Direct measurement of dissolved N2 and denitrification along a subtropical river-estuary gradient, China

The spatial pattern and seasonal variation of denitrification were investigated during 2010-2011 in the Jiulong River Estuary (JRE) in southeast China. Dissolved N-2 was directly measured by changes in the N-2:Ar ratio. The results showed that excess dissolved N-2 ranged from -9.9 to 76.4 mu mol L-1. Tidal mixing leads to a seaward decline of dissolved gaseous concentrations and water-air fluxes along the river-estuary gradient. Denitrification at freshwater sites varied between seasons, associated with changes in N input and water temperature. The denitrification process was controlled by the nitrate level at freshwater sites, and the excess dissolved N-2 observed at the tidal zone largely originated from upstream water transport. Compared to other estuaries, JRE has a relative low gaseous removal efficiency (E-d = 12% of [DIN]; annual N removal = 24% of DIN load), a fact ascribed to strong tidal mixing, coarse-textured sediment with shallow depth before bedrock and high riverine DIN input. (C) 2012 Elsevier Ltd. All rights reserved.