Factors controlling denitrification of mudflat sediments in Ariake Bay, Japan.

To investigate the seasonal variation of denitrification rate (DR) and clarify the controlling factors of denitrification in the mudflat sediments of Ariake Bay, we conducted field surveys biweekly each month from April 2006 to January 2008. NH4 +-N porewater concentration increased from summer to autumn due to the organic material mineralization under higher sediment temperatures. The seasonal pattern of NH4 +-N flux between sediments and overlying water interface indicated that the mudflat sediments were a source of NH4 +-N in summer. NO3 −+NO2 −-N porewater concentrations were low, ranging from 0.53 to 11.46 μM, and mudflat sediments were sinks of NO3 −+NO2 −-N throughout the year. The mean number of denitrifiers tended to increase in July–September (2188–75,057 MPN g−1) and to decrease in March–May (500–3740 MPN g−1). DR tended to increase in summer, ranging from 76.03 to 990.21 μmol m−2 day−1, and to decrease in winter, ranging from 25.01 to 206.07 μmol m−2 day−1. There was no significant correlation between DR and denitrifier number. Environmental factors influencing DR during the investigation period were determined by multiple regression analysis with the stepwise method. The results indicated that NO3 −+NO2 −-N flux was an important factor in denitrification of mudflat sediments in Ariake Bay. Denitrification was depended on nitrate diffusing from overlying water into sediments under reduced sediment conditions during summer-mid-autumn. On the other hand, in late autumn-winter at Eh>+200 mV and sediment temperature >10 °C, nitrate produced by sediment nitrification was thought to be denitrified subsequently; that is, the coupled nitrification-denitrification may have taken place in the upper layer of mudflat sediments.