Nitrogen Cycling And Microbial Driven Mechanism Of Jinshui River And Qi River In The Han River,China

Nitrogen(N)is one of the main elements and also determinant for primary productivity in the aquatic ecosystems.River system links the terrestrial and ocean systems on the earth surface,it plays a very important role in global nitrogen cycle.Denitrification,anaerobic ammonium oxidation(ANAMMOX)and dissimilatory nitrate reduction to ammonium(DNRA)are called dissimilatory nitrate reduction process which transforms nitrate in and even remove from the water column.Thus,understanding those processed and their relationship with environmental factors and functional genes associated with participating microorganisms is critically important for improving and sustaining ecohealth of the aquatic systems.But there are few studies combined three processes of dissimilatory nitrate reduction on freshwater rivers.The study areas were located in the Jinshui River and the Qi River,China.Jinshui River and Qi River are two tributaries of Hanjiang River Basin,the water source of the Middle Route Project of South-to-North Water Diversion,and the water resource protection is the key to the benefits of the project.We analyzed the main environment factors and the characteristics of nitrogen transformation rate.The potential of nitrogen cycle microbial was analyzed by metagenomic sequencing,and the microbial community structure of denitrification was analyzed by high-throughput sequencing,and the control experiments were set up to explore the relationship between the main environment factors and denitrification.Furthermore,the relationships among nitrogen transformation process and key microorganisms in different seasons along the longitudinal gradient(from the source to the estuary)of rivers with different human disturbance were discussed.The sampling area with high human disturbance intensity has higher water temperature,DOC and NO₃^-.The results indicated genes,microorganisms and function categories displayed significant spatial dynamics along environmental and human disturbance gradients.The relative abundances of dissimilatory nitrate reduction genes,denitrification genes,the potential of Nitrate reduction,Nitrate reduction+nitrite oxidation and Anammox in sediments increased with the increase of human disturbance,while the relative abundances of organic matter degradation and synthesis genes,and the potential of Nitrogen assimilation decreased with the increase of human disturbance.A potential major contribution to most nitrogen cycle function by Burkholderiales in both rivers.The genetic potential to close the nitrogen cycle was there,but the dentrification,ammonification and nitrogen fixtion at very low abundance as compared with other pathways in the cycle.Temperature,NO₃^-and organic carbon were identified as primary drivers in selecting nitrogen cycling microorganisms and potential functions.Generally,denitrification,ANAMMOX,and DNRA were higher in summer(August),autumn(November),and spring(March),respectively.Relative contributions of nitrate reduction from denitrification,ANAMMOX,and DNRA were quite different in different seasons.Denitrification,the dominant process that removed reactive nitrogen from the river system in summer.DNRA was the dominant pathway of nitrate reduction in autumn and spring.N₂O concentration,N₂O saturation and flux of slightly disturbed area were peak in summer.The seasonal variation of the abundance of different functional genes is inconsistent.The sediment nitrification,denitrification,ANAMMOX rate,the abundance of functional genes of nitrogen cycle,and the dissolved N₂O concentration increased with the increase of human disturbance.There was no significant correlation between sediment nitrogen transformation rate and sediment functional gene abundance.Water temperature,nitrate,ammonium,C/N,iron ion and sulfide were important factors which affect the sediment nitrification,dissimilatory nitrate reduction rates and the abundance of functional genes.C/N played an important role in regulating the sediment dissimilatory nitrate reduction processes.N₂O emission was significantly affected by sediment nitrification and denitrification processes.The correlation among sediment nitrification,denitrification,ANAMMOX and DNRA was also observed in some seasons.Sediment denitrification and DNRA contribute more than ANAMMOX to total nitrate loss.We found that different genera and species of denitrifiers in sediment had distinct response patterns to human disturbance intensity.The potential active denitrifiers with nar G and nir S gene predominantly occurred in the intensely disturbed area within the catchment.The response of denitration rate to the change of human disturbance intensity was not significant.The factors affecting sediment denitrifier community composition had spatial and temporal variability,and the primary factors were water temperature and NO₃^-.High temperature and high concentration of nitrate could inhibit the diversity of some denitrifier communities in sediment,while high concentration of DOC could increase the diversity of communities.The diversity of nar G and nir S gene-coding denitrifier communities in sediemnt were negatively correlated with sediment denitrification rate.This study will deepen the understanding of nitrogen cycle,key microbial processes and the influence of human disturbance on the geochemical cycle of nitrogen in river ecosystem.Such information will be important in control of water nitrogen pollution and emission of greenhouse gas in the river systems.