Global rates of water-column denitrification derived from nitrogen gas measurements

Abstract

Biologically available nitrogen (N) limits phytoplankton growth over much of the ocean. The rate at which N is removed from the contemporary ocean by denitrifying bacteria is highly uncertain^1,2,3. Some studies suggest that N losses exceed inputs^2,4,5,6; others argue for a balanced budget^3,7,8. Here, we use a global ocean circulation model to simulate the distribution of N₂ gas produced by denitrifying bacteria in the three main suboxic zones in the open ocean. By fitting the model to measured N₂ gas concentrations, we infer a globally integrated rate of water-column denitrification of 66±6 Tg N yr⁻¹. Taking into account isotopic constraints on the fraction of denitrification occurring in the water column versus marine sediments, we estimate that the global rate of N loss from marine sediments and the oceanic water column combined amounts to around 230±60 Tg N yr⁻¹. Given present estimates of N input rates, our findings imply a net loss of around 20 ± 70 Tg of N from the global ocean each year, indistinguishable from a balanced budget. A balanced N budget, in turn, implies that the marine N cycle is governed by strong regulatory feedbacks.