N2O Production in the Global Ocean:Controls by Nitrification and Denitrification Pathways

Abstract

Nitrous oxide (N2O) is a potent greenhouse gas with a high global warming potential, but the underlying processes behind N2O production and consumption are not well understood. The ocean is known to be a strong source of N2O to the atmosphere, yet ocean N2O production is poorly constrained at anywhere from 10 to 53% of the total N2O atmospheric source. In order to narrow down estimates of net N2O production in the ocean, I have created a steady state model to explore the uncertainties involved in the biological processes that contribute to N2O production and consumption (nitrification and denitrification). Model results show that oxygen and organic matter supply strongly control N2O production and consumption rates, and also demonstrate strong dependence on parameters such as the oxygen threshold for oxic versus suboxic remineralization, the oxygen concentration dependence of N2O reduction to N2, the proportion of suboxic and oxic remineralization in relation to oxygen, and the attenuation length scale of sinking organic matter. Additionally, I find that global net N2O production rates are heavily influenced by the oxygen data input into the model: interpolation of oxygen data or averaging of monthly oxygen data to an annual mean can lead to underestimation of denitrification. As suboxic conditions are required for denitrification to produce (and consume) N2O, precise and accurate oxygen data is integral to constraining estimates of global ocean N2O production. The results of the base case scenario of the model, as well as from a number of sensitivity experiments, indicate that the source and sink terms for N2O production are larger than suggested by many past models. N2O production and consumption by denitrification are high in oxygen deficient zones (19.2 Tg-N y-1 and 8.37 Tg-N y-1, respectively) while global total nitrification production is low (0.740 Tg-N y-1). N2O production by denitrification is primarily responsible for generating relatively high net N2O production rates in the global ocean (11.5 Tg-N y-1 for the base case scenario), and likely provide a greater proportion of global N2O emissions to the atmosphere than many previous estimates. Model representation of denitrification production and consumption rates is highly sensitive to changes in oxygen concentration and parameters related to oxygen, suggesting that altered ocean environments due to future climate change may have a large effect on N2O production in the ocean. This research isolates the uncertainties that most impact N2O production and consumption processes and suggests pathways of future research to narrow down these uncertainties. In particular, I recommend more in-situ oxygen sampling with high precision and accuracy (especially in oxygen deficient zones), experimental research to constrain important parameters, and separation of N2O production by nitrification, N2O production by denitrification, and N2O consumption by denitrification in future model development (as in this model).