| Mangrove soils,which store large amounts of carbon(C)and nitrogen(N),have been considered as "hot spots" of greenhouse gases(GHGs)and play an important role in global climate change.Recently,the effects of intensified human activities,such as invasion of Spartina alterniflora invasion and eutrophication of wetland,on ecological processes and ecological functions of mangrove ecosystem have attracted more and more attention.However,most of the studies in mangrove soil GHGs emission were conducted in a small-scale which exhibited high spatial heterogeneity.Additionally,these studies mainly focused on the relationship between mangrove soil GHGs emissions and the soil abiotic factors.As one of the most open ecosystems,mangrove wetlands are fragile to the invasion of alien plant species and nutrients over-loading.The responses of the mangrove soil GHGs emission to the exotic invasive species and eutrophication were rarely reported Therefore,this study combined meta-analysis,field investigation and laboratory simulation experiments together,1)to investigate the mangrove soil GHGs emission and their regulating factors(including biotic and abiotic factors),2)to study the responses of mangrove soil GHGs emission to S.alterniflora invasion and nutrients loading as well as their underlying mechanisms.The main results obtained are as follows:1.Meta-analysis of mangrove soil GHGs emissions worldwide revealed that more than 93%of investigated mangrove soils are sources of GHGs.The average GHGs flux in the Northern Hemisphere is higher than that in the Southern Hemisphere.The annual average air temperature is one of the most important factors influencing mangrove soil GHGs emissions.In detail,with annual average air temperature rising by 5?,mangrove soil CO2 emission will increase by about 1.3 times.However,the changes in annual average air temperature has little effects on the release of mangrove soil CH4 and N2O.Meta-analysis also revealed that another important factor in regulating mangrove soil GHGs emission was soil biogeochemical properties.2.For CH4 emission,it showed a significant seasonal variation pattern under mangrove vegetation(Kandel obovata and Avicennia marina)while no significant seasonal variation in bare mudflat.Among the investigated ecological factors,soil temperature was an important influencing factor in the seasonal variation of mangrove soil CH4 emission.Generally,higher soil CH4 emission usually occured between salinity of 15-20 and pH value of 6-8.Besides,soil CH4 emissions were positively correlated with soil C/N ratio and organic matter(SOM),while negatively correlated with redox potential(Eh).In addition to abiotic factors,mangrove soil CH4 emissions were closely linked to the functional microbial abundance.We found that soil CH4 emissions were positively correlated with methanogens population(mcrA gene).S.alterniflora invasion significantly increased soil conductivity(EC),microbial biomass carbon(MBC),total organic carbon(TOC)and mcrA gene abundance,thus promoted mangrove soil potential methane production rate(PMP)and CH4 release.In laboratory simulation experiment,exogenous addition of C and NH4+ significantly stimulated mangrove soil CH4 emission.3.For CO2 emission,it exhibited significantly seasonal variations with a CO2 sink of mangrove soil in non-growing season which partly regulated by soil temperature in Zhangjiang River Estuary Mangrove National Natural Reserve.In terms of abiotic factors,mangrove soil CO2 emissions were positively correlated with SOM,total phosphorus(TP),C/N ratio,TOC,pH and total nitrogen(TN),while negatively correlated with total carbon(TC).In biotic aspects,mangrove soil CO2 emissions were positively correlated with the gene abundance of 16S rRNA and sulfate-reducing dependent methanotroph(ANME-pmoA).S.alterniflora invasion significantly reduced the gene abundance of 16S rRNA,methanotroph(pmoA)and ANME-pmoA,thus decreased the soil potential methane oxidation rate(PMO)and CO2 emission from mangrove soils.Besides,the input of exogenous C,NH4+ and SO42-promoted mangrove soil CO2 emission.4.For N2O emissions,the positive correlation with SOM,C/N ratio,TP and TC,while negative correlattion with salinity and Eh value,were reported in the present study.S.alterniflora invasion increased soil water content,salinity and TN,changed soil microbial community structure,decreased the a diversity of microbial community while increases the abundance and diversity of denitrifying bacteria.As a result,mangrove soil nitrogen fixation,dissimilated nitrate reducrtion to ammonium and denitrification processes were enhanced after S.alterniflora invasion.On the other hand,the anaerobic ammonia oxidation process was weakened after S.alterniflora invasion.These changes in soil N dynamic lead to the increase of mangrove N2O emission after S.alterniflora invasion.In addition,the addition of exogenous C and NO3-significantly facilitated the release of mangrove soil N2O.5.With the continued intensification of global climate change,S.alterniflora invasion and wetland eutrophication,mangrove soil seems to emit more GHGs,thereby increasing regional global warmng potential(GWP)and accelerating global warming. |
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