Distinguished The Source Of Organic Matter From Mangrove Wetland Sediments In Zhangjiang Estuary

Wetlands are particularly important and productive ecosystems.Coastal wetlands in the intertidal are open,dynamic systems,dominated by tidal currents in contact with open sea.The complex of physical and chemical processes controls the organic matters dynamics in these wetlands.Being an important part of global carbon cycle,the biogeochemical process of organic matters in wetlands has been focused on for the recent decades.However there are little literatures on the organic geochemical processes in estuary wetlands.Mangroves represent a critical ecological habitat in the coastal environment of tropical and subtropical areas.To our knowledge,unfortunately, almost nothing has been published regarding the organic geochemical processes in mangrove wetlands.In this dissertation,Zhangjiang mangrove wetland which is the biggest natural distribution of mangrove in north latitude was selected as research site.The aim of this dissertation was to quantify the sources of organic matters input of the mangrove wetlands in the intertidal ecosystem,through the analysis ofδ¹³ C,δ¹⁵N isotope values,nutrient(caroon,nitrogen and sulfur),and lipoid compounds(n-alkane,fatty acids) in sediments and plants.The main results were show as followingThe concentrations of total organic carbon(TOC) in top sediments are in range from 0.76 to 1.49%.The C/N ratios in top sediments are in range of 6-11. These indicate the source of organic matters in top mangrove wetland sediments may have another origin.Tile mangrove trees contributed little to the sediment organic matters.The distribution of C,N and S elements in mangrove sediments disturb obviously than bare sediments.These may result in complicated deposited environmental characteristics and the involutedly physical and chemical properties of mangrove sediments.TOC,TN and TS have no correlation with grain sizes in Zhangjiang mangrove sediments.These show an obviously different of some other estuaries in the world which organic matters sources dominant by the suspending particles.Three mangrove pure stand contribution of sediments organic matters show as the following trend:A. comiculatum forest＞A.marina forest＞K.candel,forest.Mangrove trees contribute less than 50%of organic matters in K.candel.and A.marina sediments.The average stable carbon isotope value in the leaves of K.candel.,A. marina,A.corniculatum,is-29.50‰,-29.87‰,-29.98‰,respectively.Three mangrove leaves have the similar stable carbon isotope(δ¹³ C) values as typical C₃ plant.The average stable carbon isotope values in outside mangrove sediments are range from -23.02 to 21.61‰.However,mangrove top sediments showed a range from -22.86 to 25.53‰.Compare with wetland plant stable isotope values,top sediments concentrate the light carbon isotope. These indicate that wetland plants are not the major sources of organic matters in wetland sediments.These show the same results as to analysis the data of TOC concentrations and C/N ratios in top sediments.The average nitrogen carbon isotope values in outside mangrove sediments are range from 4.65 to7.82‰.However,mangrove top sediments showed a range from 5.23 to 6.78‰.The data of stable carbon isotope(δ¹³ C) values which significant correlate to the organic matters values can be used to analysis the sources of organic matters origin.But stable nitrogen values(δ¹⁵N) are not showing this aspect.Relative contributions of mangrove plant-derived organic matter to the sedimentary organic pool is calculated based on a two end-member mixing model and stable carbon isotope values in the core section.The result indicates that A.comiculatum,input maximum organic matters while K.candel. input the minimum.This result is similar to the trend which base on analysis of TOC concentration.The distribution of the lipid composition of n-alkanes,fatty acids indicate that the organic matters in the sediments are mainly from terrestrial-derived and see-derived organic matters.And the see-derived organic matter takes dominant in the sediments.This predominance is obviously showed in the bare beach.In mangrove forest,the plant especially fibro vascular tree significantly influences the organic matters in sediments.The evidences from fatty acids indicated that considerable unsaturated fatty acids which derive from bacteria activity.The fatty acids degraded rates increase with the depth.As indicated by organic matter source,stable carbon isotope values of see-derived organic carbon in literatures and the data of column samples in Zhangjiang estuary,the mangrove-derived organic matters were emendated.The results showed that the contribution of mangrove trees to sediments organic matters are at range from 30 to 50%.The trend of tree mangrove stands contribution follow the order of A.corniculatum forest＞A.marina forest＞K.candel,forest.