Organic Carbon Sources And Microbial Carbon Assimilation In Sediment In Mangrove Ecosystems

Mangroves are important ecosystems of sheltered tropical and subtropical coastlines,which are characterized by high productivity,high return rate and high decomposition rate,and play a critical role in the sequestration of global carbon.Sampling sites were mainly located in the Jiulong River Estuary Mangrove Nature Reserve,which is located on the southeast coast of Fujian,China.The objectives of this study were to first to quantify the spatial and temporal variations among the different mangrove community zones,second to analyze the soil organic carbon sources in the mangrove sediments,and third to analysis the microbial diversity of mangrove sediments,and to explore the relationship between soil organic carbon stocks and microbial carbon assimilation in mangrove sediments.The major results were as follows:(1).The total organic carbon(TOC)and total nitrogen(TN)contents were higher in the mangrove zone than the bare mudflat zone and and unvegetated bare mudflat.TOC and TN contents at 30 years mangrove decreased with the increase in depth in the landward zone,and the seaward and bare mudflat exhibited no obvious changes with depth increases.TOC and TN contents at natural forest mangrove decreased with increasing depth in the upper soil layer(0?60 cm),however,with increasing depth(60?100 cm),TOC and TN contents exhibited no obvious changes.In addition,there were some differences in the TOC contents in sediments from the different latitudes mangrove areas.(2).There were statistically significant differences in the ?13C and ?15N values of the sediments from the landward to the bare mudflat zones,because the soil organic carbon(SOC)was mainly derived from mangroves and other terrestrial C3 plants,while the bare mudflat zones were enriched in higher isotopes because the SOC was influenced by marine phytoplankton,marine benthic algae,and C4 marsh grasses.In addition,the ?13C values of sediments increased with increasing depth.It was possible that the SOC contents of the mangrove sediments were influenced by a variety of potential sources.(3).The combination of the ?13C,?15N,and C/N ratio values indicated that the SOC was derived from not only mangroves and other terrestrial organic matter input sources but also marine phytoplankton,marine benthic algae,and C4 marsh plants.Stable carbon and nitrogen isotopes mixture model showed that terrestrial organic matter dominated the organic carbon in the mangrove sediments.At the same time,the SOC is also influenced by the transformation of organic carbon from mangroves,aquaculture wastewater,C4 plants,and marine organic matter.The organic carbon in the surface soil layer was mainly derived from mangroves and local macrophytes,while the accumulation of organic matter in the bottom soil layer may be enhanced through the addition of autochthonous material and the trapping of suspended allochthonous matter during tidal inundation.Meanwhile,base on ternary mixing model showed that the organic matter in mangrove sediments may have other sources.(4).Bacterial community composition and diversity were analyzed by high-throughput sequencing.The bacterial communities were dominated by phylum Proteobacteria,followed by Bacteroidetes,Chloroflexi,Firmicutes,Actinobacteria,Acidobacteria,Gemmatimonadetes,Planctomycetes,Lentisphaerae,Chlorobi and Cyanobacteria.There were significant differences in the bacterial communities of the sediments among the mangrove inner,bare mudflat and unvegetated bare mudflat.The relative abundance of Elusimicrobia,Bacteroidetes,Acidobacteria,Lentisphaerae,Spirochaetes,Deferribacteres,Thaumarchaeota,Latescibacteria,Fusobacteria,Woesearchaeota_.DHVEG-6.,Candidate_division_OP3,and Hydrogenedentes in mangrove sediments were higer than bare mudflat.Meanwhile,these bacterial not only have photosynthetic carbon assimilation ability,but also were positively correlated with TOC content.Microbial carbon assimilation have higher contribution rate to the organic carbon burial in mangrove,account for over 20%of total organic carbon burial,indicating that these bacterial community had an important effect on carbon assimilation and carbon sequestration in mangrove sediments,which would directly affect the organic carbon stocks of belowground and carbon budget in mangrove ecosystems,and which was one of possible reasons that the organic carbon stocks of mangrove ecosystems were higher than other ecosystems.