Study On The Vertical Migration And Transformation Characteristics Of Soil Nitrogen In The Drawdown Zone Of The Three Gorges Reservoi

The reservoir drawdown zone has become a hot spot for nitrogen biogeochemical cycling due to its unique hydrological process,which has a significant impact on the changes in the water environment and water ecological health of the reservoir and downstream.The anti seasonal regulation mode of"winter storage and summer discharge"in the Three Gorges Reservoir Area has affected the complex processes of material migration,transformation,adsorption and release at the soil water interface of the water dissipation zone.At present,research on soil nitrogen migration in the Three Gorges Reservoir Area mainly focuses on the horizontal migration of soil nitrogen,and there is insufficient research on the vertical migration of soil nitrogen in the subsidence zone.This study simulates the hydrological situation of the Three Gorges Reservoir Area’s water level fluctuation zone,collects soil from the fluctuation zone for indoor soil column simulation experiments,sets two variable gradients:water level change and organic matter addition,and measures the changes in different forms of nitrogen content at different soil depths.To explore the effects of water level and organic matter on the vertical migration characteristics of soil nitrogen,in order to deepen the understanding of the cycling process of soil nitrogen and provide ideas for the ecological management of the water environment in the Three Gorges Reservoir drawdown area.The main results of this thesis are as follows:（1）water level fluctuations have little effect on soil water content;soil redox potential gradually decreases after water level rises,and the variation range decreases from-75～84 m V to-290～-70 m V;soil p H is overall at neutral to weakly alkaline level;The content of soil organic matter（SOM）is roughly between 11.0～22.3g/kg and 11.0～17.5g/kg,decreasing with increasing soil depth;the carbon to nitrogen（C/N）ratio was 11～12.During the experiment,the total nitrogen（TN）content in the soil decreased slightly with the increase of water level,and the variation range was 62～142 mg/g.Moreover,due to the adsorption of soil and the leaching loss of water for nitrate nitrogen（NO₃^--N）,the content of soil ammonia nitrogen（NH₄⁺-N）was always higher than that of soil NO₃^--N during the measurement,and the variation of both contents were 0.27～2.46 mg/L and 0.46～2.46 mg/L,respectively.At different incubation times,the NH₄⁺-N content of the soil always fluctuated greatly at each soil depth,while the NO₃^--N content tended to be consistent at each soil depth as the incubation time increased.（2）The addition of organic matter had little effect on soil water content;the range of soil redox potential decreased from-75～200 m V to-180～200 m V with the increase of soil organic matter content;soil p H was neutral to weakly alkaline overall;soil organic matter（SOM）content ranged from 11.0～19.9 g/kg and 13.8～22.5 g/kg,respectively;carbon to nitrogen ratio（C/N）was 0.5～0.5 g/kg;the carbon to nitrogen（C/N）ratio was 11～12.The soil TN content increased,and the variation range was 72～146 mg/g.The change pattern of NO₃^--N content in the soil during this experiment was consistent with that without the addition of organic matter,and still the NO₃^--N content tended to be consistent with the growth of incubation time at each soil depth,and the NH₄⁺-N content in the soil increased from 0.41～1.37 mg/L after the addition of organic matter to The content of soil NH₄⁺-N all had a large difference at about 6～12 cm of soil layer,but the difference in content decreased after the culture reached 23 days.This indicates that soil NH₄⁺-N does migrate in the vertical direction.Overall,organic matter had different effects on the migration of different forms of N in soil,and its effect on soil NO₃^--N was not significant,affecting the migration of soil TN and NH₄⁺-N.