Elevational Patterns And Seasonal Dynamics Of Soil Denitrifying Bacterial Communities In Alpine Grasslands On The Qinghai-Tibetan Plateau

The Qinghai-Tibetan Plateau is the highest and largest plateau in China,with alpine grasslands accounting for more than 60%of its total area.Denitrification is an important process in the nitrogen cycle of soil ecosystem,which has a significant impact on soil fertility loss and greenhouse gas N₂O emission.The denitrification process is mainly driven by a series of microorganisms.In recent years,the biogeographical study of soil denitrifying microorganisms has attracted much attention.However,the altitude distribution pattern and seasonal variation of soil denitrifying microbial community diversity and structure in alpine climate are still unclear.Therefore,in the different months of the plant growing season（June-September 2021）,we collected 6 soil samples from alpine grasslands at intervals of 200 m within an elevation of 3200-4200 m,using nitrate reductase nirK and nirS amplicons sequencing and quantitative PCR techniques to systematically study the elevational patterns,growing season dynamics and key influencing factors of soil denitrifying bacteria communities,and analyzed the microbial co-occurrence network of denitrifying bacteria communities.The main results are as follows:（1）With the increase of elevation,the denitrification rate and nirS gene abundance increased first and then decreased,reaching the maximum at 3600 m.Soil denitrification rate was positively correlated with soil total nitrogen,total carbon and moisture content,and negatively correlated with p H.The abundance of nirK gene varied significantly at different elevations,and the abundance of nirK gene was about22 times that of nirS gene.The community diversity and composition of denitrifying bacteria were significantly different at different elevations.Theα-diversity of nirK-type denitrifying bacteria increased first and then decreased with the elevation,while theα-diversity of nirS-type denitrifying bacteria showed a decreasing trend.Theα-diversity of nirK-type denitrifying bacteria was significantly negatively correlated with soil ammonium nitrogen,while theα-diversity of nirS-type denitrifying bacteria was significantly positively correlated with soil ammonium nitrogen.The communities structure of denitrifying bacteria was significantly different at different elevations,and there was the greatest difference between 3200 m and 3600 m.Soil temperature was the main factor affecting theβ-diversity of denitrifying bacteria communities at different elevations.（2）In different months of the growing season,theα-diversity of nirK-type denitrifying bacteria was different.Except for theα-diversity at 3200 m and 3800 m above sea level,theα-diversity at other elevations was significantly different in different months,with the lowestα-diversity in June.Aboveground biomass of plants was the main factor driving the change in species richness of nirK-type denitrifying bacteria during the growing season.Theα-diversity of nirS-type denitrifying bacteria was significantly different only in different months at partial elevations.The community structure of denitrifying bacteria was significantly different in different months,and the community structure of June was the most different from other months.Plant aboveground biomass and soil water content were important factors affecting the seasonal dynamics of denitrifying bacterial community structure.（3）The effects of elevation gradient and growing season on the co-occurrence network of denitrifying bacteria community were different.With the gradual decrease of elevation and the extension of growing season,the number of nodes,edges,average degree and natural connectivity of nirS-type denitrifying bacterial community co-occurrence network gradually increased,and the co-occurrence network was the most complex and stable at low elevation and in September.The co-occurrence network of nirK-type denitrifying bacterial communities was the most complex and stable at low elevation and in July.In conclusion,our results suggest that the altitude distribution patterns and growing season dynamics of denitrifying bacterial communities are heterogeneous.Soil temperature and plant aboveground biomass were important factors driving changes in denitrifying bacterial community diversity and composition.This study provides a scientific basis for understanding the biogeographic distribution and temporal variation of soil denitrifying bacterial communities in alpine grasslands,and is of great significance for evaluating the role of denitrification process in the nitrogen balance of alpine grassland ecosystems on the Qinghai-Tibetan Plateau.