Coupling the N-15/N-14 and O-18/O-16 of nitrate as a constraint on benthic nitrogen cycling

We report 15N/14N and 18O/16O ratios of nitrate in benthic chamber incubations in the continental shelf sediments of the Santa Monica Bay (SMB) to deconvolve the effects of nitrification and denitrification. Estimates of denitrification rate from benthic flux stoichiometry range from 0.9 to 2.5 mmol N m 2 d 1. Between 46% and 100% of the total denitrification can be explained by nitrate or nitrite from nitrification. In general and independent of the denitrification rate and the fraction of remineralized N being denitrified, nitrate 15N/14N and 18O/16O ratios do not change significantly with progressive nitrate depletion in the chambers. A lack of change in nitrate 15N/14N could be due to either the lack of effective N isotope fractionation associated with sedimentary denitrification or the balancing of a denitrification isotope effect by the addition of low-15N/14N nitrate from nitrification. However, the lack of an increase in nitrate 18O/16O indicates that the isotopic fractionation specifically associated with sedimentary denitrification is, in fact, negligible. The coupled N and O isotope measurements also indicate that there is no significant gross efflux of 15N-depleted nitrate from nitrification, leading to the conclusion that nitrification is closely coupled to denitrification, even in the bioturbated sediments of the SMB.