Study On The Structure And Activity Of Soil Nitrifiers In Desert Steppe,Northwestern China

Nitrification,the key process in nitrogen cycle,determines the nitrogen availability in the soil,which mainly carried out by ammonia-oxidizing bacteria(AOB),ammonia-oxidizing archaea(AOA),and nitrite-oxidizing bacteria(NOB).Ammonia oxidation,conversion of ammonia to nitrate,is the first and rate-limiting step of nitrification.More than 42% of the national territorial area of China belongs to the arid and semi-arid area,which is water and nitrogen limited,and ecologically fragile ecosystem.The activity and structure of nitrifiers in arid area is limited by many factors,but information about this is still limited.In order to explore the activity,abundance and community structure of AOA and AOB in three typical ecological types(wetland,desert steppe and sandy land)of the arid area,Northwestern China,the potential nitrification,characteristics of nitrification kinetics,as well as the abundance and high-throughput sequencing of amoA gene were measured.Furthermore,exclosure area and urea-treated fields in desert steppe were set up to explore the responses of AOA and AOB community structures and activities to grazing and nitrogen addition,which provide a scientific foundations for revealing the response mechanism of nitrifiers to global climate change in desert steppe of arid region.The main results were shown as follows:1.There was no significant differences in ammonia oxidation activity with or without fungal inhibitor,indicating that the contribution of fungi to ammonia oxidation was so low that could be ignored,AOA and AOB dominated the ammonia oxidation in three arid typical ecosystems.2.The higher activity and abundance of AOB than that of AOA suggested that AOB dominated the ammonia oxidation in desert steppe,while AOA was the main contributor to the ammonia oxidation in wetland and sandy land.Besides,the abundance and activity of AOA in the wetland were significantly higher than those in the sandy land.3.There were significant differences in the community structures of nitrifiers in three typical arid ecosystems: Group 1.1b of AOA was the dominant group in wetland and desert steppe,while ThAOA dominated in sandy land.Nitrosomonas and Nitrosospira of AOB were the main genera in wetland,but only Nitrosospira was found in desert steppe and sandy land.The dominant genus of NOB in the three ecosystems was Nitrospira.4.Soil pH,conductivity,water and organic matter content were the main environmental factors affecting the activity,abundance and community structures of ammonia oxidizers in this arid area.5.Light-grazing in desert steppe could acidify the soil and increase the organic matter content,which could stimulate the growth of heterotrophic or mixotrophic AOA and increase the activity and abundance of AOA.However,the community function of AOB changed little,which suggested that grazing may have little effects on dominant groups of AOB.6.After three years of urea addition,the conductivity and ammonia content in soil were increased.The activity and abundance of AOA were both significantly enhanced.It was supposed that the growth of AOA,which has urease activity or is suitable for high ammonia,might be promoted.While the abundance of AOB urease activity in this arid area may be too low,which was the reason AOB could not respond to urea addition.