Effects Of Nitrogen Addition And Changing Precipitation On Ch₄ Flux In Semi-arid Grassland In Northern Shanxi

The atmospheric nitrogen deposition of China is increasing rapidly since the 1980 s.In the context of climate warming,the probability of extreme precipitation events is also rising.However,the effects of changing precipitation and nitrogen addition on methane（CH₄）flux in grassland ecosystem are still unclear.Lack of research on methane flux characteristics in grassland ecosystems limits our understanding of the mechanism of CH₄ flux processes in response to climate change,especially in the Northern China pastoral farming ecotone.The in-situ observation in the station of Youyu Loess Plateau Grassland Ecosystem Research Station.Six treatments were set up as follows: control（CK）,precipitation addition（+P,50%）,precipitation reduction（-P,50%）,nitrogen addition（N）,nitrogen and precipitation addition（N+P）,nitrogen addition and precipitation reduction（N-P）.And we measured the CH₄ flux in two growing seasons（May～ October 2018 and 2019）by the method of static box-gas chromatography,and analyzed the relationship between the CH₄ flux with biotic and abiotic factors.The main results are as follows:1.Semiarid grassland ecosystem is a weak CH₄ sink in northern Shanxi.The average absorption rate of CH₄ varies from 4.62±2.58 to 9.99±3.75 μg·m-2·h-1 during the two growing seasons.Each treatment had no significant effects on the average absorption of CH₄ during the two growing seasons（P>0.05）,and there was no significant interannual fluctuations（P>0.05）.Changing precipitation,nitrogen addition,and the interaction between changing precipitation and nitrogen addition on the CH₄ flux was not found（P>0.05）.2.Nitrogen addition significantly reduced soil p H,increased above-ground biomass and the concentration of soil nitrate nitrogen（P<0.05）;increasing precipitation significantly increased the contents of soil ammonium nitrogen,nitrate nitrogen and inorganic nitrogen（P<0.05）.Nitrogen addition,precipitation addition,and the interaction of the above two treatments had no significant effects on the other biological and nonbiological factors,including the abundances of methane-producing functional gene（mcr A）and the methane oxidative functional gene（pmo A）（P>0.05）.Precipitation reduction significantly increased soil p H,soil microbial biomass nitrogen（MBN）,the content of ammonium nitrogen（P<0.05）,while significantly reduced soil microbial biomass carbon-nitrogen ratio（MBC/MBN）,the content of nitrate nitrogen and inorganic nitrogen（P<0.05）,and the abundances of methane oxide gene（pmo A）.The interaction of precipitation reduction and nitrogen addition significantly increased the content of soil ammonium nitrogen,nitrate nitrogen,and inorganic nitrogen（P<0.05）.Precipitation reduction and the interaction of precipitation reduction and nitrogen addition had no significant effects on other biological and nonbiological factors（P>0.05）.3.There are no significant linear correlations between the CH₄ uptake and biotic/abiotic factors or the abundances of key functional genes（mcr A and pmo A）（P>0.05）.The results of path analysis showed that plant root:shoot ratio（R²=0.37,P<0.05）and soil p H（R²=0.28,P<0.05）are the main factors controlling the absorption of CH₄ in semi-arid grassland ecosystem under the interaction of precipitation and nitrogen addition.The root:shoot ratio is the main factor affecting the absorption of CH₄ under the interaction of precipitation reduction and nitrogen addition,but the effect is not significant（P<0.05）.