Nitrogen Losses In Sediments Of Yangtze Estuary And Adjacent Coastal Areas:Spatiotemporal Variations,Controlling Factors And Environmental Implications

Global reactive nitrogen?N?has increased dramatically in coastal marine ecosystems over the past decades and caused numerous eco-environmental problems.Nowadays,N biogeochemical cycling in coastal marine has become a hot research topic,which has be widely concerned by several international projects,such as Internatio na l Geosphere-Biosphere Programme?IGBP?,Land-Ocean Interactions in the Coastal Zone?LOICZ?,Integrated Marine Biosphere Research?IMBER?and Future Earth.Coastal marine sediment plays a critical role for N losses via denitrification and anaerobic ammonium oxidation?anammox?,and is a potential source of nitrous oxide?N₂O?.However,both the magnitude and contributions of the denitrification,anammox,and N₂O production in sediments are still unclear,causing uncertainty in defining the N budget for coastal marine ecosystems.Here,potential rates of N-loss,and their contributions and controlling factors were investigated in surface sediments during six cruises from 429 sites in the Yangtze Estuary and adjacent coastal areas.The main findings are as following.?1?Measured potential rates of denitrification,anammox,and N₂O production as well as N₂O:N₂ ratio ranged from 0 to 36.11 nmol N g^-1 h^-1,0 to 6.46 nmol N g^-1 h^-1,0to 138.94 pmol N g^-1 h^-1,and 0 to 1.97%,respectively.All of these rates varied spatially and seasonally,but the contribution of anammmox to total N₂ production?%Anammo x?and N₂O:N₂ ratio only varied spatially.The N-loss rates were generally showed an order of ZMCM>YEM>SAN,and which were higher in summer than in winter with a marked seasonal difference.?2?The copy numbers of nirS,nosZ and Anammox 16S-rRNA genes were in the range of 1.86×10⁵–3.92×10⁶ copies g^-1,8.90×10⁴–2.52×10⁶ copies g^-1 and 1.76×10⁴–1.02×10⁶ copies g^-1,respectively.Significant spatial differences in abundance of all genes,and significant seasonal variations were found in copy number of nosZ and Anammox 16S-rRNA genes.?3?The statistical analysis shows that both organic carbon and nitrate?NO₃^–?were important factors controlling potential rates of N-loss,N₂O:N₂,and%Anammox.Also,the genes abundance of N-loss positively covaried with the associated rates,and which were remarkably affected by organic carbon.?4?The activities and genes abundance of N-loss were stimulated effectively by organic carbon?Chlorella algae?addition,especially for denitrification,but no obvious variation was found in NO₃^–addition treatment.Also,the rates of denitrification and N₂O production,as well as nirS genes abundance were influenced more by marine organic carbon than by terrestrial organic carbon.These results imply that marine organic carbon induced by eutrophication plays a critical role in stimulating reactive N removal and increasing N₂O production in summer.?5?The preferentially degraded amino acids can indicate the activities of N-loss better than organic carbon and total nitrogen.Some pigments can also been better indications of N-loss activities than amino acids,and those pigments are labile in summer.?6?The sediment N-loss caused by denitrification and anammox in this study area were estimated at approximately 2.2×10⁶ t N yr^-1 and 4.6×10⁵ t N yr^-1,respectively.These processes serve as potentially important sinks for DIN and may play an important role in maintaining the eco-environmental health in this coastal marine ecosystem havily influenced by NO₃^–-laden rivers.However,about 8×10³ t N yr^–1 N₂O was emitted from bottom sediments annually,and which remain a major source of uncertainty in the global N₂O budget.?7?Considering the systematicness and comprehensiveness of geography,other key N cycling processes?including N mineralization and immobilization,nitrificat io n,N fixation and DNRA?were clarified to provide the basic theoretical data for controlling the N pollution,and then to maintain the eco-environment health in this ecosystems.Meanwhile,we also measured the microbial production and removal pathways of NH₄⁺and NO₃^–in sediments to quantify the production and removal amount of NH₄⁺and NO₃^–and understand the environmental factors regulating these N transformation processes are critical in evaluating the N budgets and maintaining the eco-environmental health in coastal marine ecosystems.?8?Here,for the first time use membrane inlet mass spectrometry?MIMS?to determine potential rates of N mineralization and immobilization,as well as N fixat io n in the sediments of Yangtze Estuary and adjacent coastal areas,thus the application of MIMS for N cycling was promoted in various ecosystems.