Linking denitrification with ecosystem respiration in mountain streams

Rivers denitrify a portion of their nitrate ((Formula presented.)) load, but estimates are difficult using microcosm or reach-scale measurements that require specific biogeochemical and hydrologic conditions. Measuring reach-scale oxygen (O₂) respiration fluxes is easier than nitrogen (N₂) fluxes, thus we paired microcosm estimates of denitrification by N₂ production with estimates of aerobic respiration. The median molar ratio of ΔN₂:−ΔO₂ from 13 streams was 0.011 (95% credible interval 0.0002–0.027 mol:mol). We then measured diel O₂ concentrations from 11 streams and converted to ecosystem respiration (ER) using a multiday oxygen model. Given reach-scale ER of −160 mmol O₂ m⁻² d⁻¹, the estimated median denitrification was 1.5 mmol N₂ m⁻² d⁻¹ (credible interval (CI): 0.18–4.21) across our streams. Our estimates of denitrification constituted 19% of gross (Formula presented.) uptake (CI: 0–51%). In streams, ΔN₂:−ΔO₂ was lower than in estuarine and marine ecosystems. Despite multiple sources of error, this approach estimates reach-scale denitrification and variation with (Formula presented.) concentrations.