Effect Of Different Gradients Of Nitrogen Addition On The Ammonia Volatilization In Semi-arid Grassland Of The Agro-pastoral Ecotone In Northern China

The excessive use of nitrogen（N）fertilizers in modern agriculture has led to increasing atmospheric N deposition.The increase of N input will affect the soil properties and nutrient absorption of plants,which in turn will affect the use and loss of soil N.Ammonia volatilization,is one of the important ways of N loss in terrestrial ecosystems,but there is still little research on ammonia volatilization in grassland ecosystems.In this study,a random block design with 8N addition gradients(NH₄NO₃,0,1,2,4,8,16,24,and 32gNm^-2y^-1)was set in a typical semi-arid grassland ecosystem in the northern agro-pastoral ecotone（Yuyu County,Shanxi Province）.The intermittent closed chamber extraction method was used to measure the rate and accumulation of ammonia volatilization under various treatments.And we analyzed the main factors that affect the ammonia volatilization.The purpose of this study was to investigate the response mechanism and response threshold of ammonia volatilization to nitrogen input,in semi-arid grassland ecosystem of the agro-pastoral ecotone,and provide a basis for the formulation of grassland management measures in the region.The main results are as follows.1.The ammonia volatilization amount in grassland reached its maximum at10:00-12:00 a.m during a daily variation.Moreover,from morning till night,the amount of ammonia volatilization without N input increased first and then decreased gradually with time,while the amount of ammonia volatilization after nitrogen application showed a trend of fluctuation over time.The seasonal dynamic of the ammonia volatilization rate showed a trend of increasing first and then decreasing.The ammonia volatilization rate were 7.31（May）¹7.46（Aug）mg·m^-2·d^-1.2.The ammonia volatilization rate increased with the increase of N gradients,and reached a peak value at the 3rd to 4th day after fertilization.The ammonia volatilization rate in 32gNm^-2 y^-1 treatment was significantly higher than that of 4,2,1gNm^-2 y^-1 and control treatment（P<0.05）,while had no significant difference with other treatments（P>0.05）.8gNm^-2 y^-1 treatment was the threshold for the ammonia volatilization rate of N inputs,which means the ammonia volatilization rate increased slowly when N input was higher than 8 gNm^-2 y^-1.3.With the increase of nitrogen application gradients,the plant biomass of grassland was also increased gradually.However,there was no significant correlation between the ammonia volatilization rate and aboveground biomass or the net underground biomass.4.Soil temperature,p H,NH₄⁺ and NO₃^-are positively correlated with the ammonia volatilization rate（P<0.05）,while soil moisture and microbial biomass carbon were negatively correlated with ammonia volatilization rate（P<0.05）.There was no significant correlation between soil microbial biomass nitrogen and ammonia volatilization rate（P>0.05）.In conclusion,the 8gNm^-2y^-1 is the threshold of N input that affects soil ammonia volatilization;ammonia volatilization in semi-arid grassland ecosystems in the northern farming-pastoral ecotone is mainly affected by soil available N levels and soil environment,but had nothing to do with plant biomass.