Stability Of Organic Carbon Pool In Sediments Response To Warming And Flooding In Mangrove Ecosystems

Mangrove significantly contribute to global carbon storage potential,at the same time mangrove sediments and waters has been shown to be a large source of CO2 to the atmosphere and the exchange of organic matter with the adjacent estuary or ocean.In recent years,global climate change of warming and sea-level rise and other climate-change-induced disturbances threaten coastal wetland sustainability and carbon storage capacity.The main research object of this study is the mangrove sediments in Zhangjiang River Esturary,combing with solid state nuclear magnetic resonance(NMR)technology(13C CP/MAS NMR),researching the organic carbon fractions and structure vertical distribution characteristics of in-situ sediments,simulating warming and flooding conditions to the sediment through the laboratory incubation,the characteristics of organic carbon mineralization,fractions and structures of the sediments are used to understand the response of sediment organic carbon pool to warming and flooding conditions.This study is helpful to answer the carbon storage of mangroves and the source-sink problem of soil organic carbon pools in coastal wetlands such as mangroves under global warming climatic conditions,and providing scientific basis for mangrove ecosystem management.The main research conclusions are as follows:(1)The vertical distribution pattern of organic carbon fractions and structures in mangrove sediments are:TOC content decreases significantly with the increase of depth,and the vertical distribution of LOC,POC,MBC and DOC contents of organic carbon active fractions are all surface>middle>deep.The organic carbon structure in different depth sediments are similar,displaying prominent signals in the alkoxy and aromatic region,fllowed by alkyl and carboxyl region.By comparing the alkyl carbon/alkoxy carbon ratio,hydrophobic carbon/hydrophilic carbon ratio,aliphatic carbon/aromatic carbon ratio and the aromaticity indicating that the stability of the organic carbon in the deep and middle layer sediment is higher than that in the surface layer.(2)In the process of incubation,both the average mineralization rate and the cumulative mineralization amount showed the rule of warming>warming-flooding>flooding.Potential mineralization amount in the fitting results of the first-order kinetic model of one-pool for organic mineralization also show the rule of warming>warming-flooding>flooding,indicating that the characteristics of organic carbon mineralization in sediments occurred as the order of warming>warming-flooding>flooding,the potential mineralization amount of sediments increased with increasing temperature,and the mineralization capacity of organic carbon in sediments was enhanced by warming,while flooding condition was the opposite.DOC and MBC content were positively correlated with the daily mineralization amount under the warming condition,and warming enhanced the correlation between MBC and daily mineralization.The DOC content was negatively correlated with the daily mineralization,while MBC content was positively correlated with the daily mineralization.It indicates that the content of DOC leached from organic carbon in sediments by flooding makes a relatively small contribution to the organic carbon mineralization,microbial activity significantly affected organic carbon mineralization.(3)After incubated for 90 d,according to the variation results of total organic carbon in sediments,it can be seen that the loss of carbon pool presents a rule of warming>warming-flooding>flooding,which is consistent with the results of organic carbon mineralization.The loss of DOC showed a rule of warming-flooding>flooding>warming.The recalcitrant organic carbon converted to labile organic carbon as the follow rule of flooding>warming-flooding>warming,the stability of carbon pool is affected to a certain extent,and the influence of flooding is greater than that of non-flooding.The temperature sensitivity of recalcitrant carbon pool is higher than that of labile carbon pool.(4)After incubated for 90 d,the relative proportions of organic carbon functional groups in the sediments of each incubation group were:alkoxy carbon>aromatic carbon>alkyl carbon>carboxyl carbon.By comparing the alkyl carbon/alkoxy carbon ratio,hydrophobic carbon/hydrophilic carbon ratio,aliphatic carbon/aromatic carbon ratio and the aromaticity indicating that that under the condition of warming-flooding,the stability of organic carbon caused by agglomeration in sediments is the weakest,followed by warming,then flooding.To sum up,under the short-term influence,warming accelerates the release of CO2 from sediments.Although flooding inhibits the release of CO2,and warming-flooding condition has a greater impact on the stability of the overall organic carbon pool.