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

Nitrous oxide (N2O) 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 N2O saturation and air–sea flux during SOIREE (98–103%; −1.18–1.75 μmol/m2/d) were consistent with that obtained between 58°S 158°E and 49°S 162°E (99–104%; −0.3–4.7 μmol/m2/d), and confirmed predicted flux estimates for this region. Turbulent eddy diffusion across the pycnocline supplied an average 38% of the air–sea N2O flux, indicating a production mechanism in the upper 80 m. There was no significant difference in N2O saturation and flux between stations inside and outside the patch, although a N2O saturation maximum in the pycnocline at most stations inside the iron-fertilised patch was not present at stations outside. The mean N2O profile for the stations outside the patch was used as a control to identify pycnocline N2O production, which increased during SOIREE and co-varied with iron-mediated increases in phytoplankton biomass. The mechanisms for iron-mediated N2O production in the pycnocline are considered. On longer timescales, the decrease in radiative forcing resulting from carbon fixation and CO2 uptake during SOIREE may be subsequently offset by 6–12% by N2O production. Furthermore, analysis of scenarios of large-scale Southern Ocean fertilisation supports previous observations that any decrease in radiative forcing due to CO2 drawdown may be partially or totally negated by an increase in N2O production.