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

New data force us to raise previous estimates of oceanic denitrification. Our revised estimate of similar to 450 Tg N yr(-1) (Tg = 10(12) g) produces an oceanic fixed N budget with a large, deficit (similar to 200 Tg N yr(-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 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.