MEPS 326:37-47 (2006)  -  doi:10.3354/meps326037

ABSTRACT: Anammox, i.e. the anaerobic oxidation of NH₄⁺ with NO₂^– to N₂, has redefined our understanding of nitrogen cycling in aquatic ecosystems. The isotope pairing technique (IPT) is the dominant tool for quantifying denitrification in intact sediments, but it cannot distinguish anammox from denitrification as sources of N₂ and may, where anammox is significant, lead to large errors in the estimate of true N₂ production. In a previous study, the IPT was revised in theory and a solution was proposed whereby the parameter r₁₄, i.e. the ratio of ¹⁴NO₃^– to ¹⁵NO₃^– in the NO₃^– reduction zone is used to correct the IPT in the presence of anammox. We begin by exploring the limitations of the 2 indirect techniques previously proposed for estimating r₁₄. The first, based on the contribution of anammox to N₂ production (ra) in sediment slurry incubations, underestimates anammox and cannot fully correct the IPT. The second, derived from the production of ¹⁵N-N₂ gas as a function of ¹⁵NO₃^– concentration, although valid in sieved sediment, was ineffective in natural intact sediments. In contrast, a newly developed direct technique based on the ¹⁵N-labelling of N₂O, corrects the IPT in the presence of significant anammox (48% of N₂ formation) in natural sediment and can even distinguish between anammox and denitrification in estuarine sediment with a lower anammox contribution (21%). We contrast these findings with sediments where anammox is minimal (<1%). The ¹⁵N-N₂O technique allows denitrification and anammox to be quantified in sediment cores using techniques similar to those already established with the original IPT and, importantly, shows that the contribution of anammox to N₂ production is greater than previously measured using slurries.

KEY WORDS: Anammox · Denitrification · Sediments · Isotope pairing · ¹⁵N-N₂O technique