Nitrogen Cycle Processes And Influence Mechanism Research From The Typical Estuarine Area

Estuarine area which is strongly influenced by land-ocean interaction impact, is the most dynamic zone where kinds of biogeochemical processes occur. It also has a very important significance for human and social development. Nitrogen is the main element of organisms. Numerous anthropogenic nitrogen inputs have produced a bunch of serious environmental issues such as coastal eutrophication, hypoxia zone and nitrous oxide emissions. Therefore, this study set the Mississippi River and the Yangtze river estuarine area as research cases and focused on the nitrogen cycle biogeochemical processes occurred at the sediments-water interface (SWI), water column and sediment-atomosphere interface (SAI). We tried to identify and figure out the main N processes and influence factors, reveal the role of nitrogen-oxygen coupling dynamics in the hypoxia formation mechanism. The following achievements have been obtained through this research:A new approach, combining 15NH4+isotope dilution and continuous-flow techniques, provided estimates of "actual" and "net" NH4+ flux and sediment NH4+ demand (SAD) at the sediment-water interface in Mississippi river estuarine area. The SAD results indicate a rather consistent NH4+ demand at the SWI during the hypoxic season and suggest that reduced nitrogen may limit microbial dynamics in the region. The SAD concept may help us better understand the SWI NH4+-N recycling study.The gross NH4+-N regeneration fluxes (REG) and potential uptake fluxes (Upot) were both higher at hypoxic site than normoxic site and intermediate sites. All gross NH4+-N recycling fluxes were higher in hypoxic season (summer) than normoxic season (winter). The SAD values were significantly higher at hypoxic site than normoxic site when hypoxia. All SAD values were higher in summer than in winter (< 70μmol m-2 h-1). SAD values reflect the significant SWI nitrogen limitation. Nitrification could be the main NH4+-N removal process at normoxic site, but anaerobic processes such as denitrification should be the main NH4+-N removal process at hypoxic site. Direct denitrification (DDNF) but not DNRA should be the main NO3--N removal process. DDNF was the main N2 removal process. Coupled nitrification denitrification (CDNF) and potential anammox (PANA) were both insignificant at SWI. The percentage of DDNF in potential denitrificaition (PDNF) was a bit higher at hypoxic sites. There was no significantly seasonal difference for PDNF and DDNF. CDNF and PANA were generally very low and did not show any spatial or seasonal difference. Nitrogen fixation (NF) was low at hypoxic site and almost no new nitrogen was created by NF in hypoxic season.Actual uptake flux (Uact) and potential uptake flux (Upot) showed some certain interdependent relationship judged by the correlation analysis. There were significantly negative liner relationships between REG, Uact, Upot, SAD and bottom water dissolved oxygen (DO) concentrations which suggesting remineralization was the main process to replenish REG but not DNRA. Anaerobic assimilation was supposed to be the main pathway for uptake processes and SAD. There was no significant relationship between all fluxes and chlorophyll a. Bottom water NH4+-N and O-PO43- concentrations significantly correlated to all SWI NH4+-N fluxes. REG and Uact Gaussian correlated to bottom water NO3--N concentrations. Only exception of correlation analysis between SWI N2 fluxes and Bottom water hydrological characteristics was CDNF which significantly negatively correlated with bottom water DO. Further study need to be conducted since the impact factor research for SWI N cycle is still complex. The hurricane activities almost weakened all SWI N cycle fluxes.There was an obvious depth distribution pattern for water column NH4+-N recycling rate in Mississippi river estuary. Surface recycling rates were higher than bottom ones which were higher than middle ones. The site further to the Mississippi river mouth got lower recycling rates. Bottom water nitrogen recycling processes were deeply influenced by hypoxic DO condition. Potential NH4+-N uptake rates were higher than regeneration rates which indicate the water column nitrogen limitation. Upt and Reg showed significant seasonal differences. Larger seasonal variability was observed at surface than at middle and bottom. The NH4+-N regeneration and uptake turnover times also showed the same depth independent distribution pattern as recycling rates. The turnover time for surface uptake was very shot (< 0.5 d). The observation of longer regeneration turnover times than uptake also suggested the water column limitation phenomenon. The recycling turn overtimes at hypoxic site were lower in summer than in winter, and were higher than the ones at normoxic site. DON regeneration turnover times were longer than NH4+-N recycling rates in summer. This observation showed the typical water column nitrogen starvation.Upt and Reg were all deeply influenced by the freshwater from Mississippi river judged by the correlation analysis. Surface Upt and Reg significantly negatively correlated with salinity. Bottom water recycling rates signifcatnly negatively correlated with DO, suggesting low oxygen stress. Light-dark comparison study indicated that the main process for surface water NH4+-N uptake should be assimilation supported by photosynthesis. More heterotrophic microbes participated in uptake process at hypoxic site than normoxic site. Ammonia peptide enzyme consumption rate (AMP) showed the same pattern as Reg in middle and bottom water in summer indicating that ammoniation process correlated a lot to organic nitrogen hydrolysis ability. Water column NH4+-N recycling rates were far below SWI recycling fluxes which was caused by bottom seal effect. SWI nitrogen processes should play a more important role in hypoxia formation mechanism, but water column nitrification still contribute to oxygen depleting phenomenon at normoxic site or in normoxic season.There was a obvious spatial and temporal pattern in Yangtze river estuarine SAI N2O emission. All sites showed a positive average N2O flux value except at BLG. The sites influenced by fresh water got higher values than the ones influenced by sea water. Emissions at LHK and WSK in summer were far above than other sites in summer. Summer values were higher than ones in other season. The spatial distribution pattern of values depleted from fresh water control area to sea water control area only showed in summer. The Yangtze river estuarine intertidal area generally is the source of N2O emission. Denitrification or coupled denitrification were supposed to be the main generation process of N2O.Significantly correlations were observed between temperature, sediment sand percentage, sediment WFPS, summer sediment exchangeable NH4+-N concentrations, SWI NH4+-N fluxes and N2O emission respectively. It was not clear the relationship between ON and N2O emission. The main generation processes of N2O emission were supposed to be denitrification or DNRA. There was no obvious sign for nitrification contribution to N2O emission.We built a semi-empirical N2O emission model based on principle component analysis which was significantly correlated to the field measured values (R=0.63, P=0.0004). The annual N2O emission was 76.6 Mg in the sampling year based on the model we built. That number was insignificant compared to the total emission in that year from other researches. But this number might be far underestimated since we did not take the SWI N2O emission into consideration while rising tide water logging period.