Application of the isotope pairing technique in sediments where anammox, denitrification, and dissimilatory nitrate reduction to ammonium coexist

The N-15 isotope pairing technique is widely used to quantify anammox, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) in sediments. However, the effects of DNRA on anammox and denitrification in slurry incubations and on genuine N-2 production in intact core incubations have not been fully explored. We developed a mathematical model describing these effects of DNRA, and tested this model in field and computational studies. Our model calculations revealed that for slurry incubations the presence of DNRA tends to under -and overestimate the actual anammox and denitrification rates, respectively, when calculated according to Thamdrup and Dalsgaard (2002). We found this underestimate of anammox to be proportional to the (NH4+)-N-15 mole fraction (F-A), and the overestimate of denitrification to be related to both F-A and the relative contribution of anammox to the total N-2 production (ra). We propose three alternative procedures to better quantify anammox and denitrification rates in slurry incubations and two procedures for intact core incubations. Our model calculations also revealed that for intact core incubations, the presence of DNRA leads to an overestimate of the genuine N-2 production rate when calculated according to Risgaard-Petersen et al. (2003). This overestimate depends on the ra and the mole fractions of (NH4+)-N-15 and (NO3-)-N-15 in intact core incubations. The results of our field experiments and numerical modeling indicated that the overestimate of the genuine N-2 production was less than 1% at two sites in the East China Sea.