Research On Nitrogen Reduction Processes In Fujian Mangrove Sediments Using Nitrogen Stable Isotope In China

In recent years,a large number of nutrient discharge intothe aquatic system due to frequent human activities,causing serious eutrophication,threatening the growing of aquatic organisms,polluting drinking water,and destroying the recreational use of water at the same time.When the nitrogen discharging into system cannot be removed or stranded through the biogeochemical effects,excessive nitrogen will be discharged into the estuary coastal zones and into the continental shelfin the endas an important nitrogen transfer.However,the excess nitrogen will cause eutrophication of coastal waters and a series of environmental problems.Mangrove areopen estuaries intertidal ecosystems distributed in tropical and subtropical coast.Its habitat,coastal tidal wetland,as an important component of the estuarine sedimentation area,is an important source,sink converter of carbon,nitrogen,phosphorus,becoming the hot spot of research.The main research object ofthis study is the mangrove sediments in different areas of the southeast coastal in China,monitoring analysis through the ¹⁵N stable isotope technology,determinating denitrification and DNRA rates of mangrove sedimentthrough the laboratory incubation,preliminarily discussing the mainly nitrogen reduction process in southeast coastal mangrove sediments in China andthe end-result of N inputing mangrove sediments system,analyzing the main influence factors of the denitrification and DNRA,and understanding the specific functions of DNRA in mangrove sediments.The main research conclusions are as follows:（1）Average concentrations of DIN of Jiulong River estuary mangrove sediment pore water in spring,summer and autumn are 99.84 ± 108.96 μmol·L-1,28.34 ± 13.13μmol L-1 and 192.20 d± 43.42 μmol L-1,respectively.Seasonal change rule:autumn>spring>summer.Average concentrations of DIN of Zhangjiang estuary mangrove sediment pore water in autumn is 357.27 ± 40.00μmolL-1 and 43.54 ± 11.47μmol L’1 of Shacheng Bay mangrove.Spatial distribution rule:Jiulong River estuary>Zhangjiang estuary>Shacheng Bay.NH4+-N is the main form of DIN in sediment pore water,and the second is NO3--N.NO2--N has no obvious change.（2）Average concentrations of TN of Jiulong River estuary mangrove tidal waterin autumn is 228.49 ± 19.74 μmol·L-1,80.56 ± 57.31 μmol·L-1 of Zhangjiang estuary and 100.32 ± 11.92 μmol-L-1 of Shacheng Bay.Spatial distribution rule:Zhangjiang estuary>Jiulong River estuary>Shacheng Bay.Average concentrations of TP of Jiulong River estuary mangrove tidal waterin autumn is 13.52 ± 5.86 μmol·L-1,13.74 ± 3.05μmol·L-1 of Zhangjiang estuary and 5.62 ± 1.01 μmol ·L-1 of Shacheng Bay.Spatial distribution rule:Zhangjiang estuary≈Jiulong River estuary>Shacheng Bay.Average concentrations of DIN of Jiulong River estuary mangrove tidal waterin autumn is 377.94 ±45.95 μmol-L-1,478.26 ± 121.79μmol·L-1 of Zhangjiang estuary and 108.82± 39.14 μmol·L-1 of Shacheng Bay.NO3--N is the main form of DIN in sediment pore water,NH4+-N the second and NO2--N the lowest.（3）Average concentrations of dissolved N2O of Jiulong River estuary mangrove tidal waterin summer and autumn are 59.96 ± 13.84 μmol·L-1 and 118.76 ± 36.28μmol·L-1 respectively.Seasonal change rule:autumn>summer.Average concentrations of dissolved N2O of Zhangjiang estuary mangrove tidal waterin autumn are 197.53 ±125.41 μmol·L-1,and12.64 ± 0.90 μmol·L-1 of Shacheng Bay.Spatial distribution rule:Zhangjiang estuary>Jiulong River estuary>Shacheng Bay.（4）Average denitrification rates of Jiulong River estuarymangrove sediment is 199.16 ± 103.63 μmol·m-2·h-1,90.06 ± 111.03 μmol·m-2·h-1 of Zhangjiang estuary and 77.59 ± 42.14 μmoh·m-2·h-1 of Shacheng Bay.Denitrification rate is highly significantly correlated with sediment salinity and pore water NO3--N concentrations（P<0.01）,significantly associated with pH and bulk density（0.01<P<0.05）,and have no significant difference with sediment temperature,moisture content and C/N ratio（P>0.05）.（5）Average DNRA rates of Jiulong River estuary mangrove sediment is 20.00 ±17.12 pμmol·m-2·h-1,8.87 ± 12.00 μmol·m-2·h-1 of Zhangjiang estuary and 0-95 ± 0,36μmol·m-2-h-1 of Shacheng Bay.DNRA rate of Jiulong river estuary and Zhangjiang estuary is higher than Shacheng Bay.DNRA rate is highly significantly correlated with sediment temperature,sediment salinity and pore water NO3--N concentrations（P<0.01）,significantly associated with pH（0.01<P<0.05）,and have no significant difference with bulk density,moisture content and C/N ratio（P>0.05）.（6）NO3--N reduction rates of Jiulong River estuary mangrove sediment are higher than Zhangjiang estuary and Shacheng Bay.On the whole,denitrification is the major N reduction process in mangrove sediments,and denitrification rate is 80.43-98.92%of NO3--N reduction rates.Nitrogen in mangrove system remove from system mainly through denitrification.