Production,Distribution And Flux Of Nitrous Oxide In The Jiulong River Estuary

The continuous increase of the greenhouse gas nitrous oxide(N₂O)in the atmosphere is a global concern.Estuaries have been proven to be an important region for N₂O emissions.However,there is great uncertainty in the estimation of N₂O emission flux in the estuary,due to the heterogeneity of the estuarine environment and the lack of understanding of the production pathway and regulation mechanism of N₂O in the estuary.Thus,study of the production,distribution,and release of N₂O in estuaries is of great significance for understanding the biogeochemical process of nitrogen cycle,and for the accurate estimation of global N₂O emission flux.On the basis of in situ investigations and laboratory simulation incubation experiments,we studied the spatial distribution and seasonal variation of N₂O concentration,N₂O production rate and nitrification rate in surface water of Jiulong River Estuary.We analyzed the processes that affecting the concentration of N₂O in the surface waters of the Jiulong River Estuary,and discussed the main production pathways and regulation mechanisms of N₂O in the water.We also calculated the N₂O water-air fluxes,and quantitative assessment of the contribution of river input and water nitrification to N₂O in Jiulong River Estuary.The results are as follows:(1)In summer,the N₂O concentration ranged from 15.3 nmol L^-1to 44.9 nmol L^-1in surface water of Jiulong River Estuary,and the corresponding N₂O saturation ranged from259.3%to 699.1%,which showed the same distribution characteristics of high upstream and low downstream.While in winter,the N₂O concentration ranged from 17.9 nmol L^-1to 50.2 nmol L^-1,and the corresponding N₂O saturation ranged from 214.6%to 546.3%,which peak value was occurred in the middle estuary.During the studied period,the N₂O concentration in surface water of Jiulong River Estuary was oversaturated,indicating that Jiulong River Estuary was an important source of atmospheric N₂O.(2)The ammonia oxidation rate in surface water of Jiulong River Estuary ranged from 0.13?mol L^-1d^-1to 11.9?mol L^-1d^-1,which is higher in summer than in winter.And the nitrite oxidation rate ranged from 0.04?mol L^-1d^-1to 0.22?mol L^-1d^-1,which was lower than the ammonia oxidation rate..(3)The N₂O production rate ranged from 1.05 nmol L^-1d^-1to 19.53 nmol L^-1d^-1,and was no obvious seasonal variation.?N₂O was significant positively correlation with AOU and NO₃^-,indicating that nitrification was the main process of in-situ N₂O production.The N₂O yield of per unit NH₄⁺-N oxidation ranged from 0.03%to 8.9%,which increased with the increasing of salinity in the middle and low salinity areas,and decreased rapidly in the high salinity areas.(4)The box model calculation showed that the distribution of N₂O concentration in surface water of Jiulong River Estuary was mainly controlled by water nitrification and river input.The contribution of N₂O produced by water nitrification to the total estuarine N₂O was 62.5%in summer and the contribution of river input was 37.5%.In winter,the contribution of water nitrification was 90.4%,while the contribution of river input was only 9.6%.(5)The average of N₂O water-air flux in winter was 22.1±10.4?mol m^-2d^-1,which was 1.5times higher than that in summer(14.3±7.3?mol m^-2d^-1).The results of N₂O flux budget model showed that 33.6%of N₂O was released into the atmosphere in Jiulong River Estuary in summer,while the remaining 66.4%was discharged into the ocean.In contrast,65.5%of N₂O was released into the atmosphere,and 34.5%was discharged into the ocean in winter.