Abstract:

The aim of this study was to explore the patterns of functional gene abundance and community composition of microorganisms involved in ammonia oxidation and denitrification in rhizosphere soil of leguminous shrubs on the Ningxia desert steppe among different plant types and growth periods. We collected rhizosphere soil samples from typical leguminous shrub communities in the Baijitan National Nature Reserve of Ningxia， including Caragana korshinskii， Ammopiptanthus mongolicus， Oxytropis aciphylla， and Caragana tibetica at different growth stages （vegetative， flowering， and fruiting stages）. Using real-time quantitative PCR and high-throughput sequencing methods， the abundance of functional genes and the characteristics of community structure of ammonia-oxidizing and denitrifying microorganisms in rhizosphere soil of four shrubs， as well as their correlations with soil factors， were analyzed. It was found that the abundance of functional genes and community structure varied among different shrub types and growth periods. The abundance of functional genes was significantly higher in the rhizosphere soil of C. korshinskii than in the rhizosphere soil of the other shrubs， and reached the highest level during the flowering period. The diversity of the ammonia-oxidizing bacteria （AOB） community in the rhizosphere soil of C. korshinskii at different growth periods was higher than that of the AOB communities in the rhizosphere soils of the other shrub types， while the diversity of ammonia-oxidizing archaea （AOA） and nirK-type denitrifying bacteria communities was lowest in the rhizosphere soil of C. tibetica. Unclassified taxa were the predominant AOA in rhizosphere soil of each shrub with an extremely high abundance of 79.34%-98.37%， whereas Thaumarchaeota and Crenarchaeota only accounted for 0.28%-20.37% and 0.28%-2.35%， respectively. Proteobacteria was the dominant phylum of AOB in the rhizosphere of C. korshinskii， while Norank_d__Bacteria was the dominant AOB in the rhizosphere soil of the other plants. Both nirK- and nirS- type denitrifying bacteria were dominated by Proteobacteria， but the composition of nirK- type denitrifying bacteria was more significantly influenced by the plant type and growth period. The abundance of functional genes was significantly positively correlated with soil total nitrogen， available potassium， and pH. The soil total potassium， available potassium， and organic matter significantly affected the community composition of ammonia-oxidizing microorganisms， and pH was the main factor affecting the community structure of denitrifying microorganisms.

Key words: sand-fixation shrublands, ammonia oxidation, denitrification, functional genes, community composition, diversity