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

Abstract

The isotope pairing technique (IPT) is a well‐established 15N method for estimation of denitrification. Presence of anammox, the anaerobic oxidation of NH4+ to N2 with NO2− results in violation of central assumptions on which the IPT is built. It is shown that anammox activity causes overestimation of the N2 production calculated by the IPT. However, experiments with different additions of 15NO3− 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 15N‐N2 production in 15NOx−‐amended intact sediment cores and data addressing the contribution of anammox to total N2 production estimated from slurry incubations. The other procedure is based on measurements of 15N2 production in at least two parallel series of sediment cores incubated with different 15NOx− 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‐N2 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 N2 production. The overestimation of true denitrification was, however, up to several hundred percent. Our analysis suggests however that the IPT does not seriously overestimate N2 production in estuarine sediments because anammox accounts for <6% of N2 production in such sediments, according to present knowledge.