Understanding the magnitude of nitrogen (N) loss and recycling pathways is crucial for coastal N management efforts. However, quantification of denitrification and anammox by a widely-used method, the isotope pairing technique, is challenged when dissimilatory NO₃^- reduction to NH₄⁺ (DNRA) occurs. In this study, we describe a revised isotope pairing technique that accounts for the influence of DNRA on NO₃^- reduction (R-IPT-DNRA). The new calculation procedure improves on previous techniques by (1) accounting for N₂O production, (2) distinguishing canonical anammox from coupled DNRA-anammox, and (3) including the production of ³⁰N₂ by anammox in the quantification of DNRA. This approach avoids the potential for substantial underestimates of anammox rates and overestimates of denitrification rates in systems where DNRA is a significant NO₃^- reduction pathway. We apply this technique to simultaneously quantify rates of anammox, denitrification, and DNRA in intact sediments adjacent to a seagrass bed in subtropical Australia. The effect of organic carbon lability on NO₃^- reduction was also addressed by adding detrital sources with differing C:N (phytoplankton- or seagrass-derived). DNRA was the predominant pathway, contributing 49-74% of total NO₃^- reduction (mean 0.42 µmol N m^-2 h^-1). In this high C:N system, DNRA outcompetes denitrification for NO₃^-, functioning to recycle rather than remove N. Anammox exceeded denitrification (mean 0.18 and 0.04 µmol N m^-2 h^-1, respectively) and accounted for 64-86% of N loss, a rare high percentage in shallow coastal environments. Owing to low denitrification activity, N₂O production was ∼100-fold lower than in other coastal sediments (mean 7.7 nmol N m^-2 h^-1). All NO₃^- reduction pathways were stimulated by seagrass detritus but not by phytoplankton detritus, suggesting this microbial community is adapted to process organic matter that is typically encountered. The R-IPT-DNRA is widely applicable in other environments where the characterization of co-existing NO₃^- reduction pathways is desirable.