Measurements Of Methane And Nitrous Oxide Fluxes From Irrigation Rivers And Inland Aquaculture Wetlands

Carbon dioxide(CO2),methane(CH4)and nitrous oxide(N2O)are three important greenhouse gases(GHGs)in the atmosphere.Freshwater and wetland ecosystems are actively interacted with the climate system,and they are one of the major sources of the global terrestrial GHGs emissions.Currently,global warming is affecting these ecosystems and may trigger an increase in GHGs emissions,which may further enhance global warming.In recent years,researchers have raised attentions to the fluxes of GHGs from freshwater aquatic environments,and observations have been conducted in various kinds of waterbodies such as natural rivers,lakes,hydropower reservoirs,ponds,and estuary wetlands.In southeast China,agricultural irrigation river and freshwater aquaculture wetland converted from agricultural land are two typical agricultural waterbodies and active sources of GHGs emissions.The purpose of this thesis is to estimate the GHGs emissions from these two ecosystems,analyze the emission mechanisms,and amplify the GHGs emissions inventory.This thesis research also aims to constrain the observation uncertainties and assess the potential climate feedbacks by comparing the GHGs budgets with observations from various freshwater and wetland ecosystems around the globe.This thesis consists of two parts of studies,which applied different measurement methods to observe the GHGs emissions.The first study measured the CH4 and N2O fluxes from an agricultural irrigation river over the period of 2014 to 2016 in southeast China.The floating chamber technique and diffusion model method were applied at the same time for comparison.The second study is located at a fish farming wetland(FFW)converted from adjacent paddy fields in southeast China.The CH4 and N2O fluxes were measured using the floating chamber technique from 2014 to 2015.Based on the observations from the two sites,the GHGs emission intensities,patterns and driving mechanisms were analyzed.The results were also compared with other types of waterbodies around the world to evaluate the measurement uncertainties in the GHGs emissions and address their contributions to the GHGs emissions from the freshwater and wetland ecosystem.The main results and conclusions are summarized as follows:1.In the agricultural irrigation river,the diurnal variation of CH4 and N2O fluxes measured by the floating chamber technique and diffusion model method showed a similar pattern with the peak appearing during the nighttime.Chamber-measured CH4 flux was higher than the model-estimated flux at the diurnal scale,while the N2O flux measured by two methods showed a similar magnitude.The seasonal variations of CH4 and N2O fluxes measured by the two methods were similar,and the peaks of CH4 and N2O fluxes were all appeared in summer(July-August).Both annual CH4 and N2O fluxes measured by chamber were higher than the model estimate.The result suggested that the floating chamber technique is more suitable than the diffusion model method for measurements on CH4 and N2O fluxes in freshwater ecosystems.2.In the agricultural irrigation river,the CH4 and N2O fluxes showed a similar diurnal variation with the maximums occurred at night.The time interval of 9:00-11:00 a.m.was identified to be the sampling time best representing daily CH4 and N2O fluxes measurements in agricultural irrigation rivers.On the seasonal scale,CH4 and N2O fluxes varied with highest emissions in summer and lowest in winter.The annual average fluxes of CH4 and N2O was 20.94 mg m-2 h-1 and 34.27 ?g m-2 h-1,respectively.Water temperature DO,DOC and mineral nitrogen significantly affected CH4 and N2O emissions from the agricultural irrigation river.3.In the fish farming wetland(FFW),the CH4 and N2O emissions had distinguished seasonal patterns and presented a similar trend in seasonal variation during the whole breeding cycle.The average annual CH4 and N2O fluxes were 0.51 mg CH4 m-2 h-1 and 54.78 ?g N2O-N m-2 h-1,respectively.Water temperature DO,DOC and mineral nitrogen were important factors affecting the CH4 and N2O emissions in the FFW.The direct emission factor of N2O-N(EF-Y)derived from our measurement was 1.23 g kg-1(the unit yield).Using the IPCC(2013)methodology with this factor,the total annual CH4 and N2O-N emissions of the FFW in Jiangsu province,China were estimated to be 24.18 Gg and 3.02 Gg,respectively.4.Comparing with other freshwater ecosystems,the CH4 emissions from the agricultural irrigation river were significantly higher than other types of freshwater ecosystems,while the N2O emissions were within the typical range of other types of freshwater ecosystems.Due to the unique breeding management measures(e.g drainage,desilting,disinfecting and oxygenating artificially)and the good water quality,the fish farming wetland had relatively low CH4 and N2O emissions among the freshwater ecosystems.The uncertainty of GHGs emissions of freshwater ecosystems still remains large,which has limited the studies on evaluating the GHGs emissions mitigation potential in freshwater ecosystems.Therefore,further researches of direct measurements are highly needed to better constrain the uncertainty in GHGs emissions from freshwater ecosystems worldwide.Overall,the diurnal variation of CH4 and N2O fluxes measured by the floating chamber technique and diffusion model method showed a similar pattern from the agricultural irrigation river.Generally,model-estimated CH4 and N2O fluxes were lower than the chamber-measured fluxes.The result suggested that the floating chamber technique is more suitable than the diffusion model method for measurements on CH4 and N2O fluxes in freshwater ecosystems.Second,the time interval of 9:00-11:00 a.m.was identified to be the sampling time best representing daily CH4 and N2O fluxes measurements.On the seasonal scale,CH4 and N2O fluxes varied with highest emissions in summer and lowest in winter.Water temperature,DO,DOC and mineral nitrogen can significantly affected CH4 and N2O emissions from the agricultural irrigation river.Thirdly,in the fish farming wetland,the CH4 and N2O emissions had distinguished seasonal patterns and presented a similar trend in seasonal variation during the whole breeding cycle.Water temperature,DO,DOC and mineral nitrogen also can significantly affected CH4 and N2O emissions from the fish farming wetland.Using the IPCC(2013)methodology with this factor,the total annual CH4 and N2O-N emissions of the freshwater aquaculture wetlands in Jiangsu province,China were estimated to be 24.18 Gg and 3.02 Gg,respectively.Fourthly,Comparing with other freshwater ecosystems,the CH4 emissions from the agricultural irrigation river were significantly higher than other types of freshwater ecosystems,while the N2O emissions were within the typical range of other types of freshwater ecosystem.The fish farming wetland had relatively low CH4 and N2O emission rates among the freshwater ecosystems.