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Abstract

Dinitrogen (N-2) fixation is an important source of biologically
reactive nitrogen (N) to the global ocean. The magnitude of this flux,
however, remains uncertain, in part because N-2 fixation rates have been
estimated following divergent protocols and because associated levels of
uncertainty are seldom reported-confounding comparison and extrapolation
of rate measurements. A growing number of reports of relatively low but
potentially significant rates of N-2 fixation in regions such as oxygen
minimum zones, the mesopelagic water column of the tropical and
subtropical oceans, and polar waters further highlights the need for
standardized methodological protocols for measurements of N-2 fixation
rates and for calculations of detection limits and propagated error
terms. To this end, we examine current protocols of the N-15(2) tracer
method used for estimating diazotrophic rates, present results of
experiments testing the validity of specific practices, and describe
established metrics for reporting detection limits. We put forth a set
of recommendations for best practices to estimate N-2 fixation rates
using N-15(2) tracer, with the goal of fostering transparency in
reporting sources of uncertainty in estimates, and to render N-2
fixation rate estimates intercomparable among studies.