Distribution Characteristics And Metabolic Mechanisms Of Nitrous Oxide In Arctic Ocean,Chinese Estuary And Mangrove Waters

Nitrous oxide(N2O)is an important trace greenhouse gas which global warming potential is approximately 300 times that of CO2 on a per-molecule basis.The photochemical products of N2O in the atmosphere will react with ozone,resulting in the depletion of ozone in the stratosphere and the destruction of ozone layer.The environmental effects of N2O have also attracted people’s attention,so many international climate change research projects have treated it as an important research content.Ocean is an important source of atmospheric N2O in addition to continent.However,the distribution characteristics,source and sink patterns,production and consumption mechanisms,fluxes and their driving forces of N2O in the ocean are still unintelligible and with great uncertainties.In this paper,hot research areas such as polar ocean,sea ice,estuaries and mangroves are selected as subjects of research.N2O metabolism in bering sea and Chukchi Sea was analyzed for the first time in combination with the abundance and community structure of nitrogen cycling microbial functional genes.Combined with the distribution characteristics of N2O in sea ice and subglacial water,the effect of rapid ice melting on N2O distribution in the arctic ocean surface was first clarified.The distribution and driving mechanism of N2O in mangrove and estuarine waters were studied in combination with tidal cycle,which is of great significance for accurate assessment of N2O flux and coping with global climate change.The main conclusions are as follows:1.Combining with the data of ammonia oxidation function gene,the biogeochemical process of N2O metabolism in different waters of bering Sea was systematically revealed for the first time.The results show that the surface level mean saturation of bering Sea is 101.7±2.7%,which is essentially in equilibrium with the atmosphere and is a weak source of atmospheric N2O,mainly controlled by temperature.The production of N2O hardly exists without the existence of AOA and AOB,which were lost in competition with plankton for survival in aerobic seawater.The concentration of N2O in the depth range of 50-900m gradually increases with depth,and the ammonia-oxidation activities of AOA and AOB are quite intense,resulting in the active in situ formation of N2O.The existence of AOB was not detected beneath 400m.However,the substrates and products of ammonia oxidation cannot be detected beneath a depth of 1000 m in mesopelagic water.Supersaturated N2O beneath this depth is mainly generated by AOA adapted to low oxygen and low ammonia environments without obvious signals remaining from ammonia oxidation,and combined with previous studies,we speculate that the main contribution was ocean circulation within this depth range near the bottom water.However,the maximum ΔN2O on the Bering Sea shelf usually occurs in the near-bottom water rather than in the bottom water.The surface water of the Bering Sea shelf was a weak source of N2O in summer.Moreover,the oversaturated N2O below the thermocline is mainly produced by AOB via ammonia oxidation and nitrifier denitrification processes,which differs from the previous conclusion that N2O mainly originates from denitrification in sediment on the Bering Sea shelf.2.The driving mechanism of N2O distribution in chukchi continental shelf was analyzed for the first time by referring to the functional gene community and abundance of nitrogen cycling microorganisms.In the south of 70°N,the N2O saturation ranges from 97.3%to 106.7%with average saturation of 100.6 ± 4.4%,which is mainly affected by the inflow water from Pacific Ocean.In the north of 70°N,the N2O saturation ranges from 88.9%to 101.3%with average saturation of 96.2 ±3.3%,and the process of sea ice melting is the dominant control mechanism.The strong stratification of the subsurface water prevents the upward transport of supersaturated N2O,resulting in the accumulation of N2O in the bottom water with the distribution of N2O characterized by low surface layer and high bottom layer finally.the abundance of functional genes within nitrogen cycling microorganisms was nirK>AOBamoA>nirS>AO AamoA>nosZ in Chukchi shelf water.Vertical distribution characteristics of functional genes abundance was low in surface water and high in bottom water,with the nutrient concentration,salinity,and depth as important environment factors which affect those abundance.ΔN2O and abundance of all kinds of function of the nitrogen cycle genes were significantly correlated,but the effect of environmental factors on microbial community composition of nitrogen cycle is not obvious.The driving mechanism of N2O distribution in the Chukchi Shelf is extremely complex,with water mass mixing,air-sea flux exchange,sediment-seawater interface flux and N2O metabolism in water as important factors affecting distribution.The complex N2O metabolism processes include ammonia oxidation process(dominated by AOA and AOB),denitrification process dominated by denitrifying bacteria or nitrifying bacteria,and N2O consumption process in water,that is,nitrous oxide reduction process dominated by denitrifying bacteria.All these provide a theoretical basis for us to further quantify the rates of different N20 metabolic processes and more accurate flux estimation in the Chukchi Sea.3.The distribution characteristics and driving mechanism of N2O in sea ice and its underlying seawater(USW)influenced by rapid melting ice in Arctric Ocean are studied for the first time.Our results demonstrated that:There is a significant difference in the N2O concentration between first year ice(FYI)and multi-year ice(MYI).The average N2O concentration of FYI is 4.5 ±1.0 nmol kg-1,ranging from 3.7nmol kg-1 to 6.3 nmol kg-1,while MYI is 4.8±1.9 nmol kg-1,ranging from 2.1 nmol kg-1 to 7.7 nmol kg-1.The permeability of sea ice is the key factor leading to the difference of N2O concentration between FYI and MYI.The profile distribution characteristics of N2O in FYI and MY1 differ greatly.The N2O concentration in FYI was high in the surface and bottom of cores and low in the interior of sea ice,while it was low in the bottom layer and high in the subsurface layer in MYI.The exchange among atmosphere-sea iceseawater system,brine desalination and possible N2O generation process at the bottom of sea ice are the main driving mechanisms affecting the N2O distribution of FYI.However,the reasonable ventilation,the gas content of sea ice and the possible microbial denitrification process at the bottom of sea ice are the main driving mechanisms.the USW present as a sink of N2O owing to sea ice melting rapidly in Arctic Ocean,with a saturation ranging from 47.2%to 102.2%.However,due to the influence of air-sea exchange,diffusion process,possible N2O generation mechanism in USW and precipitation,the observed N2O concentration in glacial water is higher than that of theoretical mixing value(T-N2O).Since the lack of data at present,more detailed driving mechanism needs to be further studied and discussed.The Arctic climate is changing rapidly,resulting in proportion of FYI and MYI,sea ice structure and sea ice coverage also changing rapidly,which have important influence on the N2O source and sink in Arctic Ocean.Therefore,it is necessary to further strengthen studies about the effects of sea ice change on N2O distribution and metabolism in Arctic Ocean in the future,so as to better evaluate the global N2O flux in response to global climate change.4.The water samples of high and low tide were collected separately in the Jiulong River estuary(JRE)to avoid the interference of different tidal cycle stages for the first time,and the temporal distribution characteristics of N2O in the JRE were studied by anchored-site 24-hour observation.The results show that the concentration of N2O varies from 5.9～106.3 nmol L-1,and the average flux is 46.7 ±23.4 μmolm2d-1 in JRE,while the average N2O flux around Xiamen Island is 9.55±3μmol m2 d-1.The N2O concentration was significantly correlated with salinity in the section and anchored-site observation survey.The mixing process of seawater and fresh water caused by tidal cycle was the key factor affecting the temporal and spatial distribution of N2O in the JRE.The distribution of N2O along depth in JRE is different from that of other estuaries,with high at the surface and low at the bottom,which is mainly affected by the diluent water front.The N2O consumption process in the water,the sediment-water interface N2O flux,and the input of groundwater may all be the potential factors leading to the unconservative mixing of N2O in the bottom water.5.The distribution characteristics of dissolved N2O and methane(CH4)during tidal cycle were investigated in the mangroves of Golden Bay,Beihai city,Guangxi Province,China.The main influencing mechanisms of dissolved N2O and CH4 during tidal cycle were revealed in combination with the available tidal and meteorological parameters.The concentration and flux of dissolved N2O and CH4 in the GBM have distinct seasonal variations under the influence of the tidal cycle.The concentrations and fluxes of N2O and CH4 in the dry season were higher than those in the rainy season.The source and sink characteristics of dissolved N2O in the rainy season were not obvious,while the mangrove was a weak source of N2O in the dry season.Mangrove was a strong source of CH4 in both dry season and rainy season,and the flux in dry season was significantly higher than that in rainy season.Water temperature and DO are closely related to the concentrations of dissolved N2O and CH4.Due to the influence of the absorption and release of GHGs by mangrove sediments,the concentration of N2O gradually decreases from sea to shore,while the concentration of methane gradually increases from sea to while tidal water in the dry season was the source of N2O.Moreover,The fluxes of N2O and CH4 were 119±370 μmol m-2 yr-1 and 87.2±48.6 mmol m-2yr-1,respectively,flooded by tidal seawater.At present,the flux of N2O in the GBM is very small,which may be mainly affected by the nitrogen limitation of the mangroves.Once a large amount of N is input into the mangroves due to intense anthropogenic disturbance,the N2O flux may increase rapidly.Large amounts of CH4 are pumped into the atmosphere through the water-atmosphere interface and offset 22.5%of the total carbon sequestration,and a considerable part of CH4 is carried offshore by tidal transport.Thus,these problems need to be given close attention in research on mangroves as natural ’blue carbon’ sinks.