Linking denitrification with ecosystem respiration in mountain streams

Rivers denitrify a portion of their nitrate ( NO 3 − ) load, but estimates are difficult using microcosm or reach‐scale measurements that require specific biogeochemical and hydrologic conditions. Measuring reach‐scale oxygen (O 2 ) respiration fluxes is easier than nitrogen (N 2 ) fluxes, thus we paired microcosm estimates of denitrification by N 2 production with estimates of aerobic respiration. The median molar ratio of ΔN 2 :−ΔO 2 from 13 streams was 0.011 (95% credible interval 0.0002–0.027 mol:mol). We then measured diel O 2 concentrations from 11 streams and converted to ecosystem respiration (ER) using a multiday oxygen model. Given reach‐scale ER of −160 mmol O 2 m −2 d −1 , the estimated median denitrification was 1.5 mmol N 2 m −2  d −1 (credible interval (CI): 0.18–4.21) across our streams. Our estimates of denitrification constituted 19% of gross NO 3 − uptake (CI: 0–51%). In streams, ΔN 2 :−ΔO 2 was lower than in estuarine and marine ecosystems. Despite multiple sources of error, this approach estimates reach‐scale denitrification and variation with NO 3 − concentrations.