Spatio-temporal Variation In Dissolved N₂O Concentration And Diffusive Fluxes From Wenwusha

Nitrous oxide(N₂O)is not only a powerful greenhouse gas(GHG)but also a dominant stratospheric ozone-depleting substance.Increasing N₂O emission from continental aquatic ecosystems as a result of enhanced human activities on landscapes has become a global issue.As a result of the accumulation of anthropogenic wastes and the alteration of hydroecological processes,coastal reservoirs may also become the emission hotspots of N₂O.However,the spatiotemporal variations of N₂O concentration and fluxes from coastal reservoirs with intensively human activities are poorly known.In this study,we determined the surface water N₂O concentrations at a high spatial resolution across three seasons in a subtropical coastal reservoir in southeastern China,and calculated the diffusive N₂O fluxes for reservoir using the thin boundary layer method.By using lab incubation experiment,isotope fractionation and molecular biology techniques,we also determined the surface water nitrification and denitrifications rates,and analyzed the main pathways of N₂O production in the reservoir.The main results and conclusions showed as follows:(1)The hydrographical parameters(e.g.,conductivity,salinity,and nutrient)showed strong spatio-temporal across the whole reservoir.During the study period,the mean conductivity(EC),salinity and nutrient(NO₃^--N,total dissolved nitrogen,and total dissolved phosphorus)in the surface water were highest in November 2018,followed by March 2019 and June 2019.However,we observed generally higher NH₄⁺-N and P-PO₄^3-in June 2019.The EC and salinity varied spatially depending on the two reservoir zones,with higher levels of EC and salinity in the south basin(SB)than in the north basin(NB).Nutrient concentration varied spatially depending on the anthropogenic wastes discharge,with significantly lower values in sampling sites without wastewater discharge and generally higher values in sampling sites with wastewater discharge(P<0.05).(2)The dissolved N₂O concentration and diffusive fluxes varied over the ranges of6.11 to 261.27 nmol L^-1and-10.83 to 711.50?g m^-2h^-1in Wenwusha Reservoir,respectively,with mean values of 49.78 nmol L^-1and 107.02?g m^-2h^-1,respectively.Our results show that Wenwusha reservoirs are strong atmospheric N₂O sources with strong spatio-temporal variability.The mean N₂O concentration and diffusive fluxes were found to be significantly higher in June 2019 than in November 2018 and March 2019.There were large differences in N₂O concentration and diffusive fluxes between NB and SB across the three campaigns.The mean N₂O concentration and diffusive fluxes were significantly higher in NB than in SB.This could be attributed to the significantly higher nutrient concentrations and lower sanlinity found in NB.Futhermore,N₂O concentration and diffusive fluxes also varied spatially depending on the discharge types,with significantly lower values in sampling sites without wastewater discharge and generally higher values in sampling sites with wastewater discharge(P<0.05).Our results highlighted that strengthening environmental regulation,particularly for wastewater treatment and discharge,will be key to mitigate water pollution and N₂O emission in reservoir management.(3)Across all sampling sites,the denitrification and nitrification rate at the surface waters in Wenwusha reservoir ranged from 0.00 to 19.1?mol m^-2h^-1,and from 0.00 to3.32?mol m^-2h^-1,respectively.Results of Pearson correlation analysis showed that the the denitrification and nitrification rate were correlated positively with NO₃^--N concentration(P<0.05 or P<0.01)and correlated negatively with EC and salinity across all sampling sites(P<0.05 or P<0.01).The results of redundancy analysis(RDA)showed that NH₄⁺-N explained the greatest variance in N₂O concentration and diffusive fluxes,which implied that nitrification was the main microbial pathway for N₂O production in spite of the potentially increasing importance of denitrification of NO₃^--N in June 2019.