Influence of environmental factors on net N2 and N2O production in sediment of freshwater rivers

Denitrification is an important N removal process in aquatic systems but is also implicated as a potential source of global N2O emissions. However, the key factors controlling this process as well as N2O emissions remain unclear. In this study, we identified the main factors that regulate the production of net N-2 and N2O in sediments collected from rivers with a large amount of sewage input in the Taihu Lake region. Net N-2 and N2O production were strongly associated with the addition of NO3--N and NH4+-N. Specifically, NO3--N controlled net N-2 production following Michaelis-Menten kinetics. The maximum rate of net N-2 production (V-max) was 116.3 mu mol N-2-N m(-2) h(-1), and the apparent half-saturation concentration (k(m)) was 0.65 mg N L-1. N2O to N-2 ratios increased from 0.18 +/- 0.03 to 0.68 +/- 0.16 with the addition of NO3--N, suggesting that increasing NO3--N concentrations favored the production of N2O more than N-2. The addition of acetate enhanced net N-2 production and N2O to N-2 ratios, but the ratios decreased by about 59.5 % when acetate concentrations increased from 50 to 100 mg C L-1, suggesting that the increase of N2O to N-2 ratios had more to do with the net N-2 production rate rather than acetate addition in this experiment. The addition of Cl- did not affect the net N-2 production rates, but significantly enhanced N2O to N-2 ratios (the ratios increased from 0.02 +/- 0.00 to 0.10 +/- 0.00), demonstrating that the high salinity effect might have a significant regional effect on N2O production. Our results suggest that the presence of N-enriching sewage discharges appear to stimulate N removal but also increase N2O to N-2 ratios.