Direct measurement of anaerobic ammonium oxidation (anammox) and denitrification in intact sediment cores

Abstract

Anammox, i.e. the anaerobic oxidation of NH 4 + with NO 2 -to N 2 , 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 N 2 and may, where anammox is significant, lead to large errors in the estimate of true N 2 production. In a previous study, the IPT was revised in theory and a solution was proposed whereby the parameter r 14 , i.e. the ratio of 14 NO 3 -to 15 NO 3 -in the NO 3 -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 r 14 . The first, based on the contribution of anammox to N 2 production (ra) in sediment slurry incubations, underestimates anammox and cannot fully correct the IPT. The second, derived from the production of 15 N-N 2 gas as a function of 15 NO 3 -