[Simulation of Inorganic Nitrogen Fluxes at the Sediment-water Interface in a Typical Intertidal Zone, Eastern China]

Taking 12 typical intertidal zones along the eastern coast of China as the research object, indoor tide simulation experiments were conducted to measure exchange fluxes of nitrate nitrogen (NO₃^--N) and ammonia nitrogen (NH₄⁺-N) between overlying water and sediments, to investigate their spatial distribution, and to clarify controlling factors such as salinity, temperature, and organic matter. Results showed that the total NO₃^--N flux was -2.91-3.34 mmol·(m²·h)^-1, while the total flux of NH₄⁺-N was -4.36-2.34 mmol·(m²·h)^-1. The average flux, at 12℃ and 35℃, was -0.04 mmol·(m²·h)^-1, indicating that typical intertidal zone sediment is an effective sink for ammonia nitrogen and nitrate nitrogen. There was a significant difference in the spatial distribution of nitrate and ammonia nitrogen fluxes. At 12℃, the higher the latitude, the greater the ammonia nitrogen flux; results for the 25°-35°N intertidal nitrate flux were as follows:<15°-25°N < 35°-45°N at 25℃ and 35℃, while the flux of ammonia nitrogen was 25°N-35°N > 15°-25°N > 35°-45°N. The fluxes of the three intertidal zones decreased with increase in temperature, which controls the coupled nitrification-denitrification taking place in the upper layer of sediment and at the bottom of overlying water. NO₃^--N fluxes first increased and then decreased with temperature at 15°-25°N and 35°-45°N, while NO₃^--N fluxes at 25°-35°N always decreased with temperature. At each latitude, the higher the temperature, the lower the NH₄⁺-N flux. There was no single significant effect of environmental factors on fluxes. Salinity, sediment organic carbon (OC), sediment total nitrogen (TN), concentrations of ammonia nitrogen and nitrate nitrogen in pore water, and bulk density synergistically affected the spatial differentiation of exchanged NO₃^--N and NH₄⁺-N fluxes.