Impacts Of Spartina Alterniflora Invasion On Soil Organic Carbon And Nitrogen Pools In Coastal Wetland Of Eastern China

Wetlands represent the largest component of the terrestrial biological carbon(C)and nitrogen(N)pools and play an important role in global C and N cycles.Plant invasion,one of the dominant components of global change,could replace the native species and alter the biodiversity in native ecosystems,and affects native ecosystem processes and functioning.Shifts in plant community structure greatly impact the input and output of soil organic matter(SOM),and ultimately influence soil organic C and N pools.The relationship between plant invasion and soil organic C and N pools has always become the focus for scholars,because plant invasion has not always positive effect on soil organic pool,also may brings negative effect or no significant effect on soil C and N pools.We investigated the effects of soil organic C and N pools as affected by plant invasion,which could better provide scientific basis and theoretical guidance for government to implement prevention,controlling and management of plant invasion.In this study,we selected an exotic C4 perennial grass,Spartina alterniflora,as the research object,and the Jiangsu Yancheng Wetland National Nature Reserve,Rare Birds as the study area to examine the effects of S.alterniflora invasion on various soil organic C and N fractions and soil microbial community structure by soil fractionation,C and N stable isotopes and phospholipids fatty acids extraction techniques in comparison with bare flat,native C3 plant Suaeda salsa and Phragmites australis communities.The objectives of this study were to analyze the effects and mechanism of S.alterniflora invasion on soil organic C and N pools in coastal wetland of eastern China.The main conclusions are as following;(1)Impacts of Spartina alterniflora invasion on soil organic carbon dynamics in coastal wetland of eastern China:S.alterniflora invasion significantly increased soil(0-30 cm)water-soluble organic carbon(WSOC),microbial biomass carbon(MBC),cumulative CO2-C mineralization(MINC),soil organic carbon(SOC),total nitrogen(TN)compared to bare flat,native C3 plants S.salsa and P.australis soils,but considerably decreased WSOC/SOC,MBC/SOC and MINC/SOC relative to bare flat in coastal wetland of eastern China.The lower WSOC/SOC,MBC/SOC and MINC/SOC ratios in S.alterniflora soil implied that less available C and less microbial biomass were supported in per unit SOC.Thus,the large litter input,together with low proportion of labile SOC pool fractions and mineralizable carbon in per unit SOC,could have accelerated SOC pool sequestration in S.alterniflora soil of the coastal wetland ecosystems.(2)Consequence of Spartina alterniflora invasion on labile and recalcitrant soil organic carbon and nitrogen pools in coastal wetland of eastern China:S.alterniflora invasion significantly increased soil(0-30 cm)C and N concentrations and stocks in SOM,labile and recalcitrant SOM pools compared to bare flat,S.salsa and P.australis soils.The C stock of the recalcitrant SOM pool was substantially higher than that of the labile SOM pool in S.alterniflora soil.Meanwhile,the recalcitrant index for C in S.alterniflora soil was significantly higher than that in bare flat,which maybe primarily associated with large amounts of litter and root input and the decrease of SOM decomposition.The proportion of S.alterniflora-derived C in the labile SOM pool was significantly higher than that in recalcitrant SOM pool.However,the S.alterniflora-derived C stock of the recalcitrant SOM pool was much greater than that of the labile SOM pool,probably due to the higher S.alterniflora-devived C input rate accompanied by the faster decay rate for old C of the labile SOM pool in S.alterniflora soil.Meanwhile,SOM and recalcitrant SOM pools in S.alterniflora soil had higher ?15N values in comparison with bare flat,indicating an increase of N cycling and accompanied by more N losses following S.alterniflora invasion.Our results suggest that soil organic C and N pools in coastal wetland of eastern China play stronger C-and N-sink roles as affected by S.alterniflora invasion,and which might compensate the excessive greenhouse gases(e.g.,CO2,N2O)emission to the atmosphere.(3)Variations in soil organic matter dynamics as affected by Spartina alterniflora invasion in coastal wetland of eastern China:S.alterniflora invasion significantly enhanced whole soil(0-30 cm)organic C and N contents in comparison to bare flat,S.salsa and P.australis communities by primarily accumulating organic C and N contents in mineral-associated organic matter(mSOM)and intra-aggregate particulate organic matter(iPOM)of S.alterniflora soil.S.alterniflora invasion significantly increased the allocation proportion of C in iPOM compared with S.salsa and P.australis soils,but significantly decreased the allocation proportion of C in mSOM relative to S.salsa soil,revealing that a decline in physical SOC stabilization in S.alterniflora soil.The proportion of C4 S.alterniflora-derived C in free light fraction(free LF)and iPOM was significantly higher than that in mSOM of S.alterniflora soil.The highest C4 S.alterniflora-derived C content was found in iPOM of S.alterniflora soil,which is likely due to the S.alterniflora residuals being resistant to decomposition.The most enriched ?15N values were found in S.alterniflora soil,which probably due to the high S.alterlonra residuals input and the fast N loss rate in S.alterniflora soil.Increased ?15N values and decreased C:N ratios from free LF to iPOM to mSOM in S.alterniflora soil imply a rising degree of humification.Our results suggest that S.alterniflora invasion could change soil organic C and N storage and their physical distribution in SOM fractions,and ultimately affect SOM stabilization and turnover in coastal wetland of eastern China.(4)Alterations of soil microbial community composition and biomass following Spartina alterniflora invasion in coastal wetland of eastern China:S.alterniflora invasion significantly changed soil(0-30 cm)microbial community composition and biomass compared with bare flat and S.salsa and P.australis communities.The concentrations of bacterial,fungal,Gram-positive(G+)bacterial,Gram-negative(G-)bacterial,arbuscular mycorrhizal fungal(AMF),actinomycete,monounsaturated,branched and saturated straight-chain phospholipids fatty acids(PLFAs)and MBC in S.alterniflora soil considerably increased relative to bare flat and S.salsa and P.australis communities,but the ratio of monounsaturated/branched PLFAs significantly decreased compared with bare flat,suggested that S.alterniflora soil had the higher percentage of anaerobic microorganism,which may associated with a higher anaerobic condition in S.alterniflora soil.Soil moisture,pH,electrical conductivity(EC),WSOC,MBC,SOC,soil labile organic carbon(LC),recalcitrant organic carbon(RC),soil organic nitrogen(SON),labile organic nitrogen(LN)and recalcitrant organic nitrogen(RN)among the all communities explained 77.3%of the total variability of the PLFAs,among which WSOC and SON were the crucial factors related to soil microbial community composition by redundancy analysis(RDA).Moreover,the variation in soil microbial community composition was correlated with MBC among the communities.We suggests that S.alterniflora invasion could accelerate soil organic C and N accumulation not only through raising SOC,LC,RC,SON,LN and RN,but also by increasing soil microbial community composition and biomass in coastal wetland of eastern China.According to these researches,it is concluded that S.alterniflora invasion significantly increased soil organic C and N stocks,soil labile organic C fractions,labile and recalcitrant organic C and N stocks,and the C and N contents of free LF,iPOM and mSOM compared with bare flat,S.salsa and P.australis communities through increasing plant residuals(e.g.,litter and roots)input into the soil,decreasing decomposition of litter and roots,and changing soil microbial community structure in coastal wetland of eastern China.The proportion of C4 S.alterniflora-devived C was 36.62-36.62%in the total SOC,56.35%in the soil labile organic C,32.28%in the soil recalcitrant organic C,64.44%in the free LF,60.48%in the iPOM,27.89%in the mSOM of S.alterniflora soil.Although SOM in S.alterniflora soil was not all derived from the contribution of S.alterniflora,may be associated with the input of other terrestrial and/or marine organic matters(e.g.,phytoplankton),nevertheless,our results demonstrate that S.alterniflora invasion significantly altered soil organic C and N pools and make it play stronger C-and N-sink roles in coastal wetland of eastern China,and also provided evidence that plant invasion could influence ecosystem processes and functioning.