The Geographic Distribution Patterns And Assembly Mechanisms Of Nitrifying-and Denitrifying Microbial Communities In The Typical Wetland Eecosystems

Nitrogen is an indispensable element of all living organisms.The appropriate amount of nitrogen provides nutrients for macro-and microorganisms which play key roles in maintaining the stability and functions of the wetland ecosystem.High nitrogen load could cause overnutrition,which will seriously lead to the imbalance of the wetland ecosystem,and threatens the survival and health of mankind.The nitrogen cycle is mainly driven by microorganisms,so it is particularly critical for us to reveal the diversity,biogeographic distributions and community assembly mechanisms of microorganisms related to nitrogen cycle.In recent years,some studies have been focusing attention on the biogeographic distribution patterns,biodiversity,and assembly mechanisms of microbial communities.However,very few works carried out discussing the biogeographic distributions and assemblages of nitrogen cycle microorganisms.Therefore,this study selected ammonia oxidizing bacteria（AOB）and ammonia oxidizing archaea（AOA）,nir K-and nir S-type denitrifiers as representative of nitrogen cycle microorganisms in the typical wetlands in China（36 wetland sites of the Qinghai-Tibet Plateau and 30 major riverine wetland sites of Central and Eastern China）.To reveal the biogeographic distribution patterns,biodiversity,assembly mechanism of nitrifying and denitrifying microbial communities in typical wetlands and their relative contributions to ecological functions.The major contents and analysis results are showed as follows:In wetlands on the Qinghai-Tibet Plateau,for nitrifying microbial community,Nitrososphaera clade（Group I.1b）overwhelmingly dominated in AOA community（90.36%）,while Nitrosospira was the principal AOB type（64.96%）.The variations of ammonia-oxidizer communities were mainly regualted by environmental conditions,but not spatial distance.Particularly,the AOA community composition was influenced by mean annual temperature（MAT）and mean annual precipitation（MAP）,while MAT,conductivity and plant richness,p H,and TN influenced the AOB community composition.The net nitrification rate had a significant correlation to AOB,but not AOA abundance.Our results suggest a dominant role for climate factors（MAT and MAP）in shaping community composition across a wide variety of wetland sites and conditions.For denitrifying microbial community,soil moisture,carbon availability and soil temperature were the main determinants of diversity[operational taxonomic unit（OTU）number]and abundance of the nir S-type denitrifier community,while water total organic carbon,soil NO₃^–and soil moisture were important in controlling nir K-type denitrifier diversity and abundance.The nir S community composition was influenced by water electrical conductivity,soil temperature and water depth,while the nir K community composition was affected by soil electrical conductivity.Spatial distance explained more variation in the nir S community composition than in the nir K community composition.Additionally,the soil denitrification rate was positively related to the abundance,but not theα-diversity,of denitrifying communities.Our findings highlight the importance of both environmental filtering and spatial distance in explaining diversity and biogeography of soil nir communities in remote and relatively undisturbed wetlands.In 30 major riverine wetlands of Central and Eastern China,α‐andβ‐diversity of nitrifiers and denitrifiers did not differ significantly among soil types.Significant latitudinal patterns inα‐andβ‐diversity were observed for nitrifiers and denitrifiers,but these trends were not consistent between abundant and rare microbial sub-communities.Both nitrifiers and denitrifiers exhibited significant distance–decay relationship（DDR）and environmental-decay relationship（EDR）and abundant microbial taxa had steeper DDR and EDR slopes than rare ones.Further analysis showed that spatial and climate factors were the determinant of nitrifying-and denitrifying-microbial community variations,except for AOB rare one which was driven by geographic,climate and other environmental factors.Null model analysis revealed that community assemblies of nitrifiers and denitrifiers were mainly affected by dispersal limitation and undominated processes belonging to stochastic process.Interestingly,undominated processes dominated the rare nitrifying communities,while dispersal limitation governed the abundant nitrifying communities and rare and abundant denitrifying communities.Because of their significant discrepance in geographic distribution patterns,whether abundant and rare taxa have different contributions to ecological network stability and functions?Results showed that rare nitrifiers and denitrifiers exhibited broader environmental thresholds than abundant ones.And rare taxa had higher values of network topological parameters（e.g.,node degree and eigenvector centrality）than abundant taxa in co-occurrence network,implying that rare microorganisms played a vital role in maintaining the structure of nitrifying and denitrifying communities.Nitrification rates were only significantly related toβdiversity of rare amo A-type nitrifiers and denitrification rates were largely influenced byβdiversity of both rare and abundant nir K-type denitrifiers.These results suggested that rare microbial taxa had an important but previously underestimated role in biogeochemical processes.In summary,we used Sanger sequencing and high-throughput sequencing technologies to research on nitrifying and denitrifying microbial communities,and had achieved phased research results.From initially revealing the driving factors of nitrifying and denitrifying microbial communities to deciphering the community assembly mechanisms,from the study of whole microorganisms to the study of abundant and rare microbial sub-communities,our findings revealed that the community assembly mechanisms in wetlands on the Qinghai-Tibet Plateau were very different from that in wetlands of Central and Eastern China.Overall,deterministic processes contributed more to the former except for nir K-type community,and the opposite in the later.In addition,we also highlighted that rare taxa contribute more to microbial network complexity and nitrogen cycling processes than abundant taxa in river ecosystems at the continental scale.