The oceanic fixed nitrogen and nitrous oxide budgets: Moving targets as we enter the anthropocene?

Abstract

SUMMARY: New data force us to raise previous estimates of oceanic denitrification. Our revised estimate of ~ 450 Tg N yr -1 (Tg = 10 12 g) produces an oceanic fixed N budget with a large deficit ) that can be explained only by positing an ocean that has deviated far from a steady-state, the need for a major upwards revision of fixed N inputs, particularly nitrogen fixation, or both. Oceanic denitrification can be significantly altered by small re-distributions of carbon and dissolved oxygen. Since fixed N is a limiting nutrient, uncompensated changes in denitrification affect the ocean's ability to sequester atmospheric CO 2 via the "biological pump". We have also had to modify our concepts of the oceanic N 2 O regime to take better account of the extremely high N 2 O saturations that can arise in productive, low oxygen waters. Recent results from the western Indian Shelf during a period when hypoxic, suboxic and anoxic waters were present produced a maximum surface N 2 O saturation of > 8000%, a likely consequence of "stop and go" denitrification. The sensitivity of N 2 O production and consumption to small changes in the oceanic dissolved oxygen distribution and to the "spin-up" phase of denitrification suggests that the oceanic source term for N 2 O could change rapidly.

N2O cycling at the core of the oxygen minimum zone off northern Chile

“…The open water off Peru, which is certainly the most comparable area, was studied intently during the 1980s and denitrification rates were determined through values of integrated N deficit in the water column. However, this estimation method appears to underestimate the nitrate removal due to denitrification by a factor of ≈2 (Codispoti et al 2001). Moreover, the same authors mentioned the possible exclusion of reactions and interactions in the N cycle.…”

“…O 2 and organic carbon) contribute to N 2 O cycling is not yet clear, and few relevant studies have been carried out on OMZs (Law & Owens 1990, Naqvi & Noronha 1991, Naqvi et al 2000 or suboxic waters (Elkins et al 1978, Rönner & Sörensson 1985, Brettar & Rheinheimer 1992. High denitrification rates can lead to changes in the C and N exchanges between the ocean and the atmosphere, significantly influencing global climate change (Codispoti et al 2001).…”

Origin of the deep Bering Sea nitrate deficit: Constraints from the nitrogen and oxygen isotopic composition of water column nitrate and benthic nitrate fluxes

“…Isotopic studies of these regions should be able to distinguish between these explanations. Recent studies argue for the dominance of benthic denitrification in the global N budget [Brandes and Devol, 2002;Codispoti et al, 2001;Middelburg et al, 1996], and some of them [Middelburg et al, 1996] conclude that global denitrification occurring in continental slope and deep-sea sediments by far exceeds denitrification in shelf areas. Coupling of nutrient ratio data with nitrate N and O isotope measurements may allow us to test these assertions and identify the regions most responsible for nitrate loss.…”

“…There has been a drastic upward revision in estimates of the rate of denitrification in marine sediments, such that this is now the largest single flux in the N budget [Brandes and Devol, 2002;Codispoti et al, 2001;Middelburg et al, 1996]. However, the new estimates for sedimentary denitrification are highly uncertain [Deutsch et al, 2004].…”

Microbial community gene expression within colonies of the diazotroph, Trichodesmium, from the Southwest Pacific Ocean

“…Over a decade of research has highlighted the critical importance of open ocean nitrogen fixation in supporting oligotrophic microbial food web N demands (Carpenter, 1983;Carpenter and Romans, 1991;Michaels et al, 1996;Karl et al, 1997;Capone, 2000;Codispoti et al, 2001). Nitrogen fixation is carried out by a diverse suite of microorganisms, including unicellular cyanobacteria, other Bacteria, Archaea, and colonial cyanobacteria (Dugdale et al, 1961;Carpenter, 1983;Villareal, 1990;Zehr et al, 2003).…”