Nitrogen Fixation And Associated Influencing Factors In Sediments Of The Yangtze Estuary And Adjacent Coastal Areas

Reactive nitrogen input to estuarine and coastal areas has increased tremendously in the past few decades,consequently causing eutrophication,hypoxia,and other environmental issues.Anthropogenic input is generally considered a primary source for overloaded reactive nitrogen in estuarine and coastal environments,but nitrogen fixation may also be an important and internal source of reactive nitrogen.Diazotrophs have the ability to increase reactive nitrogen via fixing atmospheric nitrogen.The nitrogen fixed by diazotrophs can stimulate primary productivity,and to some extent may aggravate nitrogen pollution in estuarine and coastal ecosystems and affect nitrogen budget.Thus,an increasing attention to estuarine and coastal nitrogen fixation has been raised in recent decades.To date,nitrogen fixation in water columns has been extensively studied in estuarine and coastal environments.In contrast,benthic nitrogen fixation has not received more attention,and so its importance and associated influencing factors remain uncertain.In this study,the Yangtze estuary and adjacent coastal zone were thus selected as a typical study area to study the sediment N-fixing process,including spatial and temporal distributions of nitrogen fixation rates,and importance and ecological environmental effects of nitrogen fixation.The main findings of this study are as follows:（1）The measured rates of nitrogen fixation in the intertidal sediments of the Yangtze Estuary ranged from 0.39 to 7.74 nmol N g^-1 h^-1.Significant spatiotemporal heterogeneity of nitrogen fixation rates was observed in the study area,and sediment nitrogen fixation rates were higher in summer than in winter.The abundance of nifH gene varied from 3.64×10⁶ to 1.20×10⁸ copies g^-1.The benthic nitrogen fixation was correlated closely to the abundance of nifH gene and was affected significantly by salinity,pH,and availability of sediment organic carbon and ammonium.It is roughly estimated that approximately 1.02×10⁵ t nitrogen was fixed annually into the estuarine and coastal environment.It is estimated that sediment nitrogen fixation contributed approximately 9.3%of the total terrigenous inorganic nitrogen transported annually into the Yangtze estuarine environment.This result implies that the occurrence of benthic nitrogen fixation acts as an important internal source of reactive nitrogen and to some extent exacerbates nitrogen pollution in this aquatic ecosystem.（2）In ChongMing eastern tidal flat,the nitrogen fixation rate in rhizosphere sediment of Spartina alterniflora ranged from 1.43 to 10.12 nmol N g^-1 h^-1,while the nitrogen fixation rate of mudflats varied from 0.96 to 6.15 nmol N g^-1 h^-1.The nitrogen fixation rates in rhizosphere sediment of Spartina alterniflora was higher than that in bare flat.In agreement with our hypothesis,the nitrogen fixation rate in rhizosphere sediment of Spartina alterniflora changed seasonally with plant growth.Moreover,the nitrogen fixation rate of bare flat sediments increases with temperature.The difference in nitrogen fixation rates between the rhizosphere and bare sediments is due to the changes in environmental factors（e.g.,pH and organic carbon）caused by the"rhizosphere effect".The nitrogen fixation rate by the sulfate-reducing bacteria in rhizosphere sediment of Spartina alterniflora was 0.52⁴.00 nmol N g^-1 h^-1,with the contribution of 23.7⁵6.1%to the total nitrogen fixation.Likewise,the nitrogen fixation rate of sulfate-reducing bacteria in bare flat was 0.51².46 nmol N g^-1 h^-1,with the contribution of 20.2⁵3.6%to the total nitrogen fixation.This result indicates that sulfate-reducing bacteria play an important role in nitrogen fixation in the study area.（3）Potential rates of nitrogen fixation varied from 0.06 to 5.51 nmol N g^-1 h^-1 in the continental shelf sediments of adjacent sea areas of the Yangtze Estuary,while abundance of functional gene（nifH）ranged from 0.36×10⁶ to 5.39×10⁷ copies g^-1.Nitrogen fixation rates were not related to the abundance of nifH gene but to temperature,salinity,sulfide,iron and C/N,indicating that the sediment properties rather than microbial activity dominated the nitrogen fixation.It is also estimated that sediment nitrogen fixation annually contributed about 3.43×10⁵ to 3.10×10⁷ t nitrogen to adjacent sea areas of the Yangtze Estuary,which accounted for 8.2²2.6%of the total inorganic nitrogen input.Overall,this study highlights the importance of benthic nitrogen fixation in controlling nitrogen budget in adjacent sea areas of the Yangtze Estuary.