Application of the isotope pairing technique in sediments where anammox and denitrification coexist

The isotope pairing technique (IPT) is a well‐established 15N method for estimation of denitrification. Presence of anammox, the anaerobic oxidation of NH 4 + to N 2 with NO 2 − results in violation of central assumptions on which the IPT is built. It is shown that anammox activity causes overestimation of the N 2 production calculated by the IPT. However, experiments with different additions of 15 NO 3 − will reveal the problems posed by anammox. Two alternative calculation procedures are presented, which enable a more accurate quantification of anammox and denitrification activity in sediments where the processes coexist. One procedure is based on measurements of 15 N ‐ N 2 production in 15 NO x − ‐amended intact sediment cores and data addressing the contribution of anammox to total N 2 production estimated from slurry incubations. The other procedure is based on measurements of 15 N 2 production in at least two parallel series of sediment cores incubated with different 15 NO x − additions. The calculation procedure presented is used on field data from four studies where the IPT was used and the potential anammox rate measured. The IPT overestimated total 14N‐ N 2 production rates by 0%, 2.5%, 31%, and 82% relative to the revised estimates from the 4 different sites, where anammox accounted for 0%, 6%, 18%, and 69.8%, respectively, of N 2 production. The overestimation of true denitrification was, however, up to several hundred percent. Our analysis suggests however that the IPT does not seriously overestimate N 2 production in estuarine sediments because anammox accounts for <6% of N 2 production in such sediments, according to present knowledge.