Measuring net denitrification flux using membrane inlet mass spectrometry in mountain rivers

Plemel, Molly
Denitrification removes nitrate in streams with high anthropogenic nitrate concentrations. In subalpine forest, forest type influences the density of logjams and thereby stream complexity, which we hypothesize increases aerobic respiration and denitrification. We measured net denitrification in six streams in the Front Range, Colorado and Wyoming, USA. Sites differed in forest type and thereby logjam density and stream complexity. Study sites also had high nitrate concentrations (0.15 mg NO3 --N/L). We used dark bottle assays and oxic incubations to measure change in dissolved nitrogen and oxygen relative to argon gas concentrations to measure net denitrification flux and respiration from stream sediments. We found no relationship between organic matter content and the net denitrification flux. Net denitrification increased with respiration. With a whole stream respiration of -4 g O m-2 d-1, we estimated a whole stream net denitrification flux of 107 mg N2 m-2 d-1, which accounted for one third of whole-stream nitrate uptake.
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