Study On Fluxes Of Nitrogen And Phosphorus At Sediment-water Interface In Coastal Wetlands

As an important wetland type in nature,coastal wetlands play an important role in regulating the material circulation of terrestrial and marine ecosystems.The sediment-water interface is an important part of the wetland ecosystem.The exchange and form transformation of nitrogen and phosphorus at the interface have an important impact on the nitrogen and phosphorus cycle of the entire wetland.Nitrogen and phosphorus nutrients in the sediment can be transported to the overlying water through the sediment-water interface,acting as a"source";the nitrogen and phosphorus nutrients in the water body can also be precipitated into the sediment to act as a"sink".In order to understand the process and influencing factors of the"source"or"sink"of sediments in coastal wetlands,this study investigated 19 coastal wetlands in my country.Here not only the nitrogen and phosphorus nutrient concentration in the water inlets,outlets,open points and vegetation points of the coastal wetlands are measured and analyzed,but also the diffusive gradient in thin films device（DGT）and the high-resolution pore water sampling device（HR-Peeper）two passive sampling devices measured the vertical changes of nitrogen and phosphorus concentrations at the sediment-water interface at open points and vegetation points,and then estimated the diffusion fluxes of nitrogen and phosphorus at the sediment-water interface.The research is of great significance for the comprehensive understanding of nitrogen and phosphorus cycles at the sediment-water interface of coastal wetlands.The main results are as follows:1)The concentration ranges of NO₃-N,NH₃-N,TDN,TN,PO₄-P,TDP,and TP in coastal wetlands were 0.71±1.14（0.00～7.16）mg/L,0.35±0.70（0.00～4.05）mg/L,1.61±1.41（0.21～7.35）mg/L,1.71±1.42（0.24～7.35）mg/L,0.06±0.12（0.02～0.61）mg/L,0.08±0.16（0.02～0.77）mg/L and 0.28±0.30（0.03～1.49）mg/L.In the water samples of open points and vegetation points,the concentration of nitrogen nutrients（except NH₃-N）in vegetation points was higher than that in open points,and the concentration of phosphorus nutrients in open points was higher than that in vegetation points.NO3-N concentration in the entrance was significantly higher than that in the open area.The concentrations of TDN and TN in the water inlets were significantly higher than those in the open points and vegetation points.NO₃-N concentration was mainly affected by dissolved oxygen（DO）and PO₄-P concentration was mainly affected by salinity.NO₃-N was positively correlated with TDN and TN,NH₃-N was positively correlated with TN,and PO₄-P was positively correlated with TDP and TP.2)The contents of total organic carbon（TOC）and total organic nitrogen（TON）in surface sediments were 49.57±69.58（0.69～338.22）g/kg and 6.04±8.99（0.14～37.53）g/kg,respectively.Near the sediment-water interface,with the increase of sediment depth,the variation trend of nitrogen and phosphorus concentration is different.In general,the concentrations of DGT NO₃-N and DGT NH₃-N decreased with the increase of sediment depth,while the concentrations of Peeper NO₃-N,DGT PO₄-P,Peeper PO₄-P and DGT Fe increased with the increase of sediment depth.There was a significant positive correlation between DGT PO₄-P and DGT Fe concentration,indicating that the release of DGT PO₄-P was controlled by DGT Fe.3)The fluxes of DGT NO₃-N,DGT NH₃-N,Peeper NO₃-N,DGT PO₄-P,Peeper PO₄-P and DGT Fe in the open points of coastal wetlands were 0.67±2.43（-1.99～8.52）mg/（d·m²）,-0.06±1.58（-3.89～4.62）mg/（d·m²）,-1.77±10.79（-35.41～17.25）mg/（d·m²）,-0.88±3.65（-14.78～2.85）mg/（d·m²）,1.71±2.65（-2.42～7.35）mg/（d·m²）and 1.12±5.19（-12.79～11.52）mg/（d·m²）.The fluxes of DGT NO₃-N,DGT NH₃-N,Peeper NO₃-N,DGT PO₄-P,Peeper PO₄-P and DGT Fe at vegetation points were respectively0.14±1.36（-3.10～2.99）mg/（d·m²）,0.30±3.48（-4.67～11.60）mg/（d·m²）,3.83±13.65（-12.46～39.94）mg/（d·m²）,-0.05±2.23（-4.84～4.19）mg/（d·m²）,0.52±4.09（-6.74～11.81）mg/（d·m²）and 4.45±17.48（-46.41～27.77）mg/（d·m²）.On the whole,the DGT NO₃-N and DGT NH₃-N fluxes in open and vegetated points were mainly negative（sediments uptake）,and there were significant differences between Mangrove Wetlands and salt marshes.The fluxes of Peeper NO₃-N,Peeper PO₄-P and DGT Fe were positive（released to the overlying water）in the open and vegetative points.DGT PO₄-P flux was mainly negative in the open points and positive in the vegetation points.The nitrogen and phosphorus fluxes in open points of salt marshes and other wetlands were larger than those in mangrove wetlands,but the nitrogen and phosphorus fluxes in vegetation points were smaller than those in mangrove wetlands.In summer,DGT PO₄-P and Peeper PO₄-P were mainly released from sediments to overlying water,which aggravated the eutrophication of overlying water.The direction of bioavailable nitrogen and phosphorus fluxes at the sediment-water interface was basically determined by the concentration gradient of bioavailable nitrogen and phosphorus in overlying water and sediment.When the concentration of nitrogen and phosphorus in sediment was high,nitrogen and phosphorus will be released upward,making the flux positive,and vice versa.With the decrease of latitude of coastal wetlands,DGT NO₃-N and DGT NH₃-N fluxes of salt marshes increased first and then decreased.Peeper NO₃-N,DGT PO₄-P,Peeper PO₄-P and DGT Fe showed different geographic change trends in open and vegetative points:The fluxes of Peeper NO₃-N in the open points decreased with the decrease of the latitude of the coastal wetlands,and increased first and then decreased in the vegetation points;the fluxes of DGT PO₄-P increased first and then decreased in the open points,and increased continuously in the vegetation points;the results showed that the fluxes of Peeper PO₄-P and DGT Fe did not show a regular geographical change trend.The fluxes of DGT NO₃-N,Peeper NO₃-N,DGT PO₄-P and DGT Fe in mangrove wetlands had no obvious change in the open points,but decreased with the decrease of latitude in the vegetation points.NH₃-N fluxes decreased with the decrease of wetland latitude in both open and vegetative points,while Peeper PO₄-P flux decreased with the decrease of wetland latitude in open points.4)There was no significant correlation between DGT and Peeper.The main influencing factors of DGT NO₃-N diffusion flux were water temperature and concentration difference;the main influencing factors of DGT NH₃-N diffusion flux were water temperature,concentration difference,TN concentration in overlying water,comprehensive trophic state index and TOC content in surface sediments;the main influencing factors of DGT PO₄-P flux were DO,Chl a and TOC content in surface sediments,which were significantly positively correlated with DGT NH₃-N flux;the main influencing factor of Peeper PO₄-P flux was concentration difference.In addition,there was no significant correlation between nitrogen and phosphorus fluxes and water content and porosity.