| Located at the interaction zone of terrestrial ecosystem and marine ecosystem,coastal salt marshes are differernt from the other kind of salt marshes.Characterized by higher primary productivity and stronger potential of carbon sequestration,these salt marshes are regarded as an major carbon sink on the earth.Salt marsh plant plays an important role in the marsh ecosystem,and has a significant impact on the salt marshes carbon cycle.The changes in vegetation composition may alter ecosystem structure and processes,consequently,soil carbon pools may also be modified.Taking the tidal salt marsh of Wanggang estuary as an example,this paper focuses on the influence of spartina alterniflora(an exotic species)on organic carbon distribution distribution in horizontal and vertical direction.The tidal marsh of Wanggang estuary is located at Yanchen city,Jiangsu Province,which is a typical mud flat along Jiangsu coast.As an exotic species,Spartina alterniflora was intentionally planted in the coastal region here in 1982,after that it has gradually invaded the regions previously dominated by native C3 plants,and formed a large area of single S.alterniflora salt marsh during last 30 years.In this study,the contents of total organic carbon(TOC),total nitrogen(TN),stable carbon isotope composition(?13C),recalcitrant carbon(RC)and labile carbon(LC)of the sediments in the S.alterniflora marsh and the bare flats,were investigated with the analysis of collected sediment cores as well as the surficial sediments.Moreover,the biomass and TOC contents of S.alterniflora were measured bimonthly for the carbon storage in the plant,the sediment water content and grain size,together with 813C value of plant and suspended sediment collected from Wanggang river.Based on the data from field observation and experiments,The changes of the fractions and sources of organic carbon in surface sediments from season to season were analyzed,and the relationship between them and the litter mass varation of S.alterniflora were also identified.Subsequently the characteristics of organic carbon fractions in core sediments and the belowground biomass of S.alterniflora were studied,furthermore,the carbon fixation of plants and sediments were calculated.The results showed that,the organic carbon of the newer litters rapidly decomposed,the organic carbon storage decreased by 40%during the earlier two months,and then remained stable.The TOC contents of surface sediments in S.alterniflora salt marsh was significantly higher than that in bare flat,and the patterns were also quite different from bare flat with season changes.The seasonal variation of the TOC contents in S.alterniflora marsh was obvious,and a two-month phase shift was not only observed in the variations of the contents of TOC and the litter biomass,but also recorded in the contents and proportions of organic carbon derived from S.alterniflora and the litter biomass.However,this phenomenon of phase shift did not exist in the bare flat,indicating that the litter is a dominating source of the organic carbon of surface sediments.The similiar situation was also identified in LC fraction,and more than a half of organic carbon was LC,suggesting that the litter alter the LC contents of surface sediments,and then changed the TOC contents.Although also influenced by litter mass,RC fraction was mainly determined by the organic carbon orginated from other sources.About 60%and 33%of the belowground biomass were observed in the top 20 cm and the depth of 20-40 cm,respectively.Comparing with the bare flat,the vertical changes of TOC?RC and LC contents in S.alterniflora marsh showed a different distribution pattern.On the one hand,S.alterniflora invasion largely increased the contents of TOC?RC and LC compared with the bare flat,especially within the 0-20 cm sediment layer.The average RC and LC contents in sediments of top 20 cm depth in S.alterniflora salt marsh,in terms of the quantity 3.27 and 6.75 mg/g,were 170%and 403%that of the sediments in bare flat,respectively.On the other hand,The TOC?RC and LC contents of S.alternflora marsh sediments at depths<20 cm,together with the proportion of LC fraction in TOC,were significantly higher than those in deeper sediment layers(>20 cm),and had a decreasing trend from surface to the depth of 20 cm.In addition,the similar variation also recorded by the proportion of organic carbon derived from S.alterniflora of top 20 cm sediments.Furthermore,the fractions of organic carbon in coral sediments performed a remarkably positive correlation to S.alterniflora belowground biomass,implying that belowground biomass greatly impacted the organic carbon fractions in sediments along the profiles.The highest and lowest organic carbon storage in the plant of S.alterniflora was observed in October and April,respectively.The average carbon fixation of S.alterniflora,with quantity 2274 g.m-2.a-1,was 460%of that of the terrestrial vegetation in China.Similarly,the burial rate of organic carbon in the sediments of S,alterniflora marsh,with quantity 470 g-m-2.a-1,was significantly greater than that of the global average value.Thus,the S.alterniflora salt marsh fixed large amounts of carbon dioxide from the air,and enhanced the organic carbon accumulation in the sediments. |
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