Hydrological Control On Microbial Driven Nitrogen Transformation In Subtropical River-estuary System

Four seaward rivers(Jiulong River,Zhang River,Mulan River and Ao River)in Fujian Province were selected to study the urbanized nitrogen cycling.A series of biogeochemical measurements at different time scales were carried out in the Jiulong River estuary system by setting critical sections,under different hydrological conditions(flood and baseflow),such as high/middle/low flow and spring/neap tide.Samplings were river-estuary synchronized,fixed stations,high frequency,long time and continuous observation,using a variety of molecular biology techniques,such as chemometrics,nitrogen and oxygen isotope traceability and tracer technology,realtime PCR and high-throughput sequencing,to acquire lots of physical chemistry and microbiological data sets.The research focuses on the interaction mechanism of river nitrogen under multiple human activities,the physical-geochemical coupling mechanism of ammonia nitrogen transformation at the river-estuarine interface,and the effects of flood events on the nitrogen cycle and the transport flux to the sea in the riverestuarine system.Major findings are summarized follows:(1)In four small and medium-sized urbanized rivers,ammonium was transformed and removed after the wastewater imported,while river nitrogen fluxes still had an increasing trend to coast.Comparing the observed data of upstream and downstream stations of urban reaches,high ammonium sewage evoked river nitrification to change ammonium to nitrate and N2O.Then this nitrate was reduced by denitrification to N2O and N2.However,the concentrations of inorganic nitrogen(DIN)and dissolved total nitrogen(DTN)still increased after the river passed through urban,as for higher pollution,which implied a net increase in nitrogen flux to sea.The study found that the ratio of nitrogen isotopes of nitrate increased after passing urban,and the contribution ratio of sewage sources estimated by the isotope SIAR model increased from 50%to 60%.Meanwhile,the characteristics of downstream stations are that nitrate concentration decreased,dissolved N2 and N2O increased,and the abundance of nitrifying and denitrifying functional genes increased.These gene abundance had positively correlated with the concentration of suspended particulate matter(SPM).(2)There were significant differences in nitrogen forms,nitrogen functional bacteria abundance and diversity,and key processes of nitrogen cycle in different anthropogenic disturbed reaches(urban,agricultural and reservoir)in Jiulong River catchment.As for a large amount of fertilization,agricultural reach had the maximum nitrogen loading,while its nitrogen potential retention(3.7%PRN2)was lower than urban reach(10.4%PRN2).It is speculated that the processes of nitrification and denitrification mainly occur in agricultural soils rather than in river systems.The concentration of SPM and nitrogen in the reservoir was lower than other reaches,where had higher ratio of free-living nitrogen functional microbes(free-living nitrifers accounted for 69%).During the dry season,the nitrogen potential retention in reservoir was the highest(25.3%PRN2)than urban and agricultural reaches,which was supported by deep and anoxic water environment for denitrification and anammox.(3)Nitrification is a key nitrogen cycle process at the river-estuary interface of the Jiulong River.Nitrifiers and its nitrogen removed amount are controlled by tidal range and particulate matter.In baseflow estuary areas,AOB is the dominant ammonium-oxidizing microbes,where the abundance of NOB is relatively lower.This incomplete nitrifying process leads to nitrite accumulation in the maximum turbidity zone.High value of suspended particulate matter and particle-attached nitrifiers were observed in bottom water,spring tide,high tide and the maximum turbidity zone.This is the hot spots of estuary ammonium removal by nitrification,particularly in the warm wet season.The river-estuarine continuum had complex biogeochemical processes under the gradients of salinity and particles.During the spring tide,the ammonium flux was decreased half from the river section(9.8～22.4 t d-1)to the sea section(5～11.4 t d1).Through the integrated analysis of the data of 39 historical voyages,the linear relationship of ammonia nitrogen removal amount=0.56×tidal range-150.56(R2=0.59)was obtained.(4)Under the condition of rainstorm and flood pulse,the source,form and community of nitrogen functional microbes in the tidal river to estuary continuum of the Jiulong River are quite different,resulting in doubled DIN exported to coast during the storm event.Relatively more ammonium from river to the estuary(the ratio of NH4-N/DIN increases from 11.4%to 13.5%)during the Flooding,During the Rising and Falling,the abundance of NOB increased dramatically,resulting in the state of nitrification changed from incomplete nitrification(nitrite accumulated)to complete nitrification(without nitrite accumulated).As for most of denitrifers belong to particleattached(83%),decreased SPM and increased DO in water column would limit denitrification.Apart from river inputs,there was an additional increase of 40±8%of DIN supplied within the tidal river and estuary(far more than spring tide in baseflow),during the storm event,particularly for ammonium with 36.5 t d-1.These results indicate that human activities and climate change significantly affect the nitrogen cycling at the sea-land boundary.The increase of population(urbanization)in coastal areas promotes the river microbial nitrogen removal,while the nitrogen pollution far exceeds the self-purification capacity of rivers.Land use types(urban,agricultural,reservoir)effects river nitrogen concentration,form and dominant species of nitrogen functional microbes.The estuarine maximum turbidity zone is a naturally formed filtering barrier of terrigenous nitrogen pollution by gases N2O and N2 produced by evoked coupled nitrification and denitrification,particularly in spring tide,high water temperature and turbidity water,to cut down the nitrogen flux into the sea.However,the flood pulse changed this nitrogen removal mechanism by increased nitrification and decreased denitrification.Large amount of high-ammonium wastewater and high-nitrate groundwater are caused by rain-runoff scouring in the watershed.The part of nitrogen buried in tidal river and mangrove wetland sediments could also be scoured to the river-estuary continuum.The storm event increased the flux of DIN to the sea,or aggravates the eutrophication in the coastal area.