Major Nitrogen Removal Processes And Influence Factors In Lakes Along The Middle And Lower Reaches Of Yangtze River

About 30%of freshwater shallow lakes of China are located in the middle and lower reaches of the Yangtze River.The percent of eutrophic lakes in this reach has increased to 85.9%,as most of eutrophic lakes could be categorized into the mid-eutrophic status.Sediment denitrification is the reduction of nitrate to digitrogen gases,resulting in the permanent removal of nitrogen,while nitrification provide substrate for denitrification.Therefore,sediment nitrification-denitrification is the major nitrogen removal process in aquatic ecosystem.Sediment nitrogen removal processes are greatly influeced by abiotic and biotic fctors.Among of them,abiotic factors including available nitrogen,carbon sources and dissolved oxgen,while biotic factors including macrophyte type,plant functiong group,nitrifier,denitrifier and function gene.The effection of abiotic and biotic factors on nitrogen removal process can be divied into two categories:proximal regulatiors are those that instantaneously affect nitrogen cycling involved microbes result in immediately changes in nitrogen removal process,and distal regulatios affect the nitrogen cycling microbes over variation for a larger spatial and temporal scale than proximal regulators.Scale effect is very important in ecology studies,while human land use along the Yangtze River have an impact on the degradation of lakes.In this study,we investigated shallow lakes along the middle and lower reaches of Yangtze River,by typical lake investigation and controlling experiment.The main contents and results are as follows:1.We determined the unamended denitrification rate and nitrous oxide(N2O)production rate of sediment samples from 22 eutrophic lakes in the Yangtze River basin.We also quantified the diversity and abundance of denitrifying communities using nirK and nirS genes.The relative contribution of abiotic and biotic factors to the sediment denitrification is highly variable.The results of variance partitioning analyses showed that water physicochemical properties and nutrients but not denitrifier communities and submerged vegetation were the major factor groups predicting denitrification and N2O production rates.Path analyses further revealed that water physicochemical properties and nutrients could affect denitrification and N2O production rates both directly and indirectly,and the direct effects were considerably higher than the indirect effects mediated through changes in sediment characteristics,denitrifier communities and submerged vegetation.These findings suggest that the dominant nitrogen removal process in Yangtze lakes is largely regulated by abiotic factors rather than diversity and abundance of denitrifiers and submerged macrophytes.2.We investigated the potential nitrification rates sediment from 10 shallow lakes in the middle and lower Yangtze River,sediment potential nitrification rate in hypertrophic lake sites was slightly but not significantly higher than that in eutrophic and mesotrophic lake sites.The nitrification rates were positively related to water quality and sediment properties.Using path analysis,we found that 55-60%of the indirect effect of catchment land uses on nitrification rates was mediated via sediment nitrogen content.However,the nitrification rates were not significantly associated with diversity and abundance of ammonia-oxidizing microorganisms and submerged plants.The responses of sediment nitrification potentials and nitrifying microorganisms to water eutrophication in Yangtze lakes are inconsistent.In addition,our findings highlight the importance of multi-scale abiotic factors,especially sediment properties,in regulating the nitrification process of lake sediments.3.Decline of submerged vegetation is one of the most serious ecological problems in eutrophic lakes worldwide.Although restoration of submerged vegetation is widely assumed to enhance ecological functions(e.g.,nitrogen removal)and aquatic biodiversity,the evidence for this assumption is very limited.We investigated the spatio-temporal patterns of sediment potential nitrification,unamended denitrification and N2O production rates along a vegetation gradient in the Lake Honghu.We also used five functional genes as markers to quantify the abundance of sediment nitrifying and denitrifying microorganisms.Results showed that the absence of submerged vegetation had no significant effect on denitrification and N2O production rates.Additionally,the abundance of functional microorganisms in sediments was not significantly different among vegetation types.Season had a strong effect on both nitrogen cycling processes andmicrobial abundances.The highest nitrification rates were observed in September,while the highest denitrification rates occurred in December.The temporal variation of sediment nitrification,denitrification and N2O production rates could be due to changes in water quality and sediment properties rather than submerged vegetation and microbial abundances.Our findings suggested that vegetation restoration in eutrophic lakes improves water quality but does not enhance sediment nitrogen removal rates and microbial abundances.4.We invested the effect of macrophyte on sediment nitrogen removal process by controlling experiments in greenhouse.We selected three common macrophytes species in lakes of Yangtze River,including Hydrilla verticillata,Vallisneria natans and Potamogeton malaianus.The results showed that submerged macrophytes present and species richness had not significantly effect on sediment denitrificaiton rates.In addition,denirificaiton functional gene abundance was not associated with macrophyte types.Water quality and sediment properties were the major factors that affected sediment nitrogen removal process and associated functional gene.