Characteristics Of Greenhouse Gas Emission And Related Microbiological Mechanism Of Zhalong Wetland

Changes in precipitation patterns and thawing of frozen soils,which caused by global warming,as well as the development and utilization of wetlands have severely disturbed the ecological balance and greenhouse gas emission,and would have a feedback effect on the global climate.Inland alkaline wetlands are distributed in arid,semi-arid,and semi-arid/sub-humid transitional areas,with surface over-humidity and shallow water,the development of saline-alkali soil and halophyte communities,and constitute a geographic synthesis of unique wetland ecosystems.Inland alkaline wetlands are important parts of the wetland ecosystem.At present,the greenhouse gas emission of inland saline-alkali wetlands is not well known.Especially the research on the microbiological mechanism and regulation factors of greenhouse gas production and consumption has not yet begun.Therefore,it is necessary to investigate the greenhouse gas emission and its microbiological mechanisms for understanding the greenhouse gas emissions mechanism and providing a theoretical basis of development and protected the wetland.Zhalong Wetland is the most representative and typical inland alkaline wetland ecosystem in northern China,and it occupies an important position in global change research.This research takes Zhalong wetland as a representative,and investigates the CO₂,CH₄ and N₂O emission with static chamber-GC techniques.Then the research explores the community structure and function of the microbial floras.Based on these findings,the research investigates harvesting reed,freezing/thawing,and drying/rewetting on greenhouse gas emission and their driving factors.Get the following conclusions:Harvesting reed significantly decreased the soil carbon input.Harvesting reeds reduced the carbon and nitrogen input and significantly decreased the NO₃^--N,total nitrogen(TN),and total organic carbon(TOC).However,the soil temperature significantly increased along harvesting reeds,as wetland with harvesting reeds(HRW)received more sun radiation.The annual cumulative CO₂ emission of Zhalong wetland without harvesting reeds(NHRW)and HRW was 15198.35±88.27 kg·ha^-1and 14563.19±171.80 kg·ha^-1,respectively.CH₄ emission was significantly reduced by harvesting reeds.The annual cumulative CH₄ emission of NHRW(561.88±18.61kg·ha^-1)was 3.07 times that of HRW(183.13±18.77 kg·ha^-1).After harvesting reeds,Zhalong wetland changes from a source of N₂O to a sink.The annual cumulative N₂O emission of NHRW was 0.73±0.20 kg·ha^-1,but that of HRW reduced to-0.57±0.49kg·ha^-1.The structure equation model showed that temperature significantly influenced CO₂ and N₂O emission,and TOC was the driven factor of CH₄ emission.CO₂ source(respiration,fermentation process)and CO₂ sink in NHRW was more actived compared with HRW.Only the relative abundance of functional gene involved in CO oxidation increased after harvesting reeds.Harvesting reeds also significantly reduced the relativw abundance of methylotrophic and hydrogenotrophic methanogens,but had less impact on the aceticlastic methanogen.The relative abundance of functional genes involved in N₂O source(amo A gene and nor B gene)and N₂O sink(nos Z gene)decreased with harvesting reeds.Hence,harvesting reeds inhibited carbon and nitrogen cycle,significantly reduce the CH₄ emission,and converted the wetland from N₂O source into N₂O sink.However,increased temperature offset the effect of carbon input reduction on CO₂ emissions after reed harvesting.During the freezing and thawing cycle,soil properties,greenhouse gas emission and carbon/nitrogen cycle significantly changed along with the variational temperature.In the thawing period,NO₃^--N,TN,TOC and TS concentration was significantly increased comparing with the growth period.The average emission of CO₂ and CH₄ were significantly lower in freezing and thawing periods than that in the growth period(P<0.05).During the freezing and thawing periods,the wetland transformed into N₂O sinks and the average emission flux of N₂O was reduced to-20.7±17.93?g·m^-2·h^-1 and-27.14±19.57?g·m^-2·h^-1,respectively.Temperature was the driving factor of greenhouse gas during the freezing and thawing cycle.Freezing and thawing significantly reduced the relative abundance of functional genes involved in respiration,fermentation and CO₂ fixation within the carbon cycle.Although freezing and thawing has a little effect on mcr A gene,it significantly impacted the methanogenesis pathways.In the freezing period,the relative abundance of hydrogenotrophic and aceticlastic methanogens was highest.Whereas in the growth period,the relative abundance of methanotrophic methanogens was highest.The anaerobic methane oxidation mediated by Candidatus Mlm.oxyfera was mainly conducted in the growth period.The relative abundance of genes involved in N₂O source(amo A and nor B),and in N2O sink(nos Z)was highest in the growth period.Hence,in freezing and thawing period,the carbon/nitrogen cycle was inhibited,and the CO₂,CH₄ and N₂O emission was significantly reduced.During ecological hydration period,the variational water level not only changed the soil moisture,but also affected the redox potential,which will futher influenced the microbial growth and metabolism.During the wet,dry and flooded periods,the average CO₂ emission of the Zhalong wetland was 672.57±75.49,439.90±135.02 and194.06±99.83 mg·m^-2·h^-1,respectively(P<0.05);the average CH₄ emission was13.60±2.43,5.12±1.31 and 12.25±4.54 mg·m^-2·h^-1,respectively(P<0.05);the average N₂O emission was 71.13±12.65,65.67±10.62 and 62.63±16.76?g·m^-2·h^-1,respectively(P=0.24).The relative abundance of functional genes involved in respiration and fermentation process was highest in wet period and flooded period.The relative abundance of functional genes involved in CO₂ fix was also highest in flooded period.Although drought and rewetting had little effect on mcr A gene,it significantly inhibited the hydrogenotrophic methanogenic pathway,but had little effect on aceticlastic methanogenic pathway.The aceticlastic methanogenic pathway is dominated by methanogens belonging to Methanosarcina,which had the ability to resist oxidative stress.The relative abundance of methanotrophs was highest in the wet period.Candidatus Mlm.oxyfera utilized repair system and dormant system to resist oxidative stress.Among the N₂O source functional genes,the amo A gene is the highest in the wet period,and the nor B gene is the highest in the dry period.The abundance of the functional gene(nos Z)involved in N₂O sink is the highest during the flood period.Hence,drought and rewetting significantly influenced CO₂ and CH₄ emission,but had little effect on N₂O emission.The anaerobic microbe involved in greenhouse gas emission utilized anti-oxidation system,repair system and dormant system to resist oxidative stress.