Constraints on global oceanic emissions of N 2O from observations and models
We estimate the global ocean N 2O flux to the atmosphere and its confidence interval using a statistical method based on model perturbation simulations and their fit to a database of Δ pN 2O ( n = 6136). We evaluate two submodels of N 2O production. The first submodel splits N 2O production into oxic and hypoxic pathways following previous publications. The second submodel explicitly represents the redox transformations of N that lead to N 2O production (nitrification and hypoxic denitrification) and N 2O consumption (suboxic denitrification), and is presented here for the first time. We perturb both submodels by modifying the key parameters of the N 2O cycling pathways (nitrification rates; NH 4+ uptake; N 2O yields under oxic, hypoxic and suboxic conditions) and determine a set of optimal model parameters by minimisation of a cost function against four databases of N cycle observations. Our estimate of the global oceanic N 2O flux resulting from this cost function minimisation derived from observed and model Δ pN 2O concentrations is 2.4 ± 0.8 and 2.5 ± 0.8 Tg N yr −1 for the two N 2O submodels. These estimates suggest that the currently available observational data of surface Δ pN 2O constrain the global N 2O flux to a narrower range relative to the large range of results presented in the latest IPCC report.