The Role Of Aquatic Vegetation In Nitrous Oxide Production And Emissions In Rivers And Lakes

Greenhouse gases emissions from river and lake ecosystems have become a hot research topic in global greenhouse gaese emissions. Aquatic vegetation is an important primary producer in freshwater ecosystem. However, the effect of aquatic vegetation on nitrous oxide（N₂O） emissions from freshwater ecosystems and their impact mechanisms are not fully understood.In this study, we investigated N₂O emissions from three small rivers in Shanghai City, and from a shallow eutrophic grass-type lake in North China. We also studied the effects of filamentous algae on N₂O emissions from the three rivers with different anthropogenic activities influences, and to characterize the factors responsible for algal effects. River sediments were collected in situ and inoculated in a laboratory with filamentous algae Spirogyra spp. N₂O fluxes, N₂O concentrations, potential denitrification rate, bacteria abundances and the physicochemical characteristics of sediments and overlying water were analyzed over the course of experiment. In order to reveal the effect of submerged macrophytes on N₂O emissions in the eutrophic grass-type lake, an in situ investigation was conducted in the l. Two sampling site were selected, basing on different macrophytes densities in the lake. Overlying water and sediments were collected in situ to analysis N₂O concentrations, N₂O diffusing fluxes and potential denitrification rate.Main results of the study are as followed:1)The role of filamentous algae in N₂O production and emission in rivers:（1） N₂O fluxs from the three rivers were significantly differented（P < 0.01）. During the experiment periods, N₂O emitted from rivers in Mengqingyuan Park were ranged from 21.01±1.26 μmol m^-2 h^-1 to 1080.41±6.63 μmol m^-2 h^-1, significantly higher than that in the agricultural areas(10.41±0.24μmol m^-2 h^-1721.22±9.77 μmolm^-2 h^-1) and the Xisha Wetland Park(12.07±0.48 μmol m^-2 h^{-1 3}49.47±4.90 μmol m^-2 h^-1).（2） Filamentous algae significantly increased N₂O production and emissions from the three rivers. N₂O concentrations in overlying water and pore water of algal treatments were ranged from 66.80±0.64 μmol l^-1 to 1080.41±6.63 μmol l^-1, and from 3.07±0.12 μmol l^-1 to 48.60±1.84 μmol l^-1, respectively, which were significantly higher than those of controls groups（P<0.01）. Moreover, filamentous algae significantly increased pore water DOC concentrations during algal decomposition（P<0.01）. The pore water DOC concentration from algal treatments were ranged from 16.14±0.64 mg l^-1 to 56.89±3.29 mg l^-1, which were significantly higher than those of controls（P<0.01）. Moreover, algal treatment significantly increased the sediment potential denitrification rate in rivers（P<0.01）.（3） Results of stepwise forward multiple regression analysis showed that sediment potential denitrification rate was the main factor that determines N₂O fluxes in test columns（r²=0.712,P<0.01）. In addition, the pore water DOC concentration and NO₃^- concentration in sediment also had significant relationships with potential denitrification rate. These results implied that liable algal carbon released during algal decomposition may promote the sediment denitrification, and therefore increases the production and the emissions of N₂O.2) The role of submerged macrophytes in N₂O production and emissions in shallow eutrophic grass-type lakes:（1） We select two sampling sites according to the different submerged macrophytes densities. Reustls showed that high density of submerged macrophytes could significantly increase N₂O fluxes than the control sites（low density of submerged macrophytes）（P<0.01）. Moreover, DO concentrations were higher in the sites of high density of submerged macrophytes than the control sties（P<0.01）（2） Results of multiple regression analysis showed that the DO concentration was the main factor that determines N₂O fluxes in the grass-type lake（r²=0.848,P<0.001）. In addition, the water temperature also had significant relationships with the N₂O production and emission.