Nitrous Oxide Emission And Its Influencing Mechanisms In Intertidal Soils Of The Yangtze Estuary

Nitrous oxide?N₂O?is an important greenhouse gas,which has drawn much attention in the study of global change.As a critical zone of land-sea interaction,estuarine and intertidal wetland ecosystems are complex and diverse.Estuarine wetlands are also one of the significant natural sources of atmospheric N₂O,but its production and consumption processes are still not well understood.The effects of salinity gradient,tidal wave,salt-marsh plant and benthonic animal activities are the most prominent aspect of estuarine tidal flat wetland,which are also considered to be the most important factors regulating biogeochemical cycles.Over the past several decades,estuarine and coastal wetland ecological environments were threatened seriously due to the intensive human activities.Study on N₂O emission dynamics under the influences of salinity gradient,tidal flooding frequency,salt-marsh plant and benthonic animal activities plays a vital role in developing our knowledge of the N₂O cycling,as well as better understanding the response of environmental change such as sea-level rise,salinity intrusion,plant invasion and tidal flat degradation.Hence,Yangtze Estuary was selected as a typical study area to research the intertidal soil N₂O production and consumption processes under the effects of salinity gradient,salt plant,tidal flooding frequency and crab activities.The major drivers of N₂O emission were revealed to provide the scientific reference for nitrogen cycling and N₂O emission mitigation in estuarine wetland ecosystems.The main results are as follows:?1?Along the Yangtze Estuary salinity gradient,soil N₂O emission of tidal flat wetland varied in 0.70-2.15?mol m^-2 h^-1,which generally increased from saltwater to freshwater,and higher in summer than in winter.The?¹⁵N^bulk,?¹⁸O and SP(?¹⁵N^?-?¹⁵N^?)of emitted N₂O were-4.49-6.65‰?42.39-53.17‰?7.26-8.78‰,respectively.From saltwater to freshwater,?¹⁵N-N₂O generally decreased and?¹⁸O-N₂O increased,while SP-N₂O has no obvious regular patterns.The changes of gross N₂O production and consumption rates showed a similar spatial pattern to N₂O emission,while N₂O reduction?consumption?proportion generally decreased from saltwater to freshwater,which simultaneously controlled the variations in N₂O emission.The contributions of bacterial denitrification?nitrifier denitrification and denitrifier denitrification?to N₂O production in Reduction-Mixing scenario and Mixing-Reduction scenario ranged from 96.62–99.63%and 78.22–97.36%,respectively,while the contribution of hydroxylamine?NH₂OH?oxidation varied in 0.77–3.38%and 2.64–21.78%,respectively.The contribution of bacterial denitrification to N₂O production generally showed a decreasing trend from saltwater to freshwater,while NH₂OH oxidation contribution increased slightly.Along the salinity gradient,the variations in carbon and nitrogen substrate availability,pH,salinity and sulfide can affect nitrification,denitrification,N₂O production and consumption processes,which further regulated the N₂O emission dynamics.?2?Soil N₂O emission rates in summer were lower in Spartina alterniflora and Phragmites australis stands than in Scirpus mariqueter and bare mudflat stands,but no significant differences among these saltmarsh habitats occurred during winter.The?¹⁵N-N₂O was higher in P.australis,S.alterniflora and S.mariqueter stands than in bare mudflat stands,and the SP-N₂O was higher in Spartina alterniflora and Phragmites australis stands than in Scirpus mariqueter and bare mudflat stands,but there was no significant difference in?¹⁸O-N₂O among these saltmarsh stands.The gross N₂O production and consumption rates were higher in S.alterniflora and P.australis stands compared to S.mariqueter and bare mudflat stands.The gross consumption proportion in S.alterniflora and P.australis stands was also higher,which affected N₂O emission.Isotopic analysis?Mixing-Reduction scenario?combined with acetylene inhibition showed that NH₂OH oxidation?4.52–12.62%?and nitrifier denitrification?13.87–21.58%?were substantial importance in N₂O source,but denitrification was the dominant pathway?69.83–80.09%?.Exotic S.alterniflora invasion increased the contributions of NH₂OH oxidation and nitrifier denitrification to N₂O source slightly,but decreased the contribution of denitrification to N₂O source.Plant altered microenvironments by affecting soil water-filled pore space,pH,sulfide,carbon and nitrogen substrates,thus mediating N₂O production and consumption as well as its emission from estuarine saltmarsh.?3?Along the elevation of tidal flat,soil N₂O emission rates generally decreased with the increase of tide inundation frequency.As the inundation frequency increases,the?¹⁵N-N₂O generally increased,but the?¹⁸O-N₂O and SP-N₂O firstly decreased and then increased.Gross N₂O production and consumption rates also firstly decreased and then increased,with the increase of inundation frequency,while N₂O reduction?consumption?proportion generally showed an increasing trend.Increased N₂O consumption proportions were the key drivers for the decreases of N₂O emission with the increase of inundation frequency.With the increase of the flooding frequency,the contribution of bacterial denitrification to N₂O generally increased,while the contribution of NH₂OH oxidation decreased gradually.The variations in soil oxidation-deoxidation environment and carbon and nitrogen substrates caused by tide inundation frequency,were important factors influencing the spatial patterns of N₂O emission.?4?Laboratory short-term incubation experiments showed that crab activities generally increased soil N₂O emissions.After 30-day incubation,gross N₂O production and consumption rates in the crab groups were generally higher than in the control group,but there were no significant changes in the N₂O reduction?consumption?proportion.Crab activities generally increased the contribution of NH₂OH oxidation to N₂O production but decreased the contribution of bacterial denitrification.The changes in soil compactness and permeability,as well as substrates availability caused by crab activities,were the main factors controlling N₂O emission dynamics.?5?Overall,soil N₂O emission processes dynamics in Yangtze Estuary wetland have obvious variation characteristics under the effects of salinity gradient,salt-marsh plant,tide inundation frequency and crab activities.Natural-abundance isotopes provide valuable information about the N₂O production and consumption processes in estuarine and coastal wetlands,which helps us understand the drivers of N₂O emission.These results highlight the substantial role of nitrification and N₂O reduction?consumption?in N₂O release in redox-dynamic soils of the estuarine environment.