Nitrogen Removal Processes And Influencing Mechanisms In Coastal Tidal Flat Wetlands

Coastal wetlands are the critical zones between the ocean and the land,which have important ecological functions such as intercepting land-derived materials and improving water quality.Affected by the interaction of land and sea,the physical,chemical and biological factors in these regions are complex and changeable.Coastal wetlands are easily disturbed and destroyed by human activities,and are often regarded as typical ecologically fragile zones and sensitive areas for environmental changes.The increase of reactive nitrogen has caused a series of ecological and environmental problems in the coastal zones.The nitrogen loss processes mediated by the microorganisms can convert bioavailable reactive nitrogen into nitrogen gas,which have important ecological significance for decreasing the nitrogen load of coastal wetlands.The microbe-driving nitrogen loss processes mainly include denitrification and anammox,and the nitrogen loss processes can be divided into coupled nitrification-nitrogen loss and uncoupled nitrogen loss processes,depending on the different sources of the substrate nitrate involved in the nitrogen removal processes.At present,a large number of relevent studies have been carried out to investigate the denitrification and anammox processes of coastal wetlands at home and abroad,but these studies mainly focused on a small area.In contrast,few studies have been conducted to examine the nitrogen loss in coastal wetlands across the different latitudes.Thus,the understanding of the coupling of denitrification and anammox with nitrification processes is very limited in coastal wetlands across different latitude gradients.In this context,the core incubation experiments in combination with isotope tracing technology were conducted to investigate the denitrification and anammox processes in coastal wetlands of China across the latitude gradients,and their coupling mechanisms with nitrification at the sediment-water interface,and the community composition,distribution and abundance of nitrogen-removing microorganisms were also explored using high throughput sequencing and real-time quantitative PCR(q PCR).In addition,taking the Changjiang(Yangtze)River Estuary as a typical study area,this study explored the response of nitrogen loss processes to salinity change in coastal wetlands in combination with field and indoor control experiments.This study can strengthen the understanding of nitrogen removal processes and its influencing mechanisms in the coastal tidal flat wetlands environments,and provide basic data support for the protection of coastal environments.The main results obtained are as follows:(1)The fluxes of ammonium,nitrate and nitrite ranged from-142.21 to 37.10?mol m^-2 h^-1,from-302.21 to 148.45?mol m^-2 h^-1,and from-48.21 and 20.54?mol m^-2 h^-1,respectively,at the sediment-water interface in coastal wetlands of China.Among them,the variation range of the nitrate flux was maximal,and the nitrite flux showed the minimal range of variation.There were significant temporal and spatial changes in ammonium and nitrate fluxes across the latitude gradients.The migration characteristics of inorganic nitrogen at the sediment-water interface showed that the sediments overall were sinks of inorganic nitrogen in China's coastal wetlands,which played key roles in reducing the content of inorganic nitrogen in the overlying water.(2)The denitrification rates of sampling stations in coastal wetlands of China ranged from 3.36 to 60.93?mol N m^-2 h^-1,and the anammox rates were within the range of0.73 to 12.01?mol N m^-2 h^-1.Denitrification was predominant in total nitrogen removal,while anammox also contributed substantially to total nitrogen loss(accounting for 9.4-43.6%of total nitrogen gas production).The spatial variation characteristics of denitrification and anammox rates were generally characterized by higher nitrogen removal processes rates at the sampling stations with more eutrophication.The contribution of anammox to total nitrogen removal increased with the increase of latitude.Denitrification was mainly affected by water salinity,water nitrate content and sediment ammonium content in coastal wetlands of China,while anammox was mostly affected by water ammonium content,water nitrate content,sediment ammonium content and sediment nitrite content.On the whole,the availability of nitrogen substrates in water and sediment was the key environmental factor regulating the nitrogen loss rates of coastal wetlands in China.(3)The coupled nitrification-nitrogen loss rates and the uncoupled nitrogen loss rates varied from 1.98 to 14.93?mol N m^-2 h^-1,and from 1.68 to 67.12?mol N m^-2 h^-1,respectively,in coastal wetlands of China.As a whole,the contribution of uncoupled nitrogen removal to total nitrogen loss was higher than that of coupled nitrification-nitrogen loss,especially at sampling stations with severe eutrophication such as the Pearl River Estuary and the Changjiang River Estuary(the contribution of uncoupled nitrogen removal to total nitrogen removal could be more than 70%).The uncoupled nitrogen removal rates and the contribution of uncoupled nitrogen removal to total nitrogen loss were mainly affected by water salinity,nitrate,ammonium and sediment ferrous iron.(4)The majority of the known denitrifying nir S gene sequences obtained from the sampling sites of coastal wetlands in China belonged to the Proteobacteria(99.30%).Among them,the?-Proteobacteria accounted for the largest proportion(78.17%)of the Proteobacteria,followed by the?-Proteobacteria and the?-Proteobacteria,comprising 16.97%and 4.16%,respectively.The other known sequences were affiliated to the Firmicutes and the Actinobacteria,accounting for 0.66%and 0.04%,respectively.The results of principal coordinate analysis showed that there were obvious spatial changes in the composition of denitrifying microbial community in the sampling study areas of coastal wetlands.The structure and distribution of denitrifying microbial community were mainly influenced by water nitrate content,water p H,and sediment ferrous iron.For anammox bacteria,most of the anammox hzo gene sequences obtained in this study belonged to the unclassified anammox bacteria.In the sampling study areas of coastal wetlands,there were significant spatial differences in the community composition and distribution of anammox bacteria.In addition,there was no significant relationship between the physico-chemical factor determined in this study and the composition community of anammox bacteria,which implied that the composition and distribution of anammox bacterial community might be affected by the comprehensive impacts of multiple environmental factors.(5)The denitrifying nir S gene abundance varied from 8.81×10⁷ copies g^-1 to2.08×10⁹ copies g^-1,and the range of anammox hzo gene abundance was 5.40×10⁶copies g^-1 to 2.83×10⁸ copies g^-1 in coastal wetlands of China.The abundance of denitrification nir S gene in coastal wetlands was significantly higher than that of anammox hzo gene.There were significant correlations between denitrification rate and denitrification functional gene abundance,and between anammox rate and anammox functional gene abundance,which indicated that the nir S gene and hzo gene abundances also played important roles in regulating the activities of denitrification and anammox.(6)The denitrification rates ranged from 0.69 to 85.03?mol N m^-2 h^-1,while the anammox rates ranged from 0.19 to 7.87?mol N m^-2 h^-1 under the salinity gradients of the Changjiang River Estuary.On the whole,the denitrification and anammox activities decreased,with the increase of salinity.The correlation analysis displayed that denitrification was mainly affected by water salinity,nitrate and nitrite,and sediment ferrous iron,ferric iron and organic carbon,while anammox was mostly influenced by water salinity,nitrite,and sediment ferrous iron and organic carbon.Denitrification played a dominant role in the total nitrogen removal,overall,contributing to more than85%of the production of total nitrogen gas,and anammox played an important role in the removal of reactive nitrogen.The salinity control experiments showed that the increase of salinity in the fresh-water area generally promoted the rates of denitrification and anammox,but in the salt-water area,the increase of salinity had no significant effects on the rates of denitrification and anammox.