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

Abstract

New data forced to raise previous estimates of oceanic denitrification. The revised estimate of approx. 450 Tg N yr sup(-1) (Tg = 10 sup(12) g) produces an oceanic fixed N budget with a large deficit (approx. 200 Tg N yr sup(1)) 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 sub(2) via the "biological pump". Our concepts of the oceanic N sub(2) O regime to take better account of the extremely high N sub(2)O saturations that can arise in productive, low oxygen waters had to be modified. Recent results from the western Indian Shelf during a period when hypoxic, suboxic and anoxic waters were present produced a maximum surface N sub(2)O saturation of greater than 8000%, a likely consequence of "stop and go" denitrification. The sensitivity of N sub(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 sub(2)O could change rapidly