Seasonal discrepancies in denitrification measured by isotope pairing and N2:Ar techniques

Abstract

Isotope pairing (IP) and N2:Ar techniques for measuring sediment denitrification were compared in muddy sediments in a shallow sub-tropical embayment. At a concentration of 100 µM 15 NO 3 - (our experimental conditions), IP greatly underestimated denitrification, compared to N 2:Ar, in low-respiration sediments subject to bioturbation, where denitrification produced via coupled nitrifica- tion-denitrification (Dn) dominates overall denitrification. Consistent with this underestimation was the higher N2 flux measured using N2:Ar ratios, accounting for more of the stochimetrically predicted N mineralisation. We suggest that this underestimation is primarily due to non-homogenous mixing of the added 15 NO3 - with the endogenous 14 NO3 - in the sediment, associated with differentials in the rate of diffusion of 15 NO3 - across mucus linings in burrow structures versus rates of 14 NO3 - supply from nitri- fiers in the burrow walls. Strong support for this hypothesis was provided by the relationship between the methods discrepancy and polychaete biomass (r 2 = 0.99; p = 0.001), and the almost complete removal of the methods discrepancy when the upper heterogenous sediment layer that contained the burrow structures was experimentally removed. Due to a significant increase in potential denitrifica- tion sites in association with burrow structures, this effect can potentially cause a large underestima- tion of denitrification (up to 85%) using the IP technique. Our data show no benefit in longer pre- incubation times, suggesting that this effect may represent a fundamental limitation of the IP method in the sediments of oligotrophic (low carbon loading) systems. As such, the N2:Ar flux technique rep- resents a more realistic estimation of net denitrification in sediments with low organic carbon contents and low respiration rates and significant bioturbation. Most importantly, the present study also demon- strated that methodological comparisons need to be made over a range of environmental conditions.