Potential Denitrification In The Shelf Regions And Marginal Sea Based On ¹⁵N Tracers

Inorganic fixed nitrogen(N)is one of the vital nutrients for marine phytoplankton growth and plays an important role in sequestrating atmospheric C02 in marine ecosystem.As one of the largest microbial mediated N sinks,denitrification has long been considered to only take place under suboxic or anoxic conditions.Nevertheless,recently studies suggested that denitrifying bacteria can maintain their activity at much higher dissolved O2 level,or respire anaerobically within the suboxic micro-environments of suspended particle in the oxygenated waters,and therefore contribute potentially to marine fixed-N removal.In this study,three distinct marine habitats covering broad latitudes,including the Bering Shelf and the Chukchi Shelf in the western Arctic,the northern Beibu Gulf in the subtropics and the basin of South China Sea in the tropics,were investigated via the 15N pairing technique to evaluate the possibility of anaerobic denitrification in the oxygenated sea waters.The potential N removal flux through the denitrification,and the effects of particle content,composition and size,and dissolved O2 concentration on denitrification were quantified and discussed.As to the method study,a calibration of N2 contamination from background atmosphere was proposed according to a constant value of 29N2/28N2 in the background air,and the invariant ratio of 30N2 to 28N2 detected through the working curves.Based on this calibration,the concentrations of 29N2 and 30N2 produced during incubation,as well as denitrification rate,can be accurately obtained.After calibrating,the measured 29N2 and 30N2 concentrations were much less variable and the linearity between 15N-N2 concentration and incubation time was better improved.It suggests that the calibration of background N2 is important to a precise measurement of denitrification.15N-isotope pairing incubation was conducted over the Chukchi Shelf for the first time during an Arctic cruise in July 2012.Potentially intensive denitrification was pervasively but exclusively detected within the shallow oxygenated water column ranging from 1.8-75.9 nmol N2 L-1 h-1.The spatial pattern of denitrifying activity follows well with primary production,indicating the fresh organic matter supplied by phytoplankton plays an important role in regulating the denitrification.Under the action of hydrodynamics force,resuspension of benthic sediment coupled with particle-associated bacteria induces an active denitrification process in the oxic water column in the Chukchi Shelf during summer.When extrapolated to the whole Chukchi Shelf,a removal flux of 12.2 Tg N y-1 was estimated,which compensates for the difference between sediment cores incubation(?3 Tg N y-1)and geochemical estimation based on N deficit relative to phosphorous(?16 Tg N y-1).It implies the oxygenated Chukchi Shelf water is a great potential N sink.The particle-associated denitrification was studied in the northern Beibu Gulf during the spring and summer cruises.Our results that the potential denitrification rates in gulf water in spring(ave.3.0 ? 2.6 nmol N2 dm-3 h-1)was much lower than those in summer(ave.14.9 ± 13.0 nmol N2 dm-3 h-1).Different spatial patterns of denitrification were observed between the two seasons.Similar to the Chukchi Shelf,sediment resuspension and particle-association were the key mechanism,for occurring of anaerobic denitrification in the oxygenated water.In summer,particle-associated denitrification accounted for more than 60%of denitrifying extent in the water column.The spatial distribution of denitrification was not only regulated by particle contents,but also their compositions.A positive correlation between the denitrification rates and POC concentration or autochthonous POC was observed,indicating both quantity and quality of POM may regulate the spatial variation in denitrification.In addition,the size spectra of particles had an effect on denitrification with the small size(1.2-10 ?m)more preferentially for denitrification to take place.Based on a relationship between denitrification rate and O2 concentration,as well as comparison to the sediment denitrifying yield,an in situ N removal flux by denitrification was estimated as?4 Gg in the Beibu Gulf in summer,which corresponds to about 25%of the terrestrial inorganic N input during the same period.Denitrifying activity measurement was conducted throughout the water depth from surface to 1500 m in the South China Sea basin among three cruises in summer,autumn and winter.Besides,several kinetic features of denitrification were concurrently characterized in this region.Potential denitrification was pervasively detected among different seasons ranging from 0.1-60 nmol N2 dm-3 h-1.Unlike the two coastal seas,i.e.,Chukchi Shelf and Beibu Gulf,free-living denitrification accounted for the major potential N removal in the South China Sea basin.The spatial distribution of denitrification rate showed that the potential denitrifying rates in the eastern basin were generally higher than those of western basin with 115 °E as a boundary.The potential denitrification was mainly controlled by the POC content in the euphotic zone(0-200 m).During autumn,the maximum denitrification rates were observed both in subsurface(300-400 m)and intermediate(800-1000 m)waters with the former was significantly higher than the latter.The subsurface maximum was proposed to be related to the resuspended particles from shelf and slope sediments transported by the coastal current,while the intermediate maxima might occur due to the lateral transport of land-source particles with more abundant bacteria as well as labile organic matter.The active denitrification was also observed in the subsurface and intermediate waters during summer,but much higher than those measured in autumn.This phenomenon was possibly coupled with enhanced particle fluxes induced by the southwestern monsoon.Based on our results from the three different regions,the potential denitrification is generally distributed among the natural oxygenated sea waters,and contributes significantly to marine fixed-N sink.In the coastal areas,sediment resuspension and particle-association play an important role to induce denitrification.While in the euphotic zone,the spatial and temporal variance of denitrification activity showed a covariant relation to those of production among the three sampling areas,indicating that the autochthonous organic carbon might be more dominated in regulating marine denitrifying potentials.