The Contribution Of Complete Ammonia Oxidizers In Soil Ammonia Oxidation And Their Environmental Drivers

The discovery of complete ammonia oxidizers(comammox),a group of microorganisms that can oxidize ammonia to nitrate in a single cell,has broken the understanding that nitrification must be catalyzed stepwise by different microorganisms and is of great importance in the field of nitrogen cycle.Microbial studies related to the ammonia oxidation have focused on ammonia-oxidizing bacteria(AOB)and ammoniaoxidizing archaea(AOA),while studies on comammox in soil habitats are just beginning.This thesis selected three typical soil habitats,namely forest,grassland and cropland,to study the distribution and ecological functions of comammox,screen the key environmental factors,and investigate the effects of the key environmental factor on comammox through microcosm simulation.The results of the study are important for revealing the role of comammox in soil ammonia oxidation and analyzing the microbial mechanism of soil ammonia oxidation.The main findings are as follows.1.The distribution of comammox in typical soil habitats.The average abundance of comammox was the highest in cropland soils,but did not differ significantly among soil habitats;in all three types of soil habitats,the average abundance of comammox was the highest in the surface layer and decreased significantly along the soil depth.Comammox were the most abundant ammonia oxidizers in all three types of soil habitats studied,with an average relative abundance of 57.8% in the ammonia-oxidizing community,and have the potential to become the dominant ammonia oxidizers in terrestrial soil habitats.2.The ecological function of comammox in soil habitats.Comammox dominated the ammonia oxidation in soil habitats,especially in cropland soils,with amo A gene transcripts 33.8-42.6-fold and 5.0-14.3-fold higher than those of AOB and AOA,respectively;comammox amo A genes were highly correlated with genes of nitrite oxidation,denitrification and dissimilatory nitrate reduction,indicating high coupling of comammox with other processes of the nitrogen cycle.In the three types of soil habitats,the operational taxonomic units of comammox had an average relative abundance of 0.1-0.2% in the bacterial community,a frequency of 86.1-100% in each soil sample,and a relative abundance in the top 80%,thus inferring that comammox are core species in the bacterial community of each typical soil habitat.3.Key environmental factors on the abundance and activity of comammox.Among the six environmental factors explored,NH4+-N concentration was significantly correlated with the abundance and transcriptional activity of comammox,and significantly correlated with the community composition of ammonia oxidizers in forest and grassland soils(p<0.05);pH was significantly correlated with the abundance and transcriptional activity of comammox,and significantly correlated with the community composition of ammonia oxidizers in three types of soil habitats(p<0.05);temperature was significantly and positively correlated with the abundance of comammox in forest soils(p<0.05);C/N was significantly and positively correlated with the abundance and transcriptional activity of comammox in cropland soils(p<0.05).Collectively,pH was screened as the key environmental factor with the greatest influence.4.The response of comammox to pH changes in the microcosm simulation.Comammox adapted to a wide pH range from acidic to alkaline,but overall preferred acidic conditions.Microcosm simulation experiments revealed that after incubation under acidic conditions,the abundance of comammox increased to 4.7 times of the initial stage,and the proportion of active comammox in the active ammoniaoxidizing community was 52.9%;after incubation under alkaline conditions,the abundance of comammox decreased to 80% of the initial stage,and the proportion of active comammox in the active ammonia-oxidizing community was 12.7%.The dominant comammox differed under acidic and alkaline conditions,with amo A sequence similarity below 77.2%.