MEPS 326:37-47 (2006) - doi:10.3354/meps326037
Direct measurement of anaerobic ammonium oxidation (anammox) and denitrification in intact sediment cores
Mark Trimmer1,*, Nils Risgaard-Petersen2, Joanna C. Nicholls1, Pia Engström3
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
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