| With the rapid development of global economic integration and global tourism,invasive alien species show the trend of an increase in the number and frequency of introducing,accelerating sprawling and widening,increasing occurring hazards and economic loss.Invasive species has become one of the most difficult three environmental issues,and has become the common issue that is concerned by Governments,the international community and the academic community.It is,at the same time,widely regarded as an important part of global change.Especially,plant invasion often leads to rebuild the entire ecosystem structure,which may change their functional activity and the biogeochemical cycle.Some plants have special secretions,which not only pose a threat to local species,but also lead to the soil ecosystem changes.Those changes can make the soil fit to the growth,reproduction and diffusion of invasive plants.Spartina alterniflora is the only coastal salt marsh plants in the list of the first batch of invasive species which was announced by the State Environmental Protection Administration in early 2003.To explore the impacts of this weed invasion on soil ecosystem,we collected the soil sample of the different stages of S.alterniflora invasion in Xiapu County of Ningde City,Fujian Province,which was affected most severely by S.alterniflora.And these were the each stage of S.alterniflora invasion:? the mudflat represented the control plot without vegetation;? the mangrove Kandelia candel(Linn.)D ruce community represented the community without S.alterniflora invasion;? the mangrove K.candel-S.alterniflora symbiotic community represented the mangrove communities occupied about half by S.alterniflora;?the S.alterniflora community represented the mangrove communities completely occupied by S.alterniflora.Finally,the main results were as follows:1.S.alterniflora invasion reduced the soil fertility of mangrove wetlands and weakened carbon sink function of mangrove.But S.alterniflora could adapt to the pH better,so it was supposed to be one of its successful invasion mechanism that it could change the soil pH.S.alterniflora could increase soil bulk density and contribute to silt beach,which could help to defend storm struck.2.With the increase of soil depth,the activities of invertase,urase,alkaline phosphatase decreased for the four studied communities,and the changes were slight.But the changes of catalase among soil layers were disorder,and the soil layers had more significant effect on hydrogen peroxide for the soil S.alterniflora Community.Soil enzyme activities showed a greater difference at the different stages of S.alterniflora invasion.The activities of invertase,alkaline phosphatase and catalase were stronger for the symbiotic community and the S.alterniflora community single than that for the beach and the mangrove community,while the urease activity for mangrove community was higher than the other three communities.3.There was not significant vertical variation regularity in the soil microbial biomass for the four communities.The gross of soil microbial biomass carbon in mangrove wetland decreased after S.alterniflora invaded successfully content.However,the content of the surface showed this trend,symbiotic community>S.alterniflora community>Mangroves>the beach,which meant that S.alterniflora invasion made the soil microbial biomass carbon of surface increase.The results indicated that the soil microbial biomass carbon had been changed by the invasion of S.alteriflora,and this may be one of the mechanisms of its successful invasion.4.With the increase of soil depth,the contents of Cd and Pb elements showed the trend of ?>?>? for the S.alterniflora community,namely 2.09,1.49,1.25 and 17.38,16.1,14.65(mg/kg).And the contents of Cu and Zn elements showed the trend of ?<?<?,and they were 17.33?18.03?19.4 ? 62.18?63.65?73.97(mg/kg)?Cd,Zn and Cu in the soil for the four communities were higher than the background values in Huanan costal saline,Pb element for the symbiotic community was higher than the background values.At the different stages of S.alterniflora invasion,the content of Cd in the soil surface showed the symbiotic community>Mangroves>the beach>S.alterniflora community,and the content of Pb in the soil surface showed the same trend,and the content of Cu in the soil surface showed symbiotic community>S.alterniflora community>the beach>Mangroves,and the content of Zn in the soil surface showed S.alterniflora community>symbiotic community>the beach>Mangroves.Mangroves could make a lot of heavy metal pollutants fixed on the sediments through different ways,which can purify Fujian coastal wetland ecosystems contaminated with heavy metals,while provide a large number of clean food for consumers at all levels of the ecosystem.5.There was a very close relationship between soil enzyme activities and soil physical and chemical factors,which correlation may reflect the metabolic processes of soil.Sucrose,urease,alkaline phosphatase were the main enzymes in Fujian coastal mangrove wetlands and they can be used as the characterization indicators of soil fertility.There was not significant correlation between the soil microbial biomass carbon and the four kinds of soil enzyme activities,organic carbon,pH,soil bulk density,moisture content.There were extremely significantly positive correlations between Pb and Cd and Cu,while the pH had a significant impact on the content of heavy metals.The activities of soil enzymes,soil nutrition contents,soil microbial biomass carbon and heavy metals were closely related to one another.The activities of invertase,alkaline phosphatase and urease can be used to describe the level of soil fertility.In summary,the invasion of S.alterniflora increased the enzyme activity and decreased the soil nutrient levels,the microbial biomass carbon in surface soil and the fixed capacity of heavy metals of the mangrove community.It changed the soil ecology traits in Fujian eastern mangrove wetlands,which was not conducive to the sustainable development of mangrove community.Finally,we hypothesized it is one of the mechanisms underlying its invasion that S.alterniflora can make itself occupy ecological niche more easily than the indigenous species by changing the soil traits. |
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