Spatial-temporal Changes Of Soil Carbon Storage In Coastal Wetland Of Dafeng Milu National Nature Reserve

Du to the interaction between sea and land,coastal wetland has huge carbon storage and carbon fixation capacity,and plays a unique regulatory role in the storage,release and transformation of soil carbon.Therefore,the research on carbon sink of coastal wetland ecosystem has become a hot issue and attracted increasing attention of many scholars.The invasion of Spartina alterniflora(S.alterniflora)affects the soil carbon storage in wetland.However,the effects of S.alterniflora invasion on soil carbon storage in wetland are still controversial.Hence,in this thesis,the spatial distribution characteristics of surface soil organic carbon(SOC)were first determined based on surface soil samples collected at Dafeng Milu National Nature Reserve(DMNNR).Meanwhile,soil samples at a depth of 1 m belonging to typical coastal wetland vegetation communities over four seasons were collected to compare the horizontal and vertical distribution and seasonal dynamics of soil organic carbon(SOC),soil microbial biomass carbon(MBC)and soil dissolved organic carbon(DOC)contents.The classifications of vegetation communities included bare land,Suaeda salsa(S.salsa),S.alterniflora,Imperata cylindrica(I.cylindrica),Aeluropus sinensis(A.sinensis),Cynodon dactylon(C.dactylon)and Phragmites australis(P.australis).We also interpreted the areas of different land cover types from remote sensing images,and obtained the dynamic changes of soil carbon storage at 1 m depth in DMNNR from 1984 to2020.The main conclusions are as follows:(1)Affected by the internal soil characteristics and external human factors,SOC exhibit a medium spatial dependence,and was presented in patches in DMNNR.The higher SOC contents were mainly found in the west,central and south of the study area,and the lowest SOC was distributed in the eastern coastal area.Total nitrogen and aboveground biomass were the most important factors affecting the spatial distribution of SOC content.(2)Significant differences were detected in SOC content among different vegetation communities at 1 m depth in the coastal wetland of the study area.The results showed that the S.alterniflora ranked first in SOC content,followed by the bare land,S.salsa,C.dactylon,A.sinensis,P.australis and I.cylindrica.Distribution of SOC content over vertical gradients among different wetland classifications showed non-monotonic trends.In terms of seasonal dynamics,the SOC content of different vegetation communities showed the order of Autumn > summer > spring >winter,and there was no significant difference among seasons.(3)The characteristics of soil active carbon components of different vegetation communities in the study area were as follows: there were significant differences in soil MBC content,and the order of soil MBC content was: S.alterniflora > P.australis > S.salsa > bare land > C.dactylon >A.sinensis > I.cylindrica.Except bare land,the highest value of soil MBC content of other vegetation groups was located in the surface soil,and the seasonal pattern of soil MBC content is:Autumn > spring > summer > winter;There was no significant difference in soil DOC content among different vegetation communities.The soil DOC content of S.alterniflora was higher than that of other wetland classifications.The seasonal pattern of soil DOC content was: Autumn >summer > winter > spring.(4)The SOC density of coastal wetland vegetation community in the study area significantly differed over different soil depths.The SOC density presents an approximate “V” distribution characteristic with increasing soil depth.The highest value was found in 80～100 cm soil layer while the lowest value was detected in 20～40 cm soil layer.The annual average SOC reserves of different vegetation communities in the study area were as follows: S.alterniflora > bare land > S.salsa > A.sinensis > C.dactylon > P.australis > I.cylindrica.(5)The invasion of S.alterniflora significantly increased the soil carbon storage in DMNNR.S.alterniflora started colonizing from the south of the study area in 1993.With the expansion of S.alterniflora,the soil carbon storage of S.alterniflora showed an approximate "J" growth trend,and the soil carbon storage of the study area increased to the maximum in 2011.Affected by human disturbance,S.alterniflora began to decline from the west of the study area in 2010,but still expanded to the Yellow Sea.By 2020,the soil carbon storage in the study area increased by about26278.70 T C compared with 1984.