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 N₂ and N₂O in sediments collected from rivers with a large amount of sewage input in the Taihu Lake region. Net N₂ and N₂O production were strongly associated with the addition of NO₃(-)-N and NH₄(+)-N. Specifically, NO₃(-)-N controlled net N₂ production following Michaelis-Menten kinetics. The maximum rate of net N₂ production (V max) was 116.3 μmol N2-N m(-2) h(-1), and the apparent half-saturation concentration (k m) was 0.65 mg N L(-1). N₂O to N₂ 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 N₂. The addition of acetate enhanced net N₂ production and N₂O to N₂ 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 N₂ ratios had more to do with the net N₂ production rate rather than acetate addition in this experiment. The addition of Cl(-) did not affect the net N₂ production rates, but significantly enhanced N₂O to N₂ 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 N₂ ratios.