decrease time of soil NH₄⁺-N content（within 3 days after precipitation）was shorter under P₇₅,and soil NO₃^--N content（except for N treatment）decreased.This study revealed the dynamic characteristics of surface CO₂ and N₂O fluxes and soil CO₂,N₂O and O₂ concentrations under extreme precipitation,and clarified the relationship between surface CO₂ and N₂O fluxes and soil CO₂ and N₂O concentrations,as well as soil O₂concentrations and CO₂ and N₂O concentrations.The effects of soil O₂ concentration and carbon and nitrogen substrate on soil CO₂ and N₂O concentrations were also discussed.It provides theoretical support for further understanding the mechanism of greenhouse gas emission from farmland soil in semi-arid region.