Forms And Distribution Of Nitrogen In The Water And Sediments Of The Pearl River Estuary,China

In the past thirty years,as the rapid development of industry and agriculture,the inputs of Nr have continued to increase in Pearl River Basin.With large amount of Nr inputting the PRE waters through river discharges,eutrophication level was increasing,and caused serious environmental problems,for example bloom and hypoxia.As the transitional zone between the Pearl River and South China Sea,PRE ecosystem plays crucial roles in the removing excess reactive nitrogen,and keeping the ecological balance.In this study,water and sediment samples were collected from PRE with two voyages.By determining of DIN in the water and sedimental pore water,and Ntot,Nex,Nfix and Norg in sediments,and combined to environment parameterssuch as redox potential,depth,temperature and salt,the temporal and spatial distribution characteristics of different nitrogen formsin the water and sediments of PRE were studied.In addition,the relationship of nitrogen source-sink of PRE was investigated.The main results of this study are as following:(1)Based on the chemical reaction of oxidating NH+ 4 into NO-2 by bromite oxidant,a fast and sequential method for determing NH+ 4 was devolved in small volume.In this method,a 2 m L Oak Ridge centrifuge tube was selected as reaction vessel,and the dissolved NH+ 4 can be converted to NO-2with bromite as oxidant.The generated NO-2 was determined with Griess reagent by a Microplate Reader.The absorption blank was relatively low,the calibration curve have rigid linear regression(R2>0.99)in the range of 0~32?mol·L-1.Environmental samples assay demonstrated that this small volume method was well consistent with conventional method(in 25 m L volume)for ammonia determination.In total,this modified method improved accuracy and efficiency greatly,and would be realized a rapid and high-throughput determination of ammonia in field analysis with low cost.(2)The spatial distribution of DIN in the water of Pearl River Estuary was analysised.Water samples were collected in summer and winter.Results demonted that the nitrate(NO3-N)was the main form of the DIN in the PRE water,with an average value of 63.09?mol·L-1,accounting for 3.42% of DIN.The average content of ammonium(NH4-N) and nitrite(NO2-N)was 11.61?mol·L-1and 47.16?mol·L-1,accounting for 9.5% and 38.7% of DIN,respectively.In general,there is a general seaward decrease in DIN,especially nitrate concentration,from the estuary to the shelf,suggesting the high anthropogenic disturbance.The concentration of DIN in summer was higher than in winter.(3)The temporal and spatial distribution characteristics of different nitrogen formsin the sediments of PRE were investated.(i)Nex-NH+ 4is the main form of Nex,and Nex in surface sediments accounts for 3.42% of Ntot.(ii)Norg is the main form of Ntot,with the average content of Norg in surface sediments is 82.8% of Ntot.(iii)The distribution of Nex-NH+ 4 was greatly affected by the shift of environmental parameters.In the upper reaches of the Pearl River waters,Nex-NH+ 4 increased as the depth increasing on the surface sediment,then Nex-NH+ 4concentration decreased with the increasing of depth in the middle,in the end,Nex-NH+ 4accumulated with the deepening of depth;coastal areas Nex-NH+ 4 were generally at a very low level with no significant vertical variation.(iv)The average content of Ntot in the sediments of the Pearl River Estuary was 916.4 mg·kg-1.The distribution of Ntot in the PRE-1 sedment core showed a obvious segmentation characteristics.In the upper segment from suface to 20 cm depth,Ntot remained increasing with the depth.Defferent with upper segment,Ntot was relatively lower in thesegment between 20 cm to 40 cm,and showed a trend of decreasing with the depth increasing.Below 40 cm depth,Ntotshowed a trend of accumulation with the deepening of depth,accountingto the maximum of 1899.3 mg·kg-1.The distribution of Ntot in PRE-2,PRE-3 and PRE-4 showed a tendency of decrease with the depth increases in the vertical direction.Norg distribution is consistent with total nitrogen distribution.(v)There was no obvious change in the vertical distrubution of Nfix in the PRE sediment cores,maintaining at a level of 140 mg·kg-1.(4)The concentration of DIN in PRE water was above 100?mol·L-1,suggesting PRE water has been polluted by human beings' activity.However,the concentration of DIN in the pore waterof surface sedimentswas in the range of 17~30?mol·L-1,indicating the sediment of PREshould be nitrogen “sink” of overlying water.The nitrogen in waters deposited into the sediment was transformed into N2 by series of processes of active microbial metabolism and was output into atmosphere at last,reducing nitrogen emissions from Pearl river to South China sea.Moreover,there should be little nitrogen release from sediments to waters in the PRE,so the possibility of "endogenous nitrogen" pollution is small in short term.In conclusion,althrough the level DIN in PRE water was high because of seriously human activities,the DIN in the sedidments remained relative low level by active microbial processes.As result,most of Nr was outputted from estuarine system into atmosphere with the form of N2.Moreover,the sediment system can make response to the amount of nitrogen discharge.The high amount of nitrogen loading could promote the activity of microorganisms and accelerate the transformation of Nr.As a large natural "reactor",the estuarine systemplays an important role in reducing the nitrogen pollution and maintaining the regional nitrogen balance.