Study On The Influence Of Hydrodynamics On Nitrogen Transfer And Transformation And Microbial Communities In Hyporheic Zones

The hyporheic zone between surface water and groundwater has hydrodynamic gradient,hydrochemical properties and microbial communities,and is an intense area where biogeochemical behavior takes place.The study of nitrogen migration and transformation processes in this zone has become a hot issue in recent years.As changes in hydrodynamic conditions affect environmental conditions,nitrogen content,and differences in microbial community diversity,the three are closely related,but the research on how the microbial community responds to the transformation of nitrogen in the hyporheic zone is relatively weak.Therefore,studying hydrodynamic conditions on the nitrogen migration and transformation process in the hyporheic zone and the influence mechanism of the microbial community,and deepening the understanding of the law of nitrogen migration and transformation in the hyporheic zone,will provide a scientific basis for the prevention and control of nitrogen pollution in water bodies.Based on the National Natural Science Foundation of China（41877190）,this paper selects the Xi’an section of the Weihe River as the research area.Through the combination of field in-situ experiments,indoor soil column simulation,and microbial high-throughput sequencing,the biogeochemical characteristics of the the hyporheic zone,different waters are developed.Research on the migration and transformation law of nitrogen under dynamic conditions,and then explore in the hyporheic zone the interaction mechanism of hydrodynamic conditions on the transformation of nitrogen and microbial communities,and draw the following conclusions:（1）Through high-precision field observations,sampling tests and comprehensive studies,it is found that the nitrogen in the Xi’an section of the Weihe River increase in the concentration of NH₄⁺-N and NO₂^--N during the wet season（relative to the dry season）,and NO₃^--N concentration is reduced.The surface water in the wet season belongs to the categoryⅣstandard water,and the main nitrogen-containing pollutant is NH₄⁺-N,while the surface water quality in the dre season is better and can reach the categoryⅢstandard water;the groundwater belongs to the categoryⅣstandard water in the wet and dry seasons.It shows that the surface water is greatly affected by seasonal changes,but the impact on the groundwater is not obvious.（2）The sediments in the hyporheic zone of the Xi’an section of the Weihe River are mainly composed of fine,coarse sand and clay alternately.The dominant bacterial flora is detected in 5 phyla,9 classes,14 orders,14 families,and 11 genera.As the distance from the river bank increases,the NH₄⁺-N content changes sequentially to 14.779mg/kg,9.193mg/kg,15.464mg/kg,showing a trend of first decreasing and then increasing;NO₃^--N from70.007mg/kg gradually reduced to 29.675mg/kg.Viewed vertically,the nitrogen conversion in the shallow（above-80cm）area is more intense,and the NO₃^--N content gradually decreases as the depth increases.（3）Through indoor simulation experiments,it is found that the water level at the junction of sediments with different particle sizes in the heterogeneous hyporheic zone fluctuates greatly,while the water level and permeability coefficient in the homogeneous hyporheic zone change relatively smoothly,maintaining around 80cm and 0,05m/d,respectively.The p H and ORP are not much different,and the overall environment is weakly alkaline reduction;while the DO concentration is sightly lower than that of the homogeneous in the heterogeneity;TOC content is lower than that of the undisturbed soil,and the vertical change is larger in heterogeneity（0.006～0.062%）,and the change in homogeneity is small（0.013～0.033%）.The removal rate of nitrate in the water sample by heterogeneity is more than 95%,while the homogeneity is only above 80%;the total amount of nitrogen in the heterogeneous sediment is slightly higher than that of the average.Among them,the adsorption and retention effect of NH₄⁺-N is the most obvious（heterogeneity is increased by 2 to 11 times compared with the original state,and the homogeneity is increased by 1 to 4.5 times）.The content of NH₄⁺-N gradually decreases with the direction of water flow,while NO₃^--N and NO₂^--N content does not change much in the vertical direction.（4）Due to the different hydrodynamic conditions,when the river drains groundwater（downstream）,its DO fluctuates at 4.5～7.4mg/L,while the local sewage replenishes river water（upstream）at 2.0～4.4mg/L,which is significantly lower than the downflow;ORP is slightly lower than the downward flow,indicating that the reduction environment is stronger when the local water is used to replenish the river.The concentration of NH₄⁺-N in the downflow water sample is basically twice that of the upflow.A total of 13 phyla,24classes,37orders,46 families,and 48 genera of dominant bacteria groups were detected through microbial classification and sequencing.Changes in hydrodynamic conditions can promote the development of microorganisms in the sediments of the hyporheic zone.The dominant bacteria groups mainly include Proteobacteria,Firmicutes,Bacteroidetes.（5）The migration and transformation of nitrogen in the hyporheic zone is affected by the lithological structure and hydrodynamic conditions of the sediments,causing changes in environmental factors such as p H,DO,ORP,etc.,the microbial community also responds,and the abundance of its dominant bacteria genera.There is a big different in size in spatial distribution.The abundance and diversity of microbial communities are higher when river water drains groundwater,and the abundance at the water-soil interface is higher in the vertical direction.Low dissolved oxygen and reducing environment are conductive to denitrification or DNRA,and high p H is not conductive to the conversion of NH₄⁺-N.There are two strong reaction areas in the migration and transformation process of nitrogen in the hyporheic zone:shallow layer（river water drains groundwater）and deep layer（groundwater replenishes river water）.Due to different hydrodynamic conditions,the migration and transformation of nitrogen mainly occurs at the water-soil boundary.It gradually weakens along the direction of water flow,and in this process,denitrification,DNRA,nitrogen fixation,mineralization,nitrification and adsorption and retention of soil columns mainly occur,of which denitrification is the most important.