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Mechanisms Of Organic Matter-driven Evolutionary Processes Of Biogenic Elements In Mangrove Wetlands

Posted on:2024-02-27Degree:DoctorType:Dissertation
Country:ChinaCandidate:L YanFull Text:PDF
GTID:1521307148484234Subject:Environmental Science and Engineering
Abstract/Summary:PDF Full Text Request
Mangrove wetlands are unique ecosystems located at the interface of land and sea,which play a pivotal role in regulating the climate and ecological environment.The transport and transformation of organic matter within mangrove wetland systems are primarily controlled by surface water-groundwater interactions.As the electron donor and energy driver of the biogenic elements evolution,organic matter has a direct impact on the growth and development of mangroves and the overall health of the ecological environment.However,previous studies have focused on the plant ecology of mangrove wetlands and the spatial and temporal distribution of nutrients,heavy metals,and other pollutants.Further research is necessary to fully understand the source,composition,transport,and transformation behavior of organic matter in mangrove wetlands,as well as the underlying mechanisms driving the biogeochemical cycle.Furthermore,given the significant influence of human activities and surface-groundwater interactions,the molecular-level biochemical transformation process of organic matter in mangrove wetlands requires more accurate characterization.Specifically,there is a lack of knowledge regarding the impact of organic matter molecular composition on the biogenic element evolution.Therefore,it is imperative to address the following scientific challenges:(1)Quantitatively characterizing the contribution of sedimentary organic matter in mangrove wetlands,elucidating the burial and degradation mechanisms of sedimentary organic matter,and studying the biogeochemical behavior of dissolved organic matter;(2)Revealing the molecular transformation pathways of dissolved organic matter under surface water-groundwater interactions,and systematically identifying the microscopic mechanisms of the biogenic element evolution driven by organic matter.To address this research gap,the study selects the Dongzhai Harbor mangrove wetland as typical research area.A parallel factor analysis model and aδ13C andδ15N three-endmember mixing model were constructed to quantify the source,composition,burial,and degradation processes of sedimentary organic matter(SOM).Absorption and three-dimensional fluorescence spectroscopy were used to reveal the source,composition,and mixing behaviors of dissolved organic matter(DOM)in surface water.Additionally,we employed advanced molecular characterization techniques,including Fourier Transform Ion Cyclotron Resonance Mass Spectrometry(FT-ICR MS)and Liquid Chromatography-Organic Carbon Detection(LC-OCD),to elucidate the transport and transformation pathways of DOM and the biogenic element evolutionary processes driven by DOM,under surface water-groundwater interaction at a microscopic scale.The main research progress is summarized as follows.(1)The contribution of SOM from aquaculture(44.29%),terrestrial(62.87%),and marine plankton(56.49%)sources was quantified in the upstream,midstream,and downstream of mangrove wetlands based on theδ13C andδ15N three-endmember mixing models.The content of biogenic elements and the abundance of fine-grained sediments in the sediments decreased along a land-sea salinity gradient.Moreover,three SOM fluorescent fractions were identified,including terrestrial/microbial humic-like component(C1),tryptophan-like component(C2),and soil-derived fulvic acid(C3).Results show that in the upper and middle estuary,SOM was preserved by sedimentation due to the abundance of fine-grained aquaculture-derived biodeposits,weak hydrodynamic conditions,and the formation of Fe/Mn-humus complexes.These processes stabilize coastlines against sea level rise,subsidence,and uplift,providing favourable habitats for mangroves.However,marine plankton derived SOM can be rapidly degraded through microbial respiration in the downstream aerobic environment,resulting in the production of soluble biogenic elements that can be supplied to mangrove forests.This enables mangroves to flourish and persist even in the face of strong tides and bioturbation.Moreover,correlations betweenδ13C andδ15N values vs.C/N ratios further suggest progressive SOM degradation from initial diagenesis to post-depositional geochemical processes.(2)Surface water DOM and biogenic elements in mangrove wetlands showed spatial heterogeneity from the fluvial-mangrove zone to the main estuary zone.Three fluorescent components,comprising three humic-like components(C1,C2,and C3)and a tryptophan-like component(C4),were identified using EEM-PARAFAC modeling.Results revealed that DOM absorption at 254 nm(a254),SUVA,HIX,and the abundance of humic-like substances decreased with increasing salinity,indicating the dominance of terrestrial/anthropogenic-derived aromatic DOM in the upper estuary.Significant positive correlations were observed between SR,BIX,tryptophan-like materials,and salinity,which indicates low molecular weight autochthonous DOM prevails in the lower estuary.Notably,the estuarine mixing behaviors of each DOM component were complicated,involving photo/biogeochemical processes beyond the simple physical mixing of freshwater and seawater.DOC,TDN,TDSi,Mn,Ba,and humic-like DOM were enriched in the Yanzhou River,Sanjiang River,and Yanfeng East River fluvial-mangrove zones,located in the upper and middle estuary,which were primarily affected by terrestrial inputs,riverine runoff,and mangrove porewater exchange.In contrast,the Yanfeng West River,close to the marine endmember,had relatively higher p H,DO,EC,salinity,TDP,and tryptophan-like CDOM,suggesting saltwater dilution,tidal mixing,photo-oxidation,and microbial degradation processes might cause the phenomena.(3)The elemental ratios,molecular formulae,and chemical structure types of surface water and porewater DOM in mangrove wetlands exhibit significant spatial heterogeneity.Surface water DOM in the fluvial-mangrove zone contains more hydrophobic polycyclic aromatics and polyphenols,while the main estuary is dominated by low molecular weight neutrals,aliphatic compounds,and saturated compounds.Porewater DOM in the fluvial-mangrove zone is dominated by CHO,CHOS,CRAM,polyphenols,and aromatic compounds,while the porewater DOM in the main estuary is dominated by CHON,aliphatic and saturated compounds.The composition of porewater DOM in mangrove wetlands is controlled by sediment physicochemical properties,redox conditions,and strong microbial activities.Processes such as mixing dilution,photo-mineralization,and co-precipitation can lead to the removal of aromatic compounds and the production of more lipid compounds in the main estuary.(4)There is little accumulation of surface water DOM in the sediment porewater of mangrove wetlands,while the concentration gradient drives the transport and transformation of DOM from sediment pore water to surface water.Interactions between surface water and groundwater lead to an overall accumulation of higher NOSC and more CRAM molecules,but a loss of sulfur-containing aromatic DOM(particularly CHOS2).Porewater unique molecules exhibit higher biological activity compared to surface water DOM,characterized by lipids with high H/C ratios(H/C>1.5)and heteroatom-containing molecules with low O/C ratios(O/C<0.5).Under thermodynamic constraints,this results in the rapid aerobic biodegradation of oxygen-rich organic matter that is enriched in nitrogen,sulfur,and phosphorus.(5)The evolution of biogenic elements in mangrove wetlands is jointly influenced by factors such as the complex composition of organic matter,oxygen availability,porewater exchange,and microbial activity.In the upstream fluvial-mangrove zone,microbially mediated oxidation and degradation of highly unsaturated and oxygenated CHOS drive the enrichment of HS-in surface water.The adsorption and complexation of polyphenols and hydrophobic polycyclic aromatics with Mn promote Mn mobility.Furthermore,in the anoxic environment during tide inundation,benthic manganese-reducing bacteria utilize Mn oxides as terminal electron acceptors,oxidizing highly unsaturated oxygen-rich and aliphatic compounds,leading to Mn release into overlying surface water.In the main estuary,the deamination/decarboxylation of aliphatic and saturated compounds,as well as the mineralization of CHON,contribute to the accumulation of NH4-N in surface water.The biomineralization of highly unsaturated oxygen-rich,aliphatic and saturated CHOP compounds promotes the accumulation of TDP in downstream surface water.While the adsorption of DOM-phosphorus by highly unsaturated,polyphenolic,and aromatic CHOP in porewater immobilizes TDP from upstream surface water.Highly unsaturated,polyphenolic,and aromatic compounds immobilize Fe via adsorption/complexation in mangrove wetland sediments.Whereas the behavior of dissolved Fe is none-conservative in surface water of the fluvial-mangrove zone,which is closely related to the formation of insoluble Fe sulfides and Fe-DOM co-precipitation.Overall,the study provides insights into the transport and transformation pathways of organic matter and the evolutionary processes of biogenic elements in mangrove wetlands.The findings have important implications for the conservation and management of coastal mangrove wetland ecosystems.The distinctive and innovative aspects of this study are as follows:(1)This research quantitatively characterizes the contribution of organic matter sources in mangrove wetland sediments,uncovering the key constraining factors governing the burial and degradation processes of SOM.It also presents novel findings on biogeochemical processes of SOM that impact the environmental health of mangrove wetlands.(2)By comprehensively employing techniques,such as Absorption and fluorescence spectroscopy,FT-ICR MS and LC-OCD,this study reveals the mixing behavior and transport-transformation mechanisms of DOM under the influence of surface water-groundwater interaction and human activities.These findings lead to a new understanding of the DOM-driven biogenic element evolutionary processes in mangrove wetlands.
Keywords/Search Tags:Sedimentary organic matter, Dissolved organic matter, Biogenic element evolution, Transport and transformation, Mangrove wetland
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