Dual Isotopic Evidence For Nitrogen Biogeochemical Process In The Pearl River Estuary And Northern South China Sea

Nitrogen?N?is an indispensable element for the construction of marine organisms,and its concentration and distribution are related to marine biological productivity.N in seawater exists in two major forms,inorganic and organic.Nitrate is the main form available to marine organisms among them.With the emergence and application of isotope methods,scholars have used them to solve a series of problems,especially the use of nitrate dual isotope techniques,breaking the situation that traditional methods are difficult to explain the problems of nitrogen sources and sinks and biogeochemical processes in the ocean.To date,a majority of related works is focused on the PRE,and there is a lack of a comprehensive investigation in the N processes and cycling in water columns across a larger extent from the upper PRE to the farther south belongs to oligotrophic waters.But its nutrients source were complicated,including dilute water input,kuroshio intrusion and atmosphere deposition.In this study,we sampled different water depths in two regions:from the Pearl River Estuary to the northern part of the South China Sea?during the rainy season?and the northern part of the South China Sea?in summer?.In order to explore the spatial distribution of nitrogen sources and nitrogen conversion in the waters of the Pearl River Estuary and the northern part of the South China Sea,we determined the physicochemical parameters nitrate concentration and its nitrogen and oxygen isotope values.The results are as follows:?1?During the rainy season,nitrates from the remineralization of soil organic nitrogen enter the water increasing with freshwater-seawater mixing.Overall,sources of nitrate include reduced nitrogen?45%?,soil organic nitrogen?29%?,manure and sewage?15%?,and atmospheric deposition?11%?.In nearshore areas,incomplete nitrate assimilation results in relatively heavy nitrate nitrogen and oxygen isotopes,with a contribution of 9%to atmospheric deposition in surface waters.Besides,nitrification might be significant in the middle and bottom waters due to ammonia releases from remineralization of sinking and sedimented OM.In the offshore area,the surface nitrate is almost completely assimilated,but the nitrate nitrogen and oxygen isotopes are not heavy,indicating that the contribution of atmospheric deposition is much larger?20%?.In addition,?¹⁸O-NO3^-exhibited a clear vertical gradient,with a decrease from the surface to the bottom.A coupled nitrification-denitrification process in sediment porewaters was supposed,which left isotope imprints in bottom waters of the offshore area.?2?During summer,higher concentrations of nitrate and heavier?¹⁵N-NO₃^-values were observed in the upper waters of the near-shore area of the Pearl River Estuary,mainly due to manure and sewage?63%?,atmospheric deposition?19%?,soil organic nitrogen?12%?and reduced nitrogen?6%?.For middle layer and the upper layer of the outer zone,low concentrations of nitrate and lighter?¹⁵N-NO₃^-values were observed,but?¹⁸O-NO3^-was heavier,mainly affected by black tide intrusion?60%?,atmospheric deposition?32%?and nitrogen fixation/nitrification?8%?.Complex processes were found in the bottom water,and nitrification and phytoplankton assimilation were the reasons for the heavier?¹⁵N-NO₃^-.