Effects Of Extreme Precipitation On Soil CH₄ Fluxes In The Songnen Meadow Steppe Under Different Nitrogen Addition And Grazing Treatments

Methane?CH4?is the second most important greenhouse gas after CO2 in the atmosphere and contributes to approximately 20%of global warming.Grazing is the most common way of land use on grassland.Grazing may directly affect the soil CH4 flux,or it may adjust the influence of global factors such as nitrogen deposition on soil CH4 flux through indirect effects.In addition,as the global warming trend intensifies,the frequency,extent and duration of extreme climate?eg extreme precipitation or drought?events continue to increase;long-term sustained global change factors such as grazing and nitrogen deposition may change vegetation characteristics and soil structure to affect the response of soil CH4 flux to extreme climate events.In order to understand the effects of nitrogen addition,grazing and extreme rainfall on grassland soil CH4 flux,we conducted a field experiment?2017 and 2018?and a situ soil column microcosm experiment?2018?to study the soil CH4 flux of Songnen meadow grassland response to grassland grazing utilization patterns and global changes and their underlying mechanisms.The field experiment results showed that:?1?Due to the occurrence of extreme rainfall events,the CH4 emissions were greater than the uptake of the meadow grassland soil in the study area during the study period,and the studied meadow steppe soil acted as a CH4 source;?2?The average CH₄ fluxes were 7.8,5.8,9.3 and 7.6?g m^-22 h^-1 for the control?CK?,grazing?G?,nitrogen addition?N?and nitrogen addition interacted with grazing?NG?treatments,respectively.The cumulative CH₄ fluxes were 24.9,11.5,28.8 and 17.8?g m^-2 yr^-1 for the CK,G,N and NG treatments,respectively;?3?Grazing and nitrogen addition treatment significantly?P<0.05?affected the average soil CH4 flux,but there was no significant interaction between the grazing and nitrogen addition treatment.The nitrogen addition treatment significantly?P<0.05?increased the CH4 flux,whereas grazing significantly?P<0.05?decreased the CH4 flux.The nitrogen addition treatment increased the average CH4 flux by 19%,and the grazing treatment reduced it by 25%.The grazing treatment significantly reduced the percentage of CH4 flux increase following the extreme rainfall event?P<0.05?;?4?The soil CH4 flux was positively correlated with the 0-10 cm soil water filled pore space?WFPS?,soil NH₄⁺-N and soil NO₃^--N?P<0.01?,but negatively correlated with the 0-10 cm soil temperature?P<0.01?,except for the sampling dates that were strongly influenced by the extreme rainfall event.The average CH4 flux was positively correlated with aboveground biomass?AGB??P<0.05?,but was not significantly correlated with below-biomass?BGB?and litter mass?P>0.05?.The microcosm experiment results showed that:?1?During the experimental period,the soil average CH₄ flux of control?CK?,grazing?G?,nitrogen addition?N?and nitrogen addition interacted with grazing?NG?treatments were-10.56,-10.11,-7.99 and-8.25?g m^-2yr^-1,respectively.The soil average CH₄ flux for extreme precipitation treatment?W?,extreme precipitation treatment interacted with grazing?WG?,extreme precipitation interacted with nitrogen addition?WN?,extreme precipitation,nitrogen addition interacted with grazing?WNG?treatments were-1.15,-0.43,1.02and 0.41?g m^-2 yr^-1,respectively.Only four extreme precipitation treatments?W,WG,WN and WNG?significantly increased the soil average CH₄ flux?P<0.01?.Nitrogen addition treatments?N,NG,WN and WNG?reduced the average CH₄ flux of soil,but it was not significant?P>0.05?;?2?The cumulative CH₄flux of CK,G,N,and NG treatments were-4.60,-4.31,-3.25 and-3.64 mg m^-2,respectively.The cumulative CH₄ flux of W,WG,WN and WNG treatments were-1.67,-1.41,-0.93 and-1.31 mg m^-2,respectively.Only the extreme precipitation treatments significantly increased the cumulative CH₄ flux?P<0.01?;?3?After the extreme precipitation treatments,when the CH₄ flux was less than 0,the soil microbial biomass carbon?MBC?,soil microbial biomass nitrogen?MBN?,soil NH₄⁺-N,soil NO₃^--N and CH₄ flux had a positive correlation.When the CH₄ flux was greater than 0,there were no correlation between the MBC,MBN,soil NH₄⁺-N,soil NO₃^--N and CH₄ flux.There was a significant correlation between soil water content and CH₄ flux?P<0.01?.The dynamic changes of soil CH₄ flux in Songnen meadow grassland are affected by a combination of grassland utilization and global changes.Moderate grazing has the potential to reduce the negative impacts of N addition on CH₄ flux and can increase the capacity of the soil CH₄ sink in the studied meadow steppe.Compared with other environmental factors,the effect of soil moisture on soil CH₄ flux is decisive.With the occurrence of extreme rainfall events,the soil in the study area changed from atmospheric CH₄ sink to source.Therefore,the increasing of extreme rainfall events in the future may change the Songnen grassland from atmospheric CH₄ sink to sink.Grassland use patterns and other global factors such as nitrogen deposition can regulate the influence extent of extreme precipitation events on soil CH₄ flux.This study helps to predict whether the Songnen grassland acting as atmospheric CH₄ source or sink under the background of grassland use and global change.