Distribution And Control Factors Of Ammonia Oxidation Rate In The Northern South China Sea In Summer

Nitrogen,an important limiting factor of marine biological productivity,plays an important role in marine biological geochemistry circulation and other elements cycle(especially for carbon and phosphorus).Nitrification is an important process of marine nitrogen cycle,links the lowest reduction state(NH4+)to the highest oxidation state nitrate(NO3-)and regulates the distribution of nitrogen species.Ammonia oxidation(AO),which consumes oxygen and produces hydrogen ion,is the first step of nitrification.Thus the AO can enhance oxygen deficit and ocean acidification.Furthermore,the greenhouse gas,N2O,is a byproduct of AO,which can feedback on climate.Hence,it is a quite necessary and important to understand the AO process in marine ecosystem.By using the 15N isotope labeling method,this study focused on the(AO)rate for the first time in the northern south china sea(NSCS)during the summer of 2016.We carried out the long time series incubation experiments and the AO determination method comparison experiments(bacteria method and chemical method),what's more,we investigated the spatial distribution of ammonia oxidation rate,discussed the effect of Kuroshio,cold eddy,upwelling and nepheloid layer on the AO rate,also discuss the effect of particles on AO rate.For the AO rate determination,we found that the bacteria method has a good consistency with the chemical method.However,the chemical method has great accurate precision when the AO rate lower than 0.02 nmol L-1 d-1.Additionally,the rate keeps constant during the incubation time based on the long time series incubation,indicating that the change of community structure and the influence of the dilution effect caused by organic matter mineralization was insignificant.The AO rate ranged from 0.001 nmol L-1d-1 to 133.524 nmol L-1 d-1 in the NSCS with lower rate in the surface layer,which was mainly caused by light inhibition.At the same time,NH4+ is also a competition substrate for phytoplankton.However,ammonia oxidizers are more competitively than phytoplankton for the substrate with the depth increased since light inhibition decreased.Consequently,the AO rate maximum was often observed at the depth where light intensity is about 5-10%of surface light intensity.Besides,the primary nitrite maximum(PNM)layer always exists at the same depth,indicating that AO process is an important contributor to form the PNM layer.Finally,the AO rate decreased exponentially under the euphotic zone since the decrease of organic matter supply,which leads to the decline of NH4+ substrate.No significant correlation was observed between AO rate and AOA amoA gene abundance.We inferred that the AOA in the ocean is mainly consist of two types,the water column group A(WCA)and the water column group B(WCB).WCA is usually distributed in the upper ocean and has a good correlated with ammonia oxidation rate,whereas WCB is mainly distributed below the euphotic zone where the ammonia oxidation rate is low,has unstable activity and small contribution to ammonia oxidationBy using the isopycnal mixing model,we found that the the NH4+ concentration and AO rate were both significantly higher at the stations which have higher Kuroshio share.The degradation of organic matter was accelerated when the Kuroshio water with high temperature,high salt,high DON,low nutrient mixed with the upper NSCS water,subsequently improving the NH4+ inventory and enhancing AO.Combine potential density section figure,sea surface height anomaly figure and sea surface chlorophyll figure,we confirmed that the AO rate has response to the cold eddy and upwelling.The integrated upper 100 m AO rate in cold eddy station was 14.9 times than that in the SEATS.We inferred that the primary production was stimulated by the cold eddy and upwelling,leading to more organic matter mineralization,thus supply the substrate for AO,what's more,the upwell of high NO3-water mitigated the competition of phytoplankton for NH4+.We found the nepheloid layer in the 630 m of the D5 station based on the higher TSM and PON concentration,and the higher POC ?13C.At the same time,the AO rate also existed a peak at 630 m,which rate is 4 times higher than that in the upper layer(480 m).The percentage of particle-associated AO rate to bulk AO rate is also higher than upper layer,the particle-associated AO rate is 0.012 nmol L-1d-1,which is twelve times higher than that in the 480 m.The results show that the particles in the nepheloid layer obviously stimulate the AO rate,although the rate is still very low.The AO rate of particles in the ocean are only accounted for a small percentage(the average percentage of Free-living rate and Particle-associated in whole cruise are 68%and 32%,respectively)compared with the reported the nearshore results.The genetic data also shows that the Free-living ammonia oxidizing archaea abundance is larger in the ocean.This is mainly caused by the low concentration of particles in the ocean.This study promoted the recognition of temporal and spatial distribution of ammonia oxidation rate.