Distribution characteristics of the porewater inorganic nitrogen and their benthic exchange fluxes in coastal wetlands of Daya bay

In order to study the spatial distribution of the inorganic nitrogen in the sediment and their environmental effects, the contents and the exchange fluxes of DIN at sediment-water interface have been studied during March (dry season) and August (wet season) in 2017 in coastal wetland in Daya bay.The results showed that the average nitrogen content of overlying water near coastal wetland was far higher than that in Daya bay offshore water, which indicated the influence of human activities on the wetland was relatively serious.The river runoff is the main channel of nitrogen input.The average contents of nutrients (NH₄-N、NO₃-N and NO₂-N) were 770.60、7.63 and 7.39 μmol/L in the sediment porewater, respectively.NH₄-N was the major component and accounted for 85.82%~99.67% of DIN. The estimation of the sediment-water exchange fluxes indicated that the average fluxes of NH₄-N、NO₃-N and NO₂-N were 0.09、-0.18 and 0.36 mmol/(m²·d), respectively.Generally, the average exchange flux was 0.41 mmol/(m²·d) in dry season, as the source of DIN providing a lot of nitrogen into the overlying water.The sediment became the sink of DIN in wet season, with average flux of -0.23 mmol/(m²·d).Comparing with the original experiment group, it was found that the average exchange fluxes of NH₄-N、NO₃-N and NO₂-N in the sterilized group decreased by 76.5%, 23.7% and 50.3%, respectively.This indicates that microorganism plays a very important role in nitrogen transformation.