Microbial Community Characteristics Of The Key Processes In Soil Nitrogen Cycle In The Water-level Fluctuation Zone Of The Three Gorges Reservoir Area

The nitrogen pollution in the water-level fluctuation zone(WLFZ)of the Three Gorges Reservoir Area(TGRA)and adjacent water body is becoming a public-concern issue.The WLFZ,as an ecotone of aquatic and terrestrial ecosystem,plays an essential role in intercepting non-point source pollution,removing nitrogen and purifying water body,and has been recognized as a crucial area of natural nitrogen cycle.The influences of disturbances from water-level fluctuation and human activity on soil environment and microbial community of the WLFZ,and resulting in the changes of nitrogen cycle processes by directly or indirectly are not yet clear.This thesis set the WLFZs located in typical tributary bays of the TGRA with contrastinganthropogenic disturbance levels as study areas.Understood the spatial and temporal distribution characteristics of nitrogen transformation related soil properties and the key driving factors.The techniques of molecular biology and high-throughput sequencing combined with the ecological principles were used to understand the ecological background of the archaea and bacteria in soils of the WLFZ,and to explore the ecological characteristics of aerobic ammonia-oxidizing archaea(AOA)and bacteria(AOB),anaerobic ammonium oxidation bacteria(ANAMMOX)and nitrite-dependent anaerobic methane oxidation bacteria(N-DAMO),and the effects of water flooding and anthropogenic disturbances on their gene abundance,diversity and community composition.The major results are as follows:(1)The variation in soil properties of the WLFZ was mostly driven by water-level fluctuation(35.9%)compared with the minor role of anthropogenic disturbance(3.3%).Soil TC,TN and OM were mainly affected by anthropogenic disturbance,whereas water-level fluctuation had a major influence on soil pH,ammonium and nitrate.Along the elevation gradient of 150 to 175 m,the soil properties were mainly determined by the sampling elevation,accounting for 61.5%of the variation,followed by short-term(6.1%)and long-term flooding(6.0%).The level of 167.5 m elevation was proposed to be the threshold that divides the WLFZ into two different response zones.Soils below 167.5 m were mainly affected by water-level fluctuation,whereas soils above 167.5 m were largely impacted by anthropogenic disturbance.(2)The archaeal abundance was resilient to the disturbance of both water flooding and human impact.The community diversity and composition were susceptible to the disturbances,which were highly related to the disturbing stages.More similar community diversity and composition appeared in the recovery and control groups in the study area with low anthropogenic activity,indicating that anthropogenic disturbance can intervene the recovery of archaeal community diversity and composition.The co-occurrence network further revealed that the archaeal community in the low human impact area exhibited more interactions implying a higher ecosystem stability.The better recovery of archaeal community was associated with the high resilience ability.The Soil Crenarchaeota Group is the key taxon maintaining the better recovery of the archaeal community from the disturbances due to its high resilience index and quantitative dominance.(3)No special effects of water flooding and anthropogenic disturbance on the recovery of soil bacterial community abundance and diversity in the WLFZ were observed.Water flooding showed a great influence on the community composition of soil bacteria.However,the bacterial community compositions showed a tendency to return back to pre-disturbance levels after the flooding receded,and anthropogenic disturbance will delay the recovery of soil bacterial community composition.Soil bacterial abundance in the WLFZ with limited anthropogenic disturbance showed higer resistance against water flooding than that of in the WLFZ with stronger anthropogenic disturbance,but showed higher resilience values in both areas.Both the resistance and resilience values of bacterial community diversity in soils with different anthropogenic disturbance levels were higher.The most abundant bacterial phyla with different anthropogenic disturbance levels showed high and comparable resistance values against water flooding,but the resilience of which with strong anthropogenic disturbance were weaker than that of limited anthropogenic disturbance.(4)The AOA generally demonstrated a better recovery ability after flooding disturbance compared to AOB.In particular,AOA were more resilient after the flooding disturbance.Taxa within the AOA and AOB showed different resistance and resilience values,with the most abundant taxa of AOA and AOB,Nitrososphaera subcluster 4 sucluster(60.7%)and Nitrosospira cluster 3a subcluster(71.7%)respectively,showing in general the highest resistance values.Soil NH4+ and Fe2+/Fe3+ were the main variables controlling the key taxa of AOA and AOB,and probably influenced the resistance and resilience properties of AOA and AOB communities.The distinct mechanisms of AOA and AOB in maintaining community stability against the flooding disturbance might be linked to the different life-history strategies:the AOA community was more likely to represent r-strategists in contrast to the AOB community following a K-life strategy.(5)A wide distribution of ANAMMOX bacteria in the WLFZ ecosystem was found.The abundance of ANAMMOX bacteria in the WLFZ soils was 1.15×104 to 2.08×107 copies/g,showed an decreasing trend with the elevation increased.Soil NH4+,TC,TN,moisture and accumulated flooding time were identified as important factors influencing the abundance of ANAMMOX bacteria.ANAMMOX genera of Candidatus Brocadia and Candidatus Kuenenia co-existed in soils of the WLFZ,and in where a low diversity of ANAMMOX bacteria was found.Moreover,the Candidatus Brocadia was found to be the most abundant ANAMMOX genus.(6)N-DAMO bacterial abundance in the WLFZ with strong anthropogenic disturbance was 8.98×102 to 3.31×105 copies/g,which was generally higher than that of weak anthropogenic disturbance(4.72x102 to 5.32×104 copies/g).Besides,the abundance increased after water flooding in both sites.A linear positive correlation was observed between the abundance and accumulate flooding time in soils with weak anthropogenic disturbance.Whereas the abundance in soils with strong anthropogenic disturbance showed a significant negative correlation with soil TN.The community structure of N-DAMO bacteria was similar in parallel elevations,but greater differences were observed after flooding.The community diversity in soils of lower elevations were higher than that of higher elevations.The community structure of N-DAMO bacteria showed obvious spatial-temporal heterogeneity,and which was further increased by water flooding.Both the water flooding and human activities could affect the N-DAMO bacterial community in the WLFZ,and the human activities have the effect of increasing the abundance and diversity.