Effects Of Vegetation Distribution Under Hydrological Stress And Decay And Decomposition Of Algae On Nitrogen Cycle In River-connected Wetland

Changes of hydrological conditions caused by water conservancy projects significantly altered the hydrological rhythms of wetlands,which varied the distribution of vegetations.The nitrogen cycle of wetland ecosystem also transformed with the species of plants,and this"mediated effect"caused by the distribution of vegetation under hydrological stress further affect the migration and transformation of nitrogen.At present,wetland ecosystems such as lakes and reservoirs were considered as key areas of N₂O emissions.Many studies have shown the production of N₂O and its influencing factors.The discrepancies of vegetations under hydrological stress also affected the microbial activities involved in the nitrogen cycle,which influenced the production of N₂O.Therefore,exploring the response of N₂O emission and migration and transformation of nitrogen to the variations of vegetations under artificial hydrological regulation was significant to the study of nitrogen cycle in wetland ecosystem,global N₂O emissions and the development of hydraulic regulation methods.In this study,the real-time water level,biomass of plants,physicochemical factors in sediments,N₂O emissions and microbial community were monitored in Donging Lake from 2019 to 2020.The influence mechanism of plants on the migration and transformation of nitrogen and N₂O emission under hydrological stress at the wetlands in middle and lower reaches of the Yangtze River was investigated.It provided theoretical bases for the exploration of nitrogen cycle and production of N₂O.In addition,A control experiment in laboratory was established based on the decaying and decomposing of plants and algae,which simulated and analyzed the effects of algae blooms on nitrogen cycle and N₂O emission.The main conclusions are as follows:（1）Overall,the hydrological rhythm of Dongting Lake was opposite to that of the Three Gorges Reservoir.The wet season of Dongting Lake wetland was from May to September every year,and the dry season was from October to April the next year.The long-term hydrological rhythm results in the zonal distribution of vegetation.In Dongting Lake the water level of each vegetation zone from large to small was as follows:mud flat>nymphoides zone>phalaris zone>carex zone>reed zone.With the increase of water level change rate,the amount of carbon and nitrogen sequestration in plants increased gradually.（2）The periodic alternation of drying and wetting in the short term not only affected the content of physical and chemical factors in the sediments,but also dissimilated the carbon and nitrogen cycle in wetland.The concentration of TOC(42.743±0.384 g kg^-1)and TN(1.617±0.023 g kg^-1)of sediments in phalaris zone were the highest in the dry season,while in the wet season the concentration of TOC(66.710±1.924 g kg^-1)and TN(3.304±0.062 g kg^-1)in nymphoides zone were the highest.During the wet season,the litter and decay of plants input a large amount of carbon and nitrogen into the sediment,which served as the sink of carbon and nitrogen.During the dry season,the sediment served as the source to provide mineral nutrients for the growth of plants.The C:N ratio in the sediments also changed with the alternation of the source and sink functions.（3）The distribution of vegetation under hydrological stress has a significant effect on N₂O emission flux.Both high and low water levels,the highest N₂O emission fluxes were observed in Mud Flat(0.561±0.067μg m^-2 min^-1 and 0.988±0.267μg m^-2 min^-1,respectively).The N₂O emissions flux showed a significant difference between vegetation zones during each water level period.Moreover,when the C:N ratio>25,the N₂O emission flux gradually stabilized(0.23μg m^-2 min^-1).This suggested that the C:N ratio of sediments was the dominant factor for the N₂O emission flux,and C:N ratio=25 may be the threshold value for the increase in the N₂O emission flux.The hydrological conditions in Dongting Lake also led to the changes of the community on phylum level in the sediments,and the contents of denitrifying bacteria and anammox bacteria increased with the rise of the water level.The functional genes of denitrification and anammox were also different among different vegetation zones.（4）According to the simulation results of indoor control experiments,the algae bloom created an anaerobic environment in the water.The decay and decomposition rate of algae was significantly higher than that of aquatic plants,which provided more N and P to the sediments,thus promoting the generation and discharge of N₂O.At the same time,the microbial community structure changed with the increase of time,the contents of denitrifying bacteria and anammox bacteria decreased gradually.The addition of algae also reduced the contents of these bacteria.The high N₂O emission in mud flat and the differences of N₂O emission between vegetation zones suggested that the protection of the integrality and diversity of plant community and the collocation of plants in wetland played a significant role in reduction of greenhouse gas.Meanwhile,protecting wetland plants from cyanobacteria blooms can effectively control the emission of greenhouse gases.The change of nitrogen cycle process caused by flooding on plants in wetland further inspired that the strategies of hydrological control should pay more attention on the adverse effects of hydrological conditions on vegetations.In future management of wetland,the damage of vegetation community by strong hydrological processes should be avoided as far as possible.