| As an important wetland ecosystem,mangrove can support numerous ecosystem services,including mediating nutrient cycling in mangrove-estuary system and carbon sequestration and storage(blue carbon).Mangrove sediments may serve as a nitrogen source or sink to the hydrosphere and atmosphere at both regional and global scales.However,major mechanisms controlling the connection between the mangrove and the adjacent tidal creek and estuary(nitrogen cycling in sediments and outfluxing)remain unclear.A multidisciplinary study based on intensive investigation,time-series observation,detailed sediment profiling,multi-isotopes analysis and microbiological identification was conducted for the Yunxiao mangrove reserve and Zhangjiang Estuary in Southeast China.The objectives of the study were to explore the hydrobiogeochemical controls on nitrogen exchange between the mangrove and tidal creek and the mechanisms of nitrogen cycling in sediments,further to estimate nitrogen fluxes across mangrove and estuary interface,revealing major factors shaping mangroves as a DIN source or sink.Th e main findings and conclusions are summarized as follows.Magrove served as the source of NH4-N and the sink of NO3-N,which was driving by tidal pumping.There was no obvious difference about NO3-N concentrations between flood tide and ebb tide,but NH4-N concentration in ebb tide was higher than flood tide.Inorganic nitrogen(NH4-N,NO3-N and NO2-N)showed conservative behaviors along the river estuary gradient.In a given salinity,NH4-N in the creek was greater than in the estuary during ebb tide,while NO3-N in creek was generally lower than in the estuary,indicating mangrove served as the source of NH4-N and the sink of NO3-N,which was mainly driven by tidal pumping.There were seasonal variations about nitrogen concentrations in tidal creek.NH4-N,NO2-N,DON and DTN concentrations were highest in spring while lowest in fall,showing the influence of seasonal variation of bio-uptake.The fluxes across mangrove and estuary interface were related to nitrogen forms and showed an obvious seasonal pattern.Upstream effluents(e.g.,sewage and aquaculture pond drainage)had a significant effect on nitrogen cycling and lateral fluxes.NO3-N was the main form of nitrogen,followed by NH4-N.NH4-N and NO3-N concentrations were higher in spring while lower in summer.The sediments connecting mangrove to tidal creek were important interface for the nutrient exchanges between mangrove and the marine system,and the porewater in mangrove sediments flowed into tidal creek through the interface during ebb tide,leadinsg to higher concentrations of NH4-N,dissolved eases(CO2,CH4 and N2O)and lower concentrations of NO;-N flowed into mangrove from estuary,and the fluxes between mangrove and estuary were 68.73 kg m-2 yr-1,,33.18 kg m-2 yr-1 and 141.67 kg m-2 yr-1.NH4-N flowed from mangrove to estuary,and the flux in spring(13.6 kg m-2 yr-1)was higher than in summer(9.1 kg m-2 yr-1 and 10.1 kg m-2 yr-1).Mnagrove served as the source of NH4-N and the sink of NO3-N.Overall DTN had fluxes from estuary to mangrove and mangrove served as the sink of DTN.DRP flowed from mangrove to estuary(the fluxes were 2.5 kg m-2 yr-1,2.2 kg m-2yr-1 and 3.3 kg m-2 yr-1 respectively in three observations),indicating mangrove served as the source of DRP.During low tide when there was effluent discharge,the average concentrations of ammonium and organic nitrogen increased by 243%and 22%,and DRP,DOP and TPP increased by 216%,234%and 258%respectively compared with concentrations without sewage discharge.The fluxes from mangrove to estuary also increased significantly.Mangrove sediments are rich in organic matter,and there is strong mineralization in anaerobic environment,coupled with dissimilatory nitrate reduction to ammonium(DNRA),leading to the accumulation of ammonium in pore water.Strons;denitrification leads to the removal of NO3-N from pore water.The processes of nitrogen transformation(incomplete denitrification)and carbon-nitrogen coupling transformation(mineralization and respiration)in sediments are accompanied by the generation of greenhouse eases(CO2,CH4 and N2O).Strong mineralization led to the decrease of organic carbon and the increase of inorganic carbon,along with the accumulation of NH4-N.Average NH4-N concentraion in pore water was highest in spring(April)and lowest in fall(October)caused by the difference of bio-uptake.Strong denitrification occurred in anaerobic sediments and the potiential rate was 25.82μmol h-1 kg-1,contributing to 90%-97%of nitrate removal and leading to low NO3-N and NO2-N concentraions.Anammox contributed to a small amount of nitrogen removal in sediments with a potential rate of 0.35-3.18 mol h-1 kg-1,and DNRA contributed to NH4-N accumulation with a potential rate of 0.58-5.1 1 mol h-l kg-1 CO2,CH4 and N2O were released during nitrogen cycling processes.Isotope values showed that the main source of N2O was incomplete denitrification.The study provides a first glimpse into the hydrogeochemical process of nitrogen cycling and lateral exchange in the mangrove-estuary system as well as the main processes controlling nitrogen transformation in sediments based on intensive investigations across the mangrove-creek-estuary continuum,time-series measurement of creek water with multi-isotopes analysis,detailed sediments profiling and microbiological identification.This study could provide theoretical and scientific support for the management of coastal wetland systems and the response to climate change. |
