Nitrous oxide flux and response to increased iron availability in the Antarctic Circumpolar Current

Nitrous oxide (N 2O) profiles were obtained at stations inside and outside an area of iron-fertilised surface water at 61°S 140°E during the Southern Ocean Iron Enrichment Experiment (SOIREE). Surface N 2O saturation and air–sea flux during SOIREE (98–103%; −1.18–1.75 μmol/m 2/d) were consistent with that obtained between 58°S 158°E and 49°S 162°E (99–104%; −0.3–4.7 μmol/m 2/d), and confirmed predicted flux estimates for this region. Turbulent eddy diffusion across the pycnocline supplied an average 38% of the air–sea N 2O flux, indicating a production mechanism in the upper 80 m. There was no significant difference in N 2O saturation and flux between stations inside and outside the patch, although a N 2O saturation maximum in the pycnocline at most stations inside the iron-fertilised patch was not present at stations outside. The mean N 2O profile for the stations outside the patch was used as a control to identify pycnocline N 2O production, which increased during SOIREE and co-varied with iron-mediated increases in phytoplankton biomass. The mechanisms for iron-mediated N 2O production in the pycnocline are considered. On longer timescales, the decrease in radiative forcing resulting from carbon fixation and CO 2 uptake during SOIREE may be subsequently offset by 6–12% by N 2O production. Furthermore, analysis of scenarios of large-scale Southern Ocean fertilisation supports previous observations that any decrease in radiative forcing due to CO 2 drawdown may be partially or totally negated by an increase in N 2O production.