Nitrogen Transformations And Mechanisms Across Riparian Zones Under Manwan Reservoir Operations

Riparian zone is the hotspot of source element cycles in river ecosystems,and the transition zone of surface water and groundwater,where substance biogeochemical processes are greatly affected by river hydrology.Dam constructions have changed the hydrological regime,and potentially produced potential impacts on nitrogen transformations in the riparian zone,resulting in changes of aquatic environment and ecology.Field monitoring was conducted for 49 days on the island riparian zone of Manwan Reservoir in the Langcang River.The nitrogen composition and organic carbon were analyzed,and functional microbes were quantified.The spatial patterns of denitrification across the island were also investigated to identify the hotspot of denitrification.Hyporheic exchanges and temperature spatial-temporal patterns in the riparian zone were explored,the heat transferred across the riparian zone were calculated,and the control factor was identified by sensitivity analysis.The riparian heat transfer under Manwan Reservoir operations and the underlying mechanisms were finally revealed.The main conclusions are as follows:?1?Under reservoir operations,intentisified water level fluctuations increased the frequency of dry-wet alternates,changed redox conditions,and favored denitrification in the riparian zone in the island.Since the surface sediment of the riparian zone surffered from frequent wet-dry alternates,denitrification rate in the surface sediment layer was higher than that in the bottom sediment layer in the riparian zone,the denitrification rate was 18.2³27.1 and 6.1³7.1?mol·N/m²·h in the surface and bottom sediment,respectively.The frequency of dry-wet alternates was the highest at the island edge,which was 8 times higher than the island center.As a result,denitrification was the strongest.?2?Reservoir operation intensified water level fluctuations and drove hyporheic exchanges between the island and reservoir.In the island,the water level fluctuation exhibited hysteresis and the amplitude had a certain attenuation.The exchange volume per unit length across the island could be up to-0.12⁰.06 m²/d.A higher temperature of 0.26⁴.80?than the reservoir water was observed in the island due to the lower specific heat of island sediments.Heat was transferred from the island to the reservoir by the hyporheic flows.?3?The heat transfer from the island to the reservoir created favorable consitions for denitrifiers.After dam construtions,organic matters deposited and formed island at the reservoir bank.The rich DOC supplied carbon source,resulting in higher abundances of denitrifiers at the island edge.The abundances of nirS,nosZ,AOA,AOB and hszB were 1.76?10⁶,1.52?10⁶,2.56?10³,0.36?10³ and 8.86?10³ copies/g,respectively;and the denitrification rate and N₂O flux reached the manxiumn of 327.1and 25.8?mol/m²·h,respectively.