Denitrification rates as measured by the isotope pairing method and by the acetylene inhibition technique in continental shelf sediments of the North Sea

Abstract

Denitrification rates in continental shelf sediments of the North Sea were measured by the simultaneous application of the acetylene inhibition technique (AIT) and the isotope pairing method (IPM). Nitrification was the predominant source of nitrate in these sediments. Both techniques were able to detect coupled nitrification-denitrification activity. Denitrification rates measured by the AIT were only measurable during the first 60 min of the incubation. Rates obtained by the AIT in this period ranged between 5.4 and 7.3 pm01 N m" h-' Continued incubation of acetylene amended sediment cores resulted in decreasing denitrification rates, suggesting that accumulated nitrous oxide was consumed despite the presence of acetylene. Denitrification rates obtained by the application of the IPM ranged between 9.8 and 13.2 pm01 N m-' h-' Constant production of single-labelled ('"N1'N) relative to double-labelled (lSN''N) dinitrogen indicated a homogeneous mixlng of both isotopic nitrate species through the denitnfication zone. The present results suggest that previously published data severely underestimated denitnfication rates in North Sea continental shelf sediments. Concomitant measured fluxes of nitrate and ammonium across the sediment-water interface reveal that in these sediments most of the deposited nitrogen is returned to the water column as ammonium and nitrate.

The impact of sedimentary alkalinity release on the water column CO<sub>2</sub> system in the North Sea

“…This way, we obtain a denitrification rate of 1.94 mmol N m −2 d −1 or 2.43 mmol C m −2 d −1 , with an associated A T release of 2.29 mmol m −2 d −1 . In the North Sea, a tight coupling of nitrification and denitrification has indeed been reported by Lohse et al (1993) and Pätsch and Kühn (2008), and observed benthic denitrification rates in the SNS range from 1.6 to 4.6 mmol C m −2 d −1 (Lohse et al, 1996;Hydes et al, 1999), a range that centrally embraces our estimate here. Note that the coupling of ammonification, nitrification and subsequently denitrification does not lead to net production of A T .…”

Looking beyond stratification: a model-based analysis of the biological drivers of oxygen deficiency in the North Sea

“…This is in good agreement with the findings by Greenwood et al (2010), who observed bottom O 2 concentrations less than 6 mg O 2 L −1 in this area. As in region 3, the simulated benthic remineralisation rate lies at the lower end of the range of 5.6 to 30.6 mmol O 2 m −2 d −1 obtained from observational studies near this site (Lohse et al, 1996;Weston et al, 2008). This suggests that the model most likely underestimates benthic remineralisation rates.…”

Long-term variability of winter nitrate concentrations in the Northern Wadden Sea driven by freshwater discharge, decreasing riverine loads and denitrification

“…The imbalance was probably due to an underestimation of the used denitrification rates (van Beusekom et al 1999). Indeed, recent progress in the methodology of denitrification measurements show that early methods like the acetylene blocking method underestimate denitrification rates (van Luijn et al 1996;Lohse et al 1996;van Beusekom et al 1999). Based on available measurements and budgets, van Beusekom estimated an annual denitrification rate of about 600 mmol m -2 per year for the Wadden Sea.…”