The Effects Of Invasive Alien Plants On Ecosystem Carbon And Nitrogen Cycles: A Case Study Of Spartina Alterniflora Invasion In The Yangtze Estuary And A Meta-analysis

Invasions by alien species are one of the global environmental problems,and exert profound effects on the ecosystems they invade.More and more ecologists pay their attention to the ecosystem-level consequences of invasive alien species to the native ecosystems.Although a growing number of studies dealing with the effects of invasive alien plants on ecosystem carbon(C)and nitrogen(N)cycles have been conducted over the last two decades,the mechanisms that the invasive plants affect C and N cycling in ecosystems are still poorly understood.In particular,few generalizations have been made of how plant invasions alter ecosystem C and N cycles. One objective of this study was to explore the effects of Spartina aherniflora(L.) (Spartina)invasion in the Jiuduansha wetlands dominated by Scirpus mariqueter (Scirpus)and Phragmites australis(Phragmites)on ecosystem C pool,and litter C and N cycles in the Yangtze Estuary,China.Another objective was to quantify the extent to which ecosystem C and N cycles respond to plant invasion using meta-analysis approach.In order to examine whether or not plant invasions alter C and N stocks of ecosystems,a number of plant dominant traits responsible for ecosystem production processes were measured in the field.These traits included growing-season length,leaf area index,net photosynthetic rate,root biomass.Litter decomposition rates regulating the ecosystem C release and the ecosystem C and N stocks were also investigated.The results showed that Spartina had a longer growing-season,a higher leaf area index,a higher net photosynthetic rate,and greater root biomass than Scirpus and Phragmites. The amount of C input by net primary production was greater than that of C release during litter decomposition for all of the three species.Ecosystem C accumulation through NPP minus litter decomposition was the greatest for Spartina,i.e.,1.39kg m^-2 year^-1,which was respectively 660%and 150%of that for Scirpus and Phragmites. Ecosystem C stock was 20.9kg m^-2for Spartina.being respectively 22.7%and 7.3% greater than that for Scirpus and Phragmites in September 2004.Spartina ecosystems had a significantly greater N stock(699g m^-2)than Scirpus(597g m^-2)and Phragmites ecosystems(578g m^-2).These results suggest that Spartina invasion alters net primary production of ecosystems and litter decomposition,and hence enhance ecosystem C stock.Aerial,surficial and belowground litter of the three species was regularly collected to estimate the litter production,and litterbag method was employed to quantify litter decomposition in the aerial,surticial and belowground positions for each of the three species.Mean annual litter mass in Spartina stands was 1.99kg m^-2,being respectively 250%and 22.8%higher than that in Scirpus(0.57kg m^-2)and Phragmites (1.62kg m^-2)stands.The litter in Spartina stands was primarily distributed in the air (45%)and below the ground(48%).Scirpus and Phragmites litter was primarily allocated belowground(85%and 59%,respectively).The average decomposition rates of aerial,surficial and belowground litter were 0.80,1.69 and 1.15 year^-1,respectively, for Spartina,which were 45%,50%and 61%of that for Scirpus in corresponding positions,and 140%,141%and 77%of that for Phragmites,respectively.The differences in litter decomposition rates among the three species could be accounted for by the variations in litter qualities.The absolute amount of N increased during decomposition of Spartina stem and sheath litter in aerial and surficial positions,and root litter belowground,but that declined during decomposition of Scirpus and Phragmites litter.It was widely observed that N availability increased in Spartina stem and sheath litter during decomposition,which resulted from microbial epiphytic N fixation.These results suggest that Spartiha invasion may considerably alter litter C and N in the coastal wetlands.A meta-analysis of 106 independent studies was performed to quantify the degree to which C and N cycles respond to plant invasions.Nineteen variables associated with C and N cycles were compared between invaded and native ecosystems. These variables included C and N stocks in shoots,roots,and soils,C stock in litter, root:shoot ratio,litter and soil C:N ratios,aboveground net primary production,litter decomposition,soil net N mineralization and nitrification,plant N concentration,litter N and lignin concentrations,and Soil NH₄⁺ and NO₃^- concentrations.Our results showed that C and N stocks in plants and soils all significantly increased in invaded ecosystems relative to native ecosystems.ranging from a 4%increase in soil C stock to a 110% increase in shoot C stock.Root:shoot ratio,litter and soil C:N ratios decreased respectively by 14%,20%,and 5%in invaded ecosystems.Aboveground net primary production,rate of litter decomposition,rates of soil net N mineralization and nitrification increased on average by 41%,63%,43%,and 91%,respectively,in invaded ecosystems in comparison with native ecosystems.Plant N concentration,litter N and lignin concentrations,soil NH₄⁺ and NO₃^- concentrations were respectively 43%,25%, 8%,12%and 49%higher in invaded ecosystems than native ecosystems.Invasions by woody species usually had greater impact on C and N cycles than those by herbaceous species.Forests,grasslands,and wetlands did not show much differential response to plant invasions with respect to C and N cycles.The changes in ecosystem C and N pools, fluxes,and their associated parameters suggest that plant invasions profoundly alter ecosystem functioning.In conclusion,the results obtained from a case study and a meta-analysis suggest that plant invasions alter ecosystem C and N stocks and fluxes.Changes in plant species composition in invaded ecosystems lead to the alteration in ecophysioiogical properties, and then result in changes in ecosystem C and N stocks.Our results improve our understanding of how invasive plants change ecosystem functions and processes,and provide valuable information for management policy of invasive alien species.