Magnitudes And Environmental Drivers Of Annual Greenhouse Gas Emissions From Four Types Of Natural Wetlands In China

The increasing greenhouse gas(GHG)concentration in the atmosphere is an important cause of global warming.The saturated anaerobic conditions and rich organic matter resources of natural wetlands play a key role in the absorption and emission of greenhouse gases(GHGs).Due to the great temporal and spatial heterogeneity of wetland ecosystems in location distribution and hydrothermal conditions,there exist significant differences in the annual level and mechanism of GHG emissions between different types of wetlands.Whether natural wetlands are net sources or net sinks of GHGs remains controversial,which leads to large uncertainties in the estimates of annual GHG emissions from wetlands.Many results of in situ observational studies on wetland GHG fluxes show great differences,and the results of integrated studies on the estimation of the annual GHG emissions from wetlands on a large scale are also diverse.The incorporated observation data may affect the accuracy of the annual GHG emission accounting results due to the insufficient duration of the experiment.Therefore,this study collected the GHG flux data of different types of natural wetlands that are continuously observed throughout the whole year to verify and quantify the impact of the included data on the annual estimation results,and then calculated the warming effect scale of different types of natural wetlands on a large temporal and spatial scale.Exploring the relevant driving factors is of great significance for mitigating global climate change and managing natural wetland resources.Retrieving the research literature on wetland GHG fluxes that have been observed for at least one year,this study collated 265 sets of observational emission flux data and the related environmental variable data,and thus establishing a dataset of annual GHG fluxes of carbon dioxide(CO2),methane(CH4),and nitrous oxide(N2O)from marshes,coastal wetlands,lake wetlands,and river wetlands in China.This study firstly applied the method of meta-analysis to systematically evaluate the impact of annual and non-annual observation data on the estimation of large-scale greenhouse gas emissions,then estimated the annual emission scale and warming effect of three types of GHGs in four types of natural wetlands in China,and at the same time explored the driving factors of the effect size(ES)and the annual GHG flux level.This study obtained the following main conclusions:(1)Incorporating the observation data that lasts for the whole year is of great significance for the estimation of the annual GHG emissions from natural wetlands.Among different types of natural wetlands,significant differences existed between the CH4 flux during the plant growing season and the CH4 flux throughout the whole year(p<0.05);estimates of annual emissions using mean fluxes that are observed only from the growing season significantly overestimated the sink effect of wetlands on CO2(ES=-0.83±0.73,p<0.05)and significantly overestimated the emission effect of CH4(ES=1.43±0.38,p<0.0001),while for N2O,the overall difference was not significant(ES=0.19±0.34,p=0.28).There are significant heterogeneities in the subgroups of natural wetland types,observation frequencies,and observation durations.(2)Based on the unbiased data that are observed over the whole year,we found that the annual net CO2 emission fluxes(FCO2)of the four types of natural wetlands in China showed significant differences(p<0.05)while their CH4 and N2O emission fluxes(FCH4,FN2O)were not significantly different.Coastal wetlands had the highest carbon sequestration capacity(FCO2=-182.36 g C m-2 year-1),the annual CH4 emission flux of lake wetlands was relatively the lowest(FCH4=0.12 g C m-2 year-1),and the river wetlands showed the relatively highest fluxes of all three GHGs(FCO2=211.11 g C m-2 year-1,FCH4=22.45 g C m-2 year-1,FN2O=1.10 g N m-2 year-1).When the GHG fluxes were converted into global warming potential(GWP),the estimation results of this study showed that the cumulative annual warming effect of the four natural wetlands in China reached 426.68 Tg CO2-eq year-1.Among the four wetland types,only coastal wetlands were net sinks of warming effect(GWP=-1 1.59 Tg CO2-eq year-1),while river wetlands had the highest warming effect(GWP=243.38 Tg CO2-eq year-1).Among the three GHG types,CH4 had the highest warming effect(GWP=325.01 Tg CO2-eq year-1).(3)The results of meta-regression analysis and correlation analysis of GHG fluxes showed that wetland geographical location,climate variables,natural conditions,and physiochemical properties of soil and water jointly drive the ES values and the annual flux levels of GHGs,among which the physiochemical properties of wetlands serve as the main influencing factors.At the management level of natural wetlands,different management measures should be given to various wetlands according to the influencing factors proposed in this study.More focus should also be paid to the protection of coastal wetland resources as well as the reduction of wetland CH4 emission.This study verifies the necessity of incorporating observation data that last at least one year to estimate the scale of annual GHG emissions,establishes the dataset of GHG fluxes covering four types of natural wetlands,reveals the important carbon sequestration of coastal wetlands and the significant warming effect of wetland CH4,and uncovers the impacts of soil water content and the concentration of nitrogen and phosphorus on the wetland annual GHG emissions.This research can be used as a method reference and data case for large-scale ecosystem warming effect accounting,and also provides a decision-making basis for mitigating global warming and managing natural wetland resources.