Effects Of Nitrogen And Phosphorus Input And Heavy Metals On The Spectral Characteristics Of Dissolved Organic Matter In Mangroves

Mangrove ecosystem is a unique ecosystem connecting land and sea.Due to human activities,a large number of heavy metals,nitrogen and phosphorus nutrients are collected here.Under the action of microorganisms,dissolved organic matter(DOM)producing complex coupling effects that have a profound effect on the circulation of mangrove ecosystem material.DOM is widely distributed in mangrove waters and sediments,and has rich ecological effects.The profound impact is on the biogeochemical processes of metals and other pollutants.The DOM structure in mangrove wetlands is complex,with both endogenous sources of natural origin and exogenous artificial inputs.The analysis of different DOM sources,influencing factors and their coupling mechanism of heavy metals in mangroves has important theoretical and practical significance.It can characterize the environmental behavior of DOM in offshore mangrove system and predicting its effect and environmental convergence mechanism.DOM contains conjugated organic compounds,many of which are amino acids,proteins and aromatic hydrocarbons with fluorescence characteristics.In this research,Parallel factor analysis(PARAFAC)was combined with Execitation-emission Matrix Spectroscopy(EMMs)to analyze the spectral characteristics of DOM and explore the changes of DOM composition and content under the input of heavy metals,nitrogen and phosphorus.The ion exchange resin was used to separate the fulvic acid from DOM in typical mangrove area,and its binding behavior with typical heavy metals was studied by fluorescence quenching titration,it provides theoretical and data for understanding the ecological behavior of DOM and typical heavy metals in mangrove ecosystems.The main conclusions are as follows:1.The DOM of Taiwan,Guangxi and Hainan mangrove areas are widely affected by human input.The DOM fluorescence components of the three mangrove areas mainly include ultraviolet terrestrial humus(C peak),a mixture of ultraviolet marine humus and ultraviolet terrestrial humus(mixed peaks of M and C peaks),ultraviolet fulvic acicd(A peak)and protein-like protein(T peak),in which the T peak is the characteristic peak of Taiwan mangroves,the DOM source in Guangxi mangroves contains less terrestrial humus and microbial products,and the fluorescence intensity of A and C peaks in Guangxi and Hainan ate higher,sewage may be an important source of pollutant discharge.2.Fluorescence index(FI),Humification index(HIX)and Biogenic index(BIX)in sediments indicating the main source of DOM,the degree of humification of DOM,and the influence degree of humans on DOM,respectively.The FI of the DOM in this study area is 1.53-4.1 1,with an average of 2.03,which is similar to the endogenous characteristic value of 1.9,indicating that the biodegradation product is the main source of DOM humic components.The HIX of the DOM is 0.72?9.97,which is four times that of the natural water body,indicating that the mangrove sediment DOM has a high degree of humification.The BIX of the DOM is 0.25?1.36,with an average of 0.6,indicating that the DOM in the sediment is affected by the land source input.3.The factors affecting the fluorescence characteristics of mangrove DOM are nitrogen and phosphorus input,salinity,microorganisms,plant communities,litter and so on.Nitrogen and phosphorus nutrient input increased the degree of aromatization and humification of DOM.Ammonium nitrogen(NH4+-N)is more conducive to plant growth than nitrate nitrogen(NO3--N),which indirectly affects the fluorescence characteristics of DOM.Salinity was significantly negatively correlated with HIX(P<0.05).Different plant communities contained different types of DOM,the highest DOM content in sediments of Spartina alterniflora,Avicennia ma,rina,and Aegiceras corniculatum were UV fulvic acid,humic-like and tyrosine-like,respectively.Under the action of microorganisms,the fluorescence intensity decrease of DOM is more than 90%after one week,and more humus is produced during the degradation process,which increases the degree of refractory degradation of DOM.The addition of Avicennia marina litter in sediments can rapidly increase the fluorescence intensity of DOM.Under the action of microorganisms,the fluorescence intensity increased by more than 100 times in the initial stage(1 d),the addition of Aegiceras corniculatum litter and Spartina alterniflora litter increased the DOM fluorescence intensity by about 10 times in the initial stage,and the litter input decreased the fluorescence intensity of humic acid in the sediment.The overall degree of influence is Avicennia marina>Aegiceras corniculatum>Spartina alterniflora.4.Heavy metal ions have a quenching effect on DOM,and there are many protein-like components in areas with more serious human pollution.Heavy metal ions have a good fluorescence quenching effect on humic-like substances.Among the fulvic acids extracted from Avicennia marina,the decrease in fluorescence intensity of copper,cobalt and lead is copper(45%)>cobalt(27%)>lead(12%).When quenching with Cu,the fluorescence intensity of the four natural furic acids decreased order is Spartina alterniflora(50%)>Avicennia marina(44%)>Kandelia candel(40%)>Aegiceras corniculatum(27%).