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Night and day: short-term variation in nitrogen chemistry and nitrous oxide emissions from streams.

Baulch, H.M., Dillon, P.J., Maranger, R., Venkiteswaran, J.J., Wilson, H.F., and Schiff, S.L. (2012). "Night and day: short-term variation in nitrogen chemistry and nitrous oxide emissions from streams.", Freshwater Biology, 57(3), pp. 509-525. doi : 10.1111/j.1365-2427.2011.02720.x  Access to full text

Abstract

1. Diel variation in metabolism contributes to variation in oxygen (O2) concentrations in streams. This variation in O2 and other parameters (e.g. pH) can in turn affect the rates of microbial nitrogen (N) processing, concentrations of nitrogenous solutes and production of the greenhouse gas nitrous oxide (N2O). We investigated diel variability in emissions of N2O and the magnitude of short-term variability in N solutes across 10 streams. 2. Nitrous oxide fluxes varied on average 2.3-fold over diel cycles. Concentrations would be underestimated by sampling around noon, but N2O fluxes would not show a consistent bias. Time-weighted mean daily N2O flux was strongly related to nitrate concentration (r2 = 0.58). Diel patterns in N2O and dissolved N species were often complex (rather than simple sinusoidal curves), probably reflecting complex underlying processes. 3. Reliance on samples obtained around noon would overestimate daily mean nitrate concentrations by 5% and underestimate ammonium by 32% (average bias across all streams and dates). 4. Dissolved organic N did not show consistent day–night variation. However, the magnitude of diel variability was similar to that observed for dissolved inorganic N. Organic and inorganic N concentrations were often similar. Both appear to be dynamic components of stream N budgets. 5. The Intergovernmental Panel on Climate Change (IPCC) relies upon an emission factor to estimate indirect agricultural N2O emissions from streams and ground water. The measured emission factor (defined as the ratio of concentrations of N2O-N to NO-3-N) was typically below the recently revised IPCC default figure. Measured values varied on average 1.8-fold over approximately 24-h periods and were slightly higher at night than by day. The emission factor was actually highest in streams that were net sinks for N2O, highlighting a conceptual problem in the current IPCC method. 6. Typical sampling programmes rely on daytime-only sampling, which might cause bias in results. In our study streams, the bias was generally small. Diel variation in nitrate concentrations was related to mean temperature; variation in ammonium and N2O concentrations was greatest at low concentrations of nitrite and ammonium.

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