Hydrological Control On Ammonium Nitrogen Transformation And Export Across The Jiulong River Estuary

Estuaries currently considered as hot sposts on the interaction between terrestrial and marine ecosystems under global climate change.Affected by river discharges and marine tides,hydrology and nutrient biogeochemical processes in the estuaries are highly dynamic.It is still unclear how estuarine hydrology influences the concentration,composition and flux of nutrient to the coast area.Understanding the transformation and export of inorganic nitrogen(N)especially ammonium N and its hydrological control mechanisms is important to support the integrated nutrient pollution management from land to sea.A medium subtropical estuary--the Jiulong River Estuary(JRE)in the Fujian Province in southeast China were selected as an example to examine the sources,transformation and export of inorganic nitrogen across the river-sea interface(including the estuarine turbidity maximum zone,ETM),to understand the mechanism of hydrological control on the non-conservative behaviours(addition and removal)of ammonium N under different hydrological conditions,and to estimate the function of estuaries on affecting the fluxes of inorganic N from the river to the coastal area.Based on those previous studies,stable nitrogen and oxygen isotope technique by multiple dissolve inorganic nitrogen species were applied in the comparison of nitrogen biogeochemical processes in the different channels of the JRE.Major findings are summarized as follows:(1)The removal of ammonium N(nitrification and particulate adsorption)in the water column of the estuary was mainly controlled by the status of tidal currents and suspended particles,while the addition of ammonium N was mainly from the release of sediment porewater caused by sediment erosion during strong water currents.Nitrite accumulation was constantly observed in the low salinity area of the JRE,which was related to the higher ammonium oxidation rates(7.40?25.16 ?mol N/(L·d))than the nitrite oxidation rates(0.48?0.82 nol N/(L-d)).There was larger nitrite addition during spring tides(16.5?25.8 ?mol/L)than the neap tides(4.0-14.4 ?mnol/L),which indicated that there was stronger nitrification during spring tides than the neap tides.Tidal strength could effectively change the magnitude and composition of suspended particles in the water column.During spring tides,the total suspended materials(76.3?363.6 mg/L)was significantly higher than that during the neap tides(16.5?115.0 mg/L).There was also more fine particles during the spring tides(87%)than neap tides(82%).A higher concentration of suspended particles in the water column could enhance nitrification and ammonium adsorption onto particles,leading to stronger ammonium removal.Sediment pore water in the JRE was enriched in ammonium(116.3?653.4?mol/L)produced by mineralization of organic nitrogen,which was released during the sediment resuspension.In addition,the ammonium storage in the sediment could also affect the amount of ammonium release from the sediment.Previous strong sediment disturbance during early flooding period caused a large amount of ammonium release leading to decreasing storage of ammonium in the sediment.Therefore,there would be less ammonium addition during the late flooding period during sediment resuspension by the strong tidal currents during ebb and flood.The ammonium increase during freshwater period(late ebb)might due to the porewater inputs from the adjacent wetland and sewage discharge from the city around the ETM.(2)The magnitude of ammonium nitrogen in the estuarine water column is controlled by the balance of addition and removal processes.Under baseflow conditions,ammonium tended to be added in the estuary during or close to neap tides with smaller tidal range and less suspended particles leading to weaker nitrification and ammonium adsorption but still considerable ammonium release from the sediment;while it tended to be removed during or closed to spring tides with larger tidal range and more suspended particles enhancing the stronger nitrification and adsorption than the release of ammonium from the sediment erosion by tidal currents during ebb or flood period.Under flood-affected periods,ammonium N was found significantly added(9.6-71.5 ?mol/L)in the estuary,especially during the first storm of the year,due to stronger sediment erosion by large flood current.But too large river discharge would significantly reduce the water residence time,which limited the N transformation like nitrification driven by microbes.In addition,there were both strong nitrification in the JRE during the summer with high temperature,leading to the removal of ammonium and nitrate(-6.9?13.4 ?mol/L and-25.9?37.2 pmol/L);while low temperature and stronger tidal strength during the winter could still maintain considerable nitrification rates,but inhibited the denitrification rates,leading to less ammonium addition(-2.3?-5.2 ?mol/L)and nitrate addition(45.1-48.5?nol/L).(3)The magnitude of all N fluxes across the Estuary-Coast Interface(ECI)is controlled by the N fluxes across the River-Estuary Interface and the estuarine retention capacities.Under flood-affected conditions,293.0?374.2 t/d of dissolved inorganic nitrogen(DIN)(including 5.9?35.5 t/d of ammonium N)were exported to the sea,which was 2-4 times larger than the DIN fluxes of 68.9?176.6 t/d(including 52.4?134.8 t/d of ammonium N)under baseflow conditions.Large flood events could increase M fluxes of inorganic nitrogen but reduce the estuarine retention capacities,leading to more ammonium and DIN fluxes to the sea.Moreover,tidal ammonium N fluxes during the freshwater period contributed over 85%of the total tidal fluxes.With longer duration of this period caused by stronger tidal asymmetry,there were a larger total tidal ammonium N and DIN fluxes.(4)Estuarine dissolved inorganic nitrogen was mainly originated from river nitrogen inputs.Dual-isotope analysis of nitrate indicated 56%?58%of nitrite contributed by manure and sewage in the JRE.The discharge of high 815N-NH4 contaminated water from the South River significantly influenced the 815N-NH4 values of surface water in the middle JRE estuary.The major biogeochemical processes differed in the different channels of the JRE.Nitrification was dominated in the southern channel evidenced by raised ?15N-NH4 but lowered ?15N-NO2,?15N-NO3 and ?18O-NO3.Nitrification and denitrification were both important in the northern channel evidenced by much-raised ?15N-NH4,raised ?15N-NO2,?15N-NO3 and?18O-NO3.In conclusion,river discharges(especially storm discharge)was the major hydrological factors controlling the estuarine inorganic N exports,while tidal currents could affbect the release of inorganic nitrogen stored in the estuaries and the intensity of major biogeochemical processes like nitrification and denitrification by controlling the intensity of sediment resuspension and magnitude of suspended particles.