Processes And Factors Controlling Organic Carbon Burial In A Common Reed Wetland In The Liaohe Delta

As an important part of blue carbon pools,coastal wetlands play a critical role in carbon sequestration and mitigation of global warming effects.However,there has been a long-standing controversy on burial of organic carbon through sedimentation.Understanding the processes of burial,decomposition and storage of organic carbon in coastal wetlands will help to precisely evaluate the efficiency of carbon burial,and then put forward effective countermeasures for the protection and management of these carbon pools.This work focuses on dissolved organic matter?DOM?and the microbial communities in sediments of coastal wetlands in the Liaohe Delta.The technologies involved High-throughput sequencing,Fluorescence quantitative Polymerase Chain Reaction,3D Exitation-Emission-Matrix,and Fourier transform ion cyclotron resonance mass spectrometry.Combined with geological background and sediment profile dating,chronological succession of DOM,the microbial communities and their interactions in sediments profile among different habitats and seasons were explored.The effects of microbial taxa and environmental factors on organic carbon sequestration and its controlling mechanism were examined.The highlights of this study are:1.There was a positive correlation between the concentration of dissolved organic carbon and salinity across various habitats in the coastal wetlands of the Liaohe delta.Compared with the surface water,the pore water in sediments generally has higher molecular weight,aromaticity,unsaturation,humic-like components and abundance of heteroatom compounds,which likely relates to the photodegradation,biodegradation and sulfidation of dissolved organic matters.2.In the common reed wetland of the Liaohe delta,compared with the surface layer,the ?-diversity was lower at greater depths,where the rate of succession of microbial community was slower,and microbial taxa shifted from actinomycetes and sulfur oxidizing bacteria to sulfur reducing bacteria and Pseudomonas.The relative contents of TOC and humic-like components decreased with the increase of depth,and was negatively correlated with the relative abundance of Pseudomonas and the expression of aromatic ring hydroxyl dioxygenase.3.The structure of microbial community in wetland sediments is strictly restricted by geographical space,the salinity of a tidal flat wetland was relative high,the dominant microorganisms were oceanic halophilic bacteria,and the rate of organic carbon decomposition was much lower than that of a common reed?Phragmites australis?wetland with low salinity.4.The rates of microbial decompositon of organic matter in the Liaohe common reed wetland was significantly affected by seasons,the degradation ability of humic-like components by microorganisms in a non-growing season was significantly highter than those in a growing season.High salinity and high water table are the dominant factors affecting microbial community function and redox conditions,and restricting the decomposition of organic matter,especially inert components.5.The results of the chronological succession of dissolved organic matter and microbial communities in a common reed wetland of the Liaohe Delta showed that there was a significant gradient of organic matter degradation along sediment profile,with aerobic degradation at 0–20 cm,and anaerobic degradation at 20–35 cm,where the decomposition rate of organic carbon was significantly reduced.Remarkably,there was a strict anaerobic zone at the depths of 35–50 cm,which showed a very slow rate of organic carbon decomposition.Our results also demonstrated that the stable organic carbon layer likely shifted to a greater depth due to oxygen being transported by aerenchyma of vascular plants Phragmites australis.The rates of organic carbon decomposition ranged from 1.42% yr^-1 to 1.70% yr^-1 at 0–20 cm,0.45% yr^-1 at 20–35 cm,and 0.15% yr^-1 at 35–50 cm.6.In a common reed wetland with high productivity,the dominant source of organic matter was plant residues which had undergone a rapid humification.The study on the rate of sedimentation carbon flux revealed that only ?14% of highly humic organic matter can enter a long-term carbon pool??221 years?after microbial mineralization during a sedimentation process.